Rainbow Electronics MAX1865 User Manual

General Description
The MAX1864/MAX1865 power-supply controllers are designed to address cost-conscious applications such as cable modem Consumer Premise Equipment (CPE), xDSL CPE, and set-top boxes. Operating off a low-cost, unregulated DC supply (such as a wall adapter output), the MAX1864 generates three positive outputs, and the MAX1865 generates four positive outputs and one neg­ative output to provide a cost-effective system power supply.
The MAX1864 includes a current-mode synchronous step-down controller and two positive regulator gain blocks. The MAX1865 has one additional positive gain block and one negative regulator gain block. The main synchronous step-down controller generates a high-current output that is preset to 3.3V or adjustable from 1.236V to 0.8 ✕ VINwith an external resistive­divider. The 100kHz/200kHz operating frequency allows the use of low-cost aluminum-electrolytic capaci­tors and low-cost power magnetics. Additionally, the MAX1864/MAX1865 step-down controllers sense the voltage across the low-side MOSFET’s on-resistance to efficiently provide the current-limit signal, eliminating the need for costly current-sense resistors.
The MAX1864/MAX1865 generate additional supply rails at low cost. The positive regulator gain blocks use an external PNP pass transistor to generate low-voltage rails directly from the main step-down converter (such as 2.5V or 1.8V from the main 3.3V output) or higher voltages using coupled windings from the step-down converter (such as 5V, 12V, or 15V). The MAX1865’s negative gain block uses an external NPN pass transis­tor in conjunction with a coupled winding to generate
-5V, -12V, or -15V.
All output voltages are externally adjustable, providing maximum flexibility. Additionally, the MAX1864/ MAX1865 feature soft-start for the step-down converter and all the positive linear regulators, and have a power­good output that monitors all of the output voltages.
Applications
xDSL, Cable, and ISDN Modems
Set-Top Boxes
Wireless Local Loop
Features
4.5V to 28V Input Voltage Range
Master DC-DC Step-Down Converter
Preset 3.3V or Adjustable (1.236V to 0.8 ✕ V
IN
)
Output Voltage
Fixed-Frequency (100kHz/200kHz) PWM Controller
No Current-Sense Resistor
Adjustable Current Limit
95% Efficient
Two (MAX1864)/Four (MAX1865) Analog
Gain Blocks
Positive Analog Blocks Drive Low-Cost PNP Pass Transistors to Build Positive Linear Regulators
Negative Analog Block (MAX1865) Drives a Low-Cost NPN Pass Transistor to Build a Negative Linear Regulator
Power-Good Indicator
Soft-Start Ramp for All Positive Regulators
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
________________________________________________________________ Maxim Integrated Products 1
Pin Configurations
Ordering Information
19-2030; Rev 0; 4/01
Pin Configurations continued at end of data sheet.
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
PART
MAX1864TEEE -40°C to +85°C 16 QSOP 200
MAX1864UEEE -40°C to +85°C 16 QSOP 100
MAX1865TEEP -40°C to +85°C 20 QSOP 200
MAX1865UEEP -40°C to +85°C 20 QSOP 100
TEMP.
RANGE
PIN­PACKAGE
TOP VIEW
1
POK IN
COMP
2
OUT
3
MAX1864
4
FB
B2
5
FB2
6
B3
7
FB3
8
16
15
VL
14
BST
13
DH
12
LX
DL
11
10
GND
9
ILIM
f
OSC
(kHz)
16 QSOP
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN= 12V, ILIM = FB = GND, V
BST
- VLX= 5V, TA= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
IN, B2, B3, B4 to GND............................................-0.3V to +30V
B5 to OUT...............................................................-20V to +0.3V
VL, POK, FB, FB2, FB3, FB4, FB5 to GND ...............-0.3V to +6V
LX to BST..................................................................-6V to +0.3V
BST to GND ............................................................-0.3V to +36V
DH to LX....................................................-0.3V to (V
BST
+ 0.3V)
DL, OUT, COMP, ILIM to GND......................-0.3V to (V
L
+ 0.3V)
VL Output Current ...............................................................50mA
VL Short Circuit to GND...................................................100ms
Continuous Power Dissipation (T
A
= +70°C)
16-Pin QSOP (derate 8.3mW/°C above +70°C)...........666mW
20-Pin QSOP (derate 9.1mW/°C above +70°C)...........727mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
GENERAL
Operating Input Voltage Range (Note 1)
V
IN
4.5 28 V
MAX1864 1.0 2
Quiescent Supply Current I
IN
VFB = 0, V
OUT
= 4V,
V
FB2
= V
FB3
= V
FB4
= 1.5V,
V
FB5
= -0.1V
MAX1865 1.4 3
mA
VL REGULATOR
Output Voltage VL 6V < VIN < 28V, 0.1mA < I
LOAD
< 20mA
V
Power-Supply Rejection PSRR VIN = 6V to 28V 3 %
Undervoltage Lockout Trip Level
VL rising, 3% hysteresis (typ) 3.2 3.5 3.8 V
Minimum Bypass Capacitance
)
10m < ESR < 500m F
DC-DC CONTROLLER
Output Voltage (Preset Mode) V
OUT
FB = GND
V
Typical Output Voltage Range (Adjustable Mode) (Note 2)
V
OUT
V
FB Set Voltage (Adjustable Mode)
V
SET
FB = COMP
V
FB Dual Mode Threshold 50
mV
FB Input Leakage Current I
FB
VFB = 1.5V
nA
FB to COMP Transconductance g
m
FB = COMP, I
COMP
= ±5µA 70
µS
Current-Sense Amplifier Voltage Gain
A
LIM
VIN - VLX = 250mV
4.9
V/V
Current-Limit Threshold (Internal Mode)
V
ILIM
= 5.0V
mV
Current-Limit Threshold (External Mode)
V
ILIM
= 2.5V
mV
Dual Mode is a trademark of Maxim Integrated Products, Inc.
V
UVLO
C
BYP(MIN
V
VALLEY
V
VALLEY
4.75 5.00 5.25
3.272 3.314 3.355
1.236 0.8 x V
1.221 1.236 1.252
100 150
0.01 100
100 140
4.46
190 250 310
440 530 620
IN
5.44
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VIN= 12V, ILIM = FB = GND, V
BST
- VLX= 5V, TA= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Switching Frequency f
Maximum Duty Cycle D
Soft-Start Period t
Soft-Start Steps V
DH Output Low Voltage I
DH Output High Voltage I
DL Output Low Voltage I
DL Output High Voltage I
DH, DL On-Resistance 310
Output Drive Current Sourcing or sinking, VDH or VDL = VL/2 0.5 A
LX, BST Leakage Current V
POSITIVE ANALOG GAIN BLOCKS
FB2, FB3, FB4 Regulation Voltage
FB2, FB3, FB4 to B_ Transconductance
Feedback Input Leakage Current
Driver Sink Current IB_
NEGATIVE ANALOG GAIN BLOCK
FB5 Regulation Voltage
FB5 to B5 Transconductance ∆V
Feedback Input Leakage Current
Driver Source Current I
POWER GOOD (POK)
OUT Trip Level (Preset Mode) FB = GND, falling edge, 1% hysteresis (typ) 2.88 3 3.12 V
FB Trip Level (Adjustable Mode) Falling edge, 1% hysteresis (typ) 1.070 1.114 1.159 V
FB2, FB3, FB4 Trip Level Falling edge, 1% hysteresis (typ) 1.070 1.114 1.159 V
FB5 Trip Level Rising edge, 35mV hysteresis (typ) 368 500 632 mV
POK Output Low Level I
POK Output High Leakage V
THERMAL PROTECTION (Note 3)
Thermal Shutdown Rising temperature 160 °C Thermal Shutdown Hysteresis 15 °C
OSC
SOFT
V
I
I
MAX
FB
FB5
B5
MAX186_T 160 200 240
MAX186_U 80 100 120
77 82 90 %
1024 1/f
/64 V
REF
= 10mA, measured from DH to LX 0.1 V
SINK
= 10mA, measured from BST to DH 0.1 V
SOURCE
= 10mA, measured from DL to GND 0.1 V
SINK
= 10mA, measured from DL to GND VL - 0.1 V
SOURCE
= VLX = VIN = 28V, VFB = 1.5V 0.03 20 µA
BST
VB2 = VB3 = VB4 = 5V, I
= IB3 = IB4 = 1mA (sink)
B2
V
= VB3 = VB4 = 5V, IB2 = IB3 = IB4 =
B2
_
FB
0.5mA to 5mA (sink)
_V
FB2
V
FB2
V
FB4
V
B5
= V
FB3
= V
FB3
= 1.188V
= V
OUT
= V
FB4
=
- 2V, V
= 1.5V 0.01 100 nA
VB2 = VB3 = VB4 = 2.5V 10 23
V
= VB3 = VB4 = 4.0V 26
B2
= 3.5V, IB5 = 1mA
OUT
(source)
VB5 = 0, IB5 = 0.5mA to 5mA (source) -13 -20 mV
FB5
V
= -100mV 0.01 100 nA
FB5
V
= 200mV, VB5 = V
FB5
OUT
- 2.0V, V
OUT
3.5V
= 1mA 0.4 V
SINK
= 5V 1 µA
POK
1.226 1.240 1.257 V
-1 -1.75 %
-20 -5 +10 mV
=
10 25 mA
kHz
OSC
mA
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS
(VIN= 12V, ILIM = FB = GND, V
BST
- VLX= 5V, TA= -40°C to +85°C, unless otherwise noted.) (Note 4)
PARAMETER SYMBOL CONDITIONS MIN MAX UNITS
GENERAL
Operating Input Voltage Range (Note 1)
Quiescent Supply Current I
VL REGULATOR
Output Voltage VL 6V < VIN < 28V, 0.1mA < I
Power-Supply Rejection PSRR VIN = 6V to 28V 3 %
Undervoltage Lockout Trip Level V
DC-DC CONTROLLER
Output Voltage (Preset Mode) V
Feedback Set Voltage (Adjustable Mode)
Current-Sense Amplifier Voltage Gain
Current-Limit Threshold (Internal Mode)
Current-Limit Threshold (External Mode)
Switching Frequency f
Maximum Duty Cycle D
POSITIVE ANALOG GAIN BLOCKS
FB2, FB3, FB4 Regulation Voltage
V
IN
VFB = 0, V
IN
UVLO
OUT
V
SET
A
LIM
V
VALLEYVILIM
V
VALLEYVILIM
OSC
MAX
= V V
VL rising, 3% hysteresis (typ) 3 4 V
FB = GND 3.247 3.380 V
FB = COMP 1.211 1.261 V
VIN - VLX = 250mV 4.12 5.68 V/V
MAX186_T 160 240
MAX186_U 80 120
VB2 = VB3 = VB4 = 5V, IB2 = IB3 = IB4 = 1mA (sink)
FB3
= -0.1V
FB5
= 5V 150 350 mV
= 2.5V 400 660 mV
= V
OUT
FB4
= 4V, V
= 1.5V,
FB2
MAX1864 2
MAX1865 3
<20mA 4.75 5.25 V
LOAD
4.5 28 V
74 90 %
1.215 1.265 V
mA
kHz
FB2, FB3, FB4 to B_ Transconductance
NEGATIVE ANALOG GAIN BLOCK
FB5 Regulation Voltage
FB5 to B5 Transconductance ∆V
V
FB
FB5
V
= VB3 = VB4 = 5V, IB2 = IB3 = IB4 =
B2
_
0.5mA to 5mA (sink)
V
= V
OUT
- 2V, V
B5
(source)
VB5 = 0, IB5 = 0.5mA to 5mA (source) -30 mV
= 3.5V, IB5 = 1mA
OUT
-25 +10 mV
-2.25 %
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
_______________________________________________________________________________________ 5
Note 1: Connect VL to IN for operation with VIN< 5V. Note 2: See Output Voltage Selection section. Note 3: The internal 5V linear regulator (VL) powers the thermal shutdown block. Shorting VL to GND disables thermal shutdown. Note 4: Specifications to -40°C are guaranteed by design, not production tested.
ELECTRICAL CHARACTERISTICS (continued)
(VIN= 12V, ILIM = FB = GND, V
BST
- VLX= 5V, TA= -40°C to +85°C, unless otherwise noted.) (Note 4)
Typical Operating Characteristics
(Circuit of Figure 1, VIN= 12V, V
OUT
= 3.3V, TA = +25°C, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN MAX UNITS
POWER GOOD (POK)
OUT Trip Level (Preset Mode) FB = GND, falling edge, 1% hysteresis (typ) 2.85 3.15 V
FB Trip Level (Adjustable Mode)
FB2, FB3, FB4 Trip Level Falling edge, 1% hysteresis (typ) 1.058 1.17 V
FB5 Trip Level Rising edge, 35mV hysteresis (typ) 325 675 mV
Falling edge, 1% hysteresis (typ) 1.058 1.17 V
EFFICIENCY vs. LOAD CURRENT
(PRESET MODE)
100
VIN = 6.5V
90
80
70
EFFICIENCY (%)
60
50
0.01 1010.1 LOAD CURRENT (A)
EFFICIENCY vs. LOAD CURRENT
(ADJUSTABLE MODE)
100
VIN = 6.5V
90
80
70
EFFICIENCY (%)
60
50
0.01 1010.1 LOAD CURRENT (A)
VIN = 8V
VIN = 12V
VIN = 18V
VIN = 24V
V
= 5.0V
OUT
VIN = 8V
VIN = 12V
VIN = 18V
VIN = 24V
OUTPUT VOLTAGE vs. LOAD CURRENT
(PRESET MODE)
3.33
3.32
MAX1864/65 toc01
3.31
3.30
3.29
OUTPUT VOLTAGE (V)
3.28
V
= 3.3V
OUT
3.27 0 1.0 1.50.5 2.0 2.5 3.0
LOAD CURRENT (A)
MAX1864/65 toc02
MAX1864/65 toc03
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
6 _______________________________________________________________________________________
E
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN= 12V, V
OUT
= 3.3V, TA = +25°C, unless otherwise noted.)
4.95
4.97
4.99
5.01
5.03
5.05
01.00.5 1.5 2.0 2.5 3.0
OUPUT VOLTAGE vs. LOAD CURRENT
(ADJUSTABLE MODE)
MAX1864/65 toc04
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
4.95
4.97
4.99
5.01
5.03
5.05
0105 15202530
INTERNAL 5V LINEAR REGULATOR
vs. LOAD CURRENT
MAX1864/65 toc05
LOAD CURRENT (mA)
VL (V)
1ms/div
LOAD TRANSIENT
(STEP-DOWN CONVERTER)
3.5V
3.1V
B
A
MAX1864/65 toc06
1A
0
3.3V
A. V
OUT
= 3.3V (PRESET), 200mV/div
B. I
OUT
= 10mA TO 1A, 500mA/div
V
IN
= 12V
SWITCHING WAVEFORMS
(STEP-DOWN CONVERTER)
3.35V
3.30V
1.5A
1A
0.5A
10V
0
A. V
= 3.3V (PRESET), I
OUT
B. INDUCTOR CURRENT, 500mA/div
, 10V/div
C. V
LX
= 12V
V
IN
2µs/div
OUT
MAX1864/65 toc07
= 1A, 50mV/div
SOFT-START
5V
A
B
C
0
4V
2V
0
1A
0
, 5V/div
A. V
L
= 3.3V (PRESET), 2V/div
B. V
OUT
C. INDUCTOR CURRENT, 1A/div
= 0 TO 12V
V
IN
1ms/div
MAX1864/65 toc08
A
B
C
DRIVE CURRENT vs. BASE-DRIVE VOLTAG
40
V
= 1.0V
35
30
25
20
15
10
BASE-DRIVE SINK CURRENT (mA)
5
0
0246810
FB_
V
= 0.96V
FB_
(MAX1865) ONLY
BASE VOLTAGE (V)
REF
B2, B3 AND B4
MAX1864/65 toc09
POSITIVE LINEAR REGULATOR BASE-
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
_______________________________________________________________________________________ 7
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN= 12V, V
OUT
= 3.3V, TA = +25°C, unless otherwise noted.)
2.50
0.01 10.1 100
POSITIVE LINEAR REGULATOR
OUTPUT VOLTAGE vs. LOAD CURRENT
(Q
LDO
= 2N3905)
2.42
2.44
2.46
2.48
MAX18664/65 toc10
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
10 1000
V
SUP(POS)
= 5.0V
V
SUP(POS)
= 3.3V
2.42
2.44
2.46
2.48
2.50
243 5678
POSITIVE LINEAR REGULATOR
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
(Q
LDO
= 2N3905)
MAX1864/65 toc11
SUPPLY VOLTAGE (V)
OUTPUT VOLTAGE (V)
I
OUT2
= 1mA
I
OUT2
= 100mA
80
70
60
50
40
30
20
10
0
0.1 10 1001 1000
POSITIVE LINEAR REGULATOR
POWER-SUPPLY REJECTION RATIO
(Q
LDO
= 2N3905)
MAX1864/65 toc12
FREQUENCY (kHz)
PSRR (dB)
I
OUT2
= 50mA
POSITIVE LINEAR REGULATOR
LOAD TRANSIENT
(Q
LDO
= 2W3905)
MAX1864/65 toc13
100mA
2.457V
2.467V
A. I
OUTZ
= 1mA TO 100mA, 50mA/div
B. V
OUTZ
= 2.5V, 5mV/div
C
LDO(POS)
= 10µF CERAMIC, V
SUP(POS)
= 3.3V
CIRCUIT OF FIGURE 1
0
B
A
10µs/div
2.50
0.01 0.1 1000100
POSITIVE LINEAR REGUALTOR
OUTPUT VOLTAGE vs. LOAD CURRENT
(Q
LDO
= TIP30)
2.42
2.44
2.46
2.48
MAX1864/65 toc14
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
110
V
SUP(POS)
= 5.0V
V
SUP(POS)
= 3.3V
2.42
2.44
2.46
2.48
2.50
24681012
POSITIVE LINEAR REGULATOR
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
(Q
LDO
= TIP30)
MAX1864/65 toc15
SUPPLY VOLTAGE (V)
OUTPUT VOLTAGE (V)
I
OUT2
= 1mA
I
OUT2
= 100mA
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
8 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN= 12V, V
OUT
= 3.3V, TA = +25°C, unless otherwise noted.)
-12.00
-12.06
-12.12
-12.18
-12.24
0.01 1
10
1000.1 1000
NEGATIVE LINEAR REGULATOR
OUTPUT VOLTAGE vs. LOAD CURRENT
(Q
LDO
= TIP29)
MAX1864/65 toc19
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
V
SUP(NEG)
= -15V
V
OUT3
= 5V
-12.24
-12.18
-12.12
-12.06
-12.00
-20 -18 -16 -14 -12 -10
NEGATIVE LINEAR REGULATOR
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
(Q
LDO
= TIP29)
MAX1864/65 toc20
SUPPLY VOLTAGE (V)
OUTPUT VOLTAGE (V)
I
LDO(NEG)
= 100mA
I
LDO(NEG)
= 1mA
80
70
60
50
40
30
20
10
0
0.1 10 10011000
POSITIVE LINEAR REGULATOR
POWER-SUPPLY REJECTION RATIO
(Q
LDO
= TIP30)
MAX1864/65 toc16
FREQUENCY (kHz)
PSRR (dB)
I
OUT2
= 150mA
POSITIVE LINEAR REGULATOR
LOAD TRANSIENT
(Q
LDO
= TIP30)
MAX1864/65 toc17
250mA
2.453V
2.473V
A. I
OUT2
= 10mA TO 250mA, 200mA/div
B. V
OUT2
= 2.5V, 10mV/div
C
LDO(POS)
= 10µF CERAMIC, V
SUP(POS)
= 3.3V
CIRCUIT OF FIGURE 1
0
B
A
10µs
0
10
5
25
20
15
40
35
30
45
0426810
NEGATIVE LINEAR REGULATOR BASE-
DRIVE CURRENT vs. BASE-DRIVE VOLTAGE
MAX1864/65 toc18
V
OUT
- VB5 (V)
BASE-DRIVE SOURCE CURRENT (mA)
V
FB5
= 250mV
V
FB5
= 50mV
V
OUT
= 5.0V
V
OUT
= 3.3V
B5 (MAX1865) ONLY
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
_______________________________________________________________________________________ 9
Pin Description
PIN
MAX1864 MAX1865
1 1 POK
2 2 COMP
3 3 OUT
44FB
55B2
6 6 FB2
77B3
NAME FUNCTION
Open-Drain Power-Good Output. POK is low when the output voltage is more than 10% below the regulation point. POK is high impedance when the output is in regulation. Connect a resistor between POK and VL for logic-level voltages.
Compensation Pin. Connect a series RC to GND to compensate the control loop. Typical values are 47k and 8.2nF.
Regulated Output Voltage High-Impedance Sense Input. Internally connected to a resistive-divider and negative gain block (MAX1865).
Dual-Mode Switching-Regulator Feedback Input. Connect to GND for the preset 3.3V output. Connect to a resistive-divider from output to FB to GND to adjust the output
V
voltage between 1.236V and 0.8
Open-Drain Output PNP Transistor Driver (Regulator #2). Internally connected to the drain of a DMOS. B2 connects to the base of an external PNP pass transistor to form a positive linear regulator.
Analog Gain-Block Feedback Input (Regulator #2). Connect to a resistive-divider between the positive linear regulators output and GND to adjust the output voltage. The feedback set point is 1.24V.
Open-Drain Output PNP Transistor Driver (Regulator #3). Internally connected to the drain of a DMOS. B3 connects to the base of an external PNP pass transistor to form a positive linear regulator.
. The feedback set point is 1.236V.
IN
Analog Gain-Block Feedback Input (Regulator #3). Connect to a resistive-divider
8 8 FB3
9B4
10 FB4
11 B5
12 FB5
between the positive linear regulators output and GND to adjust the output voltage. The feedback set point is 1.24V.
Open-Drain Output PNP Transistor Driver (Regulator #4). Internally connected to the drain of a DMOS. B4 connects to the base of an external PNP pass transistor to form a positive linear regulator.
Analog Gain-Block Feedback Input (Regulator #4). Connect to a resistive-divider between the positive linear regulators output and GND to adjust the output voltage. The feedback set point is 1.24V.
Open-Drain Output NPN Transistor Driver (Regulator #5). Internally connected to the drain of a P-channel MOSFET. B5 connects to the base of an external NPN pass transistor to form a negative linear regulator.
Analog Gain-Block Feedback Input (Regulator #5). Connect to a resistive-divider between the negative linear regulators output and a positive reference voltage, typically one of the positive linear regulator outputs, to adjust the output voltage. The feedback set point is at GND.
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
10 ______________________________________________________________________________________
Detailed Description
The MAX1864/MAX1865 power-supply controllers pro­vide system power for cable and xDSL modems. The main step-down DC-DC controller operates in a cur­rent-mode pulse-width-modulation (PWM) control scheme to ease compensation requirements and pro­vide excellent load- and line-transient response.
The MAX1864 includes two analog gain blocks to regu­late two additional positive auxiliary output voltages, and the MAX1865 includes four analog gain blocks to regulate three additional positive and one negative aux­iliary output voltages. The positive regulator gain blocks can be used to generate low-voltage rails directly from the main step-down converter or higher voltages using coupled windings from the step-down converter. The negative gain block can be used in conjunction with a coupled winding to generate -5V, -12V, or -15V.
DC-DC Controller
The MAX1864/MAX1865 step-down converters use a pulse-width-modulated (PWM) current-mode control scheme (Figure 2). An internal transconductance amplifier establishes an integrated error voltage at the COMP pin. The heart of the current-mode PWM con­troller is an open-loop comparator that compares the
integrated voltage-feedback signal against the ampli­fied current-sense signal plus the slope compensation ramp. At each rising edge of the internal clock, the high-side MOSFET turns-on until the PWM comparator trips or the maximum duty cycle is reached. During this on-time, current ramps up through the inductor, sourc­ing current to the output and storing energy in a mag­netic field. The current-mode feedback system regulates the peak inductor current as a function of the output voltage error signal. Since the average inductor current is nearly the same as the peak inductor current (assuming that the inductor value is relatively high to minimize ripple current), the circuit acts as a switch­mode transconductance amplifier. It pushes the output LC filter pole, normally found in a voltage-mode PWM, to a higher frequency. To preserve inner loop stability and eliminate inductor stair-casing, a slope-compensa­tion ramp is summed into the main PWM comparator.
During the second-half of the cycle, the high-side MOS­FET turns off and the low-side N-channel MOSFET turns on. Now the inductor releases the stored energy as its current ramps down, providing current to the output. Therefore, the output capacitor stores charge when the inductor current exceeds the load current and dis­charges when the inductor current is lower, smoothing
Pin Description (continued)
PIN
MAX1864 MAX1865
9 13 ILIM
10 14 GND Ground
11 15 DL Low-Side Gate-Driver Output. DL swings between GND and VL.
12 16 LX
13 17 DH High-Side Gate-Driver Output. DH swings between LX and BST.
14 18 BST
15 19 VL
NAME FUNCTION
Dual-Mode Current-Limit Adjustment Input. Connect to VL for the default 250mV current-limit threshold. In adjustable mode, the current-limit threshold voltage is 1/5th the voltage present at ILIM. Connect to a resistive-divider between VL and GND to adjust V default value is approximately VL - 1V.
Inductor Connection. Used for current sense between IN and LX, and used for current limit between LX and GND.
Boost Flying Capacitor Connection. Connect BST to the external boost diode and capacitor as shown in the standard application circuit (Figures 1 and 6).
Internal 5V Linear-Regulator Output. Supplies the IC and powers the DL low-side gate driver and external boost diode and capacitor. Bypass with a 1µF or greater ceramic capacitor to GND.
between 1V and 2.5V. The logic threshold for switchover to the 250mV
ILIM
16 20 IN
Input Supply Voltage, 4.5V to 28V. Bypass to GND with a 1µF or greater ceramic capacitor close to the IC.
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
______________________________________________________________________________________ 11
the voltage across the load. Under overload conditions, when the inductor current exceeds the selected cur­rent-limit (see Current Limit), the high-side MOSFET is not turned on at the rising edge of the clock and the low-side MOSFET remains on to let the inductor current ramp down.
The MAX1864/MAX1865 operate in a forced-PWM mode, so even under light loads the controller main­tains a constant switching frequency to minimize cross­regulation errors in applications that use a transformer. The low-side gate-drive waveform is the complement of the high-side gate-drive waveform, which causes the inductor current to reverse under light loads.
Current-Sense Amplifier
The MAX1864/MAX1865s current-sense circuit ampli­fies (A
V
= 5) the current-sense voltage generated by
the high-side MOSFETs on-resistance (R
DS(ON)
I
INDUCTOR
). This amplified current-sense signal and the internal slope compensation signal are summed together (V
SUM
) and fed into the PWM comparator’s inverting input. The PWM comparator turns-off the high­side MOSFET when V
SUM
exceeds the integrated feed-
back voltage (V
COMP)
. Place the high-side MOSFET no further than 5mm from the controller, and connect IN and LX to the MOSFET using Kelvin sense connections to guarantee current-sense accuracy and improve stability.
Figure 1. Standard MAX1864 Application Circuit
1µF
1µF
INPUT
D1 CENTRAL CMPSH-3
BST
OUT GND
FB2
FB3
R
DH
10
DH
C
BST
0.1µF
LX
R
DL
10
DL
C
BE2
2200pF
R
BE2
R3
30k
10k
R1
10k
R2
10k
4700pF
R4
220
C
BE3
R
BE3
220
B2
B3
POK
COMP
IN
VL
ILIM
MAX1864
POK
FB
COMP
C1
C2
R
100k
R
47k
C
COMP
8.2nF
9V TO 18V
C
IN
470µF
N
H
T1
1
N
L
Q1
Q2
C7 10µF
C4 10µF
C3 10µF
C6 10µF
V
V
OUT3
OUT2
300mA
100mA
V
= 2.5V
= 5.0V
NL, NH: INTERNATIONAL RECTIFIER
Q1: TIP30 Q2: 2N3905
= 3.3V
OUT
1A
C
OUT
470µF
T1
D2 NIHON EP05Q03L
C5 470µF
IRF7303
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
12 ______________________________________________________________________________________
Figure 2. Functional Diagram
BIAS
MAX1864 MAX1865
OK
V
REF
COMP
OUT
FB
100mV
FB_
B_
0.9V
REF
FB1
SOFT­START
FB1
V
REF
1.236V
CLK
ENABLE
VL LDO
5V
1.114V
3.5V
IN
VL
BST
DH
LX
DL
GND
THERMAL
SHDN
= 5
A
V
SLOPE COMP
= 5
A
V
100k
0.9V
REF
400k
ILIM
VL
B5*
FB5*
*MAX1865 ONLY
OUT
250mV
0.9
ENABLE
500mV
POK
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
______________________________________________________________________________________ 13
Current-Limit Circuit
The current-limit circuit employs a unique valley cur­rent-limiting algorithm that uses the low-side MOSFET’s on-resistance as a sensing element (Figure 3). If the voltage across the low-side MOSFET (R
DS(ON)
I
IN-
DUCTOR
) exceeds the current-limit threshold at the beginning of a new oscillator cycle, the MAX1864/ MAX1865 will not turn on the high-side MOSFET. The actual peak current is greater than the current-limit threshold by an amount equal to the inductor ripple current. Therefore, the exact current-limit characteristic and maximum load capability are a function of the low­side MOSFET on-resistance, inductor value, input volt­age, and output voltage. The reward for this uncertainty is robust, loss-less overcurrent limiting.
In adjustable mode, the current-limit threshold voltage is 1/5th the voltage seen at ILIM (I
VALLEY
= 0.2 ✕V
ILIM
). Adjust the current-limit threshold by connecting a resis­tive-divider from VL to ILIM to GND. The current-limit threshold can be set from 106mV to 530mV, which cor­responds to ILIM input voltages of 500mV to 2.5V. This adjustable current limit accommodates MOSFETs with a wide range of on-resistance characteristics (see Design Procedure). The current-limit threshold defaults to 250mV when ILIM is connected to VL. The logic threshold for switchover to the 250mV default value is approximately VL - 1V.
Carefully observe the PC board layout guidelines to ensure that noise and DC errors dont corrupt the cur­rent-sense signals seen by LX and GND. The IC must be mounted close to the low-side MOSFET with short (less than 5mm), direct traces making a Kelvin sense connection.
Synchronous Rectifier Driver (DL)
Synchronous rectification reduces conduction losses in the rectifier by replacing the normal Schottky catch diode with a low-resistance MOSFET switch. The MAX1864/MAX1865 also use the synchronous rectifier to ensure proper startup of the boost gate-driver circuit and to provide the current-limit signal.
The DL low-side drive waveform is always the comple­ment of the DH high-side drive waveform (with con­trolled dead time to prevent cross-conduction or shoot-through). A dead-time circuit monitors the DL output and prevents the high-side FET from turning on until DL is fully off. For the dead-time circuit to work properly, there must be a low-resistance, low-induc­tance path from the DL driver to the MOSFET gate. Otherwise, the sense circuitry in the MAX1864/ MAX1865 will interpret the MOSFET gate as “off” when gate charge actually remains. Use very short, wide
traces (50mil to 100mil wide if the MOSFET is 1 inch from the device). The dead time at the other edge (DH turning off) is determined by a fixed internal delay.
High-Side Gate-Drive Supply (BST)
Gate-drive voltage for the high-side N-channel switch is generated by a flying-capacitor boost circuit (Figure 1). The capacitor between BST and LX is alternately charged from the VL supply and placed parallel to the high-side MOSFETs gate-source terminals.
On startup, the synchronous rectifier (low-side MOS­FET) forces LX to ground and charges the boost capacitor to 5V. On the second half-cycle, the switch­mode power supply turns on the high-side MOSFET by closing an internal switch between BST and DH. This provides the necessary gate-to-source voltage to turn on the high-side switch, an action that boosts the 5V gate-drive signal above the battery voltage.
Internal 5V Linear Regulator (VL)
All MAX1864/MAX1865 functions, except the current­sense amplifier, are internally powered from the on­chip, low-dropout 5V regulator. The maximum regulator input voltage (VIN) is 28V. Bypass the regulators output (VL) with at least a 1µF ceramic capacitor to GND. The VIN-to-VL dropout voltage is typically 200mV, so when VINis less than 5.2V, VL is typically VIN- 200mV.
The internal linear regulator can source up to 20mA to supply the IC, power the low-side gate driver, charge the external boost capacitor, and supply small external loads. When driving particularly large FETs, little or no regulator current may be available for external loads. For example, when switched at 200kHz, a large FET with 40nC total gate charge requires 40nC x 200kHz, or 8mA.
Figure 3. “Valley” Current-Limit Threshold Point
(VIN - V
INDUCTOR CURRENT
TIME
I
PEAK
= I
VALLEY
OUT
+
[
L
-I
PEAK
I
LOAD
I
VALLEY
)
V
OUT
()
]
VINf
OSC
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
14 ______________________________________________________________________________________
Undervoltage Lockout
If VL drops below 3.5V, the MAX1864/MAX1865 assume that the supply voltage is too low to make valid decisions, so the undervoltage lockout (UVLO) circuitry inhibits switching, forces POK low, and forces the DL and DH gate drivers low. After VL rises above 3.5V, internal digital soft-start is initiated (see Soft-Start).
Startup Sequence
Externally, the MAX1864/MAX1865 starts switching when VL rises above the 3.5V undervoltage lockout threshold. However, the controller is not enabled unless all four of the following conditions are met: 1) VL exceeds the 3.5V undervoltage lockout threshold, 2) the internal reference exceeds 90% of its nominal value (V
REF
> 1.114V), 3) the internal bias circuitry powers up, and 4) the thermal limit is not exceeded. Once the MAX1864/MAX1865 assert the internal enable signal, the step-down controller starts switching and enables soft-start.
Soft-Start
Upon power-up, the MAX1864/MAX1865 begin a start­up sequence. First, the reference powers up. Then, the main DC-DC step-down converter and positive linear regulators power up with soft-start enabled. Once the regulators reach 90% of their nominal value and soft­start is complete, the active-high ready signal (POK) goes high (see Power-Good Output).
Soft-start gradually ramps up to the reference voltage in order to control the rate of rise of the output voltages and reduce input surge currents during startup. The soft-start period is 1024 clock cycles (1024/f
OSC
), and the internal soft-start DAC ramps up the voltage in 64 steps. The output reaches regulation when soft-start is completed, regardless of output capacitance and load.
Power-Good Output
The power-good output (POK) is an open-drain output. The MOSFET turns on and pulls POK low when any out­put is less than 90% of its nominal regulation voltage or during soft-start. Once all of the outputs exceed 90% of their nominal regulation voltages and soft-start is com­pleted, POK goes high impedance. To obtain a logic voltage output, connect a pullup resistor from POK to VL. A 100kresistor works well for most applications. If unused, leave POK grounded or unconnected.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipa­tion in the MAX1864/MAX1865. When the junction tem­perature exceeds TJ= +160°C, a thermal sensor shuts down the device, forcing DL and DH low, allowing the IC to cool. The thermal sensor turns the part on again after the junction temperature cools by 10°C, resulting in a pulsed output during continuous thermal-overload conditions. If the VL output is short circuited, thermal­overload protection is disabled.
During a thermal event, the main step-down converter and the linear regulators are turned off, POK goes low, and soft-start is reset.
Design Procedure
DC-DC Step-Down Converter
Output Voltage Selection
The step-down controllers feedback input features dual-mode operation. Connect the output to OUT and connect FB to GND for the preset 3.3V output voltage. Alternatively, the MAX1864/MAX1865 output voltage may be adjusted by connecting a voltage-divider from the output to FB to GND (Figure 4). Select R2 in the 5kto 50krange. Calculate R1 with the following equation:
where V
SET
= 1.236V, and V
OUT
may range from
1.236V to approximately 0.8 x VIN(up to 20V). If V
OUT
> 5.5V, then connect OUT to GND (MAX1864) or to one of the positive linear regulators (MAX1865) with an out­put voltage between 2V and 5V.
Inductor Value
Three key inductor parameters must be specified: inductance value (L), peak current (I
PEAK
), and DC resistance (RDC). The following equation includes a constant LIR, which is the ratio of inductor peak-to­peak AC current to DC load current. A higher LIR value allows smaller inductance but results in higher losses and higher output ripple. A good compromise between size and losses is a 30% ripple-current to load-current ratio (LIR = 0.3). The switching frequency, input volt­age, output voltage, selected LIR determine the induc­tor value as follows:
V
RR
12 1=
L
=
VI LIR
IN SW LOAD MAX
OUT
V
SET
VVV
()
OUT IN OUT
ƒ
-
 
-
()
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
______________________________________________________________________________________ 15
where fSWis 200kHz for MAX186_T and 100kHz for MAX186_U. The exact inductor value is not critical and can be adjusted to make trade-offs among size, cost, and efficiency. Lower inductor values minimize size and cost, but they also increase the output ripple and reduce the efficiency due to higher peak currents. On the other hand, higher inductor values increase effi­ciency, but at some point resistive losses due to extra turns of wire will exceed the benefit gained from lower AC current levels.
Find a low-loss inductor having the lowest possible DC resistance that fits in the allotted dimensions. Ferrite cores are often the best choice, though powdered iron is inexpensive and can work well at 200kHz. The cho­sen inductors saturation rating must exceed the peak inductor current:
Setting the Current Limit
The minimum current-limit threshold must be high enough to support the maximum load current at the minimum tolerance level of the current-limit circuit. The
valley of the inductor current occurs at I
LOAD(MAX)
minus half of the ripple current:
where R
DS(ON)
is the on-resistance of the low-side MOSFET (NL). For the MAX1864/MAX1865, the mini­mum current-limit threshold is 190mV (for the typical 250mV default setting). Use the worst-case maximum value for R
DS(ON
) from the MOSFET NLdata sheet, and
add some margin for the rise in R
DS(ON)
over tempera­ture. A good general rule is to allow 0.5% additional resistance for each °C of the MOSFET junction temper­ature rise.
Connect ILIM to VL for the default 250mV (typ) current­limit threshold. For an adjustable threshold, connect a resistive-divider from VL to ILIM to GND. The 500mV to
2.5V external adjustment range corresponds to a 106mV to 530mV current-limit threshold. When adjust­ing the current limit, use 1% tolerance resistors and a 10µA divider current to prevent a significant increase in the current-limit tolerance.
Figure 4. Adjustable Output Voltage
* FOR OUTPUT VOLTAGES > 5V, SEE "OUTPUT VOLTAGE SELECTION."
INPUT
D1
MAX1864
IN
C1
C2
R
POK
R
COMP
C
COMP
MAX1865
VL
ILIM
POK
COMP
BST
DH
OUT GND
C
BST
LX
DL
FB
4.5V TO 28V
C
IN
N
H
L
N
L
R1
R2
C
OUT
OUTPUT
1.25V TO 5V*
LIR
II
=+
PEAK LOAD MAX LOAD MAX
() ()
I
2
V
VALLEY LOW
R
()
DS ON
()
>
I
LOAD MAX LOAD MAX
() ()
LIR
-
I
2
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
16 ______________________________________________________________________________________
MOSFET Selection
The MAX1864/MAX1865s step-down controller drives two external logic-level N-channel MOSFETs as the cir­cuit switch elements. The key selection parameters are:
On-resistance (R
DS(ON)
)
Maximum drain-to-source voltage (V
DS(MAX)
)
Minimum threshold voltage (V
TH(MIN)
)
Total gate charge (Qg)
Reverse transfer capacitance (C
RSS
)
The high-side N-channel MOSFET must be a logic-level type with guaranteed on-resistance specifications at VGS≤ 4.5V. Select the high-side MOSFET’s R
DS(ON)
so
I
PEAK
x R
DS(ON)
225mV for the current-sense range.
For maximum efficiency, choose a high-side MOSFET (NH) that has conduction losses equal to the switching losses at the optimum input voltage. Check to ensure that the conduction losses at minimum input voltage dont exceed the package thermal limits or violate the overall thermal budget. Check to ensure that the con­duction losses plus switching losses at the maximum input voltage dont exceed package ratings or violate the overall thermal budget.
The low-side MOSFET (NL) provides the current-limit signal, so choose a MOSFET with an R
DS(ON)
large enough to provide adequate circuit protection (see Setting the Current-Limit):
Use the worst-case maximum value for R
DS(ON)
from the MOSFET NLdata sheet, and add some margin for the rise in R
DS(ON)
over temperature. A good general rule is to allow 0.5% additional resistance for each °C of the MOSFET junction temperature rise. Ensure that the MAX1864/MAX1865 DL gate drivers can drive NL; in other words, check that the dv/dt caused by NHturning on does not pull up the NLgate due to drain-to-gate capacitance, causing cross-conduction problems.
MOSFET package power dissipation often becomes a dominant design factor. I2R power losses are the great­est heat contributor for both high-side and low-side MOSFETs. I2R losses are distributed between NHand NLaccording to duty factor as shown in the equations below. Generally, switching losses affect only the high­side MOSFET since the low-side MOSFET is a zero-volt­age switched device when used in the buck topology.
Gate-charge losses are dissipated by the driver and do not heat the MOSFET. Calculate the temperature rise according to package thermal-resistance specifications
to ensure that both MOSFETs are within their maximum junction temperature at high ambient temperature. The worst-case dissipation for the high-side MOSFET (P
NH
) occurs at both extremes of input voltage, and the worst-case dissipation for the low-side MOSFET (PNL) occurs at maximum input voltage.
where I
GATE
is the DH driver peak output current capa­bility (1A typ), and 20ns is the DH driver inherent rise/fall-time. To reduce EMI caused by switching noise, add a 0.1µF ceramic capacitor from the high­side switch drain to the low-side switch source, or add resistors (47max) in series with DL and DH to increase the switches turn-on and turn-off times (Figure
5).
The minimum load current should exceed the high-side MOSFETs maximum leakage current over temperature if fault conditions are expected.
Input Capacitor
The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on the input caused by the circuits switching. The input capacitor must meet the ripple current requirement (I
RMS
) imposed by the switching currents
defined by the following equation:
For most applications, nontantalum capacitors (ceram­ic, aluminum, polymer, or OS-CON) are preferred due to their robustness with high inrush currents typical of systems with low-impedance battery inputs. Additionally, two (or more) smaller value low-ESR capacitors can be connected in parallel for lower cost. Choose an input capacitor that exhibits less than +10°C temperature rise at the RMS input current for optimal circuit long-term reliability.
V
R
DS ON
()
=
VALLEY
I
VALLEY
V
Duty Cycle D
PVI
()
NH SWITCHING IN LOAD OSC
PIRD
() ()
NH CONDUCTION LOAD DS ON NH
PP
() ( )
NH TOTAL NH SWITCHING
P
NH CONDUCTION
PI R D
NL LOAD DS ON NL
:
=+
()
=
OUT
=
V
IN
=
2
()
2
1-
()
VC
IN RSS
I
GATE
 
II
=
RMS LOAD
VVV
OUT IN OUT
-
()
V
IN
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
______________________________________________________________________________________ 17
Output Capacitor
The key selection parameters for the output capacitor are the actual capacitance value, the equivalent series resistance (ESR), and voltage-rating requirements, which affect the overall stability, output ripple voltage, and transient response.
The output ripple has two components: variations in the charge stored in the output capacitor, and the voltage drop across the capacitors ESR caused by the current into and out of the capacitor:
The output voltage ripple as a consequence of the ESR and output capacitance is:
where I
P-P
is the peak-to-peak inductor current (see Inductor Selection). These equations are suitable for initial capacitor selection, but final values should be set by testing a prototype or evaluation circuit. As a gener­al rule, a smaller ripple current results in less output rip­ple. Since the inductor ripple current is a factor of the inductor value and input voltage, the output voltage rip­ple decreases with larger inductance but increases with lower input voltages.
With low-cost aluminum electrolytic capacitors, the ESR-induced ripple can be larger than that caused by the current into and out of the capacitor. Consequently, high-quality low-ESR aluminum-electrolytic, tantalum, polymer, or ceramic filter capacitors are required to minimize output ripple. Best results at reasonable cost are typically achieved with an aluminum-electrolytic capacitor in the 470µF range, in parallel with a 0.1µF ceramic capacitor.
Since the MAX1864/MAX1865 use a current-mode con­trol scheme, the output capacitor forms a pole that affects circuit stability (see Compensation Design). Furthermore, the output capacitors ESR also forms a zero.
The MAX1864/MAX1865s response to a load transient depends on the selected output capacitor. After a load transient, the output instantly changes by ESR
I
LOAD
. Before the controller can respond, the output will sag further, depending on the inductor and output capacitor values.
After a short period of time (see Typical Operating Characteristics), the controller responds by regulating the output voltage back to its nominal state. For appli­cations that have strict transient requirements, low-ESR high-capacitance electrolytic capacitors are recom­mended to minimize the transient voltage swing.
Do not exceed the capacitors voltage or ripple-current ratings.
Compensation Design
The MAX1864/MAX1865 controllers use an internal transconductance error amplifier whose output com­pensates the control loop. Connect a series resistor and capacitor between COMP and GND to form a pole­zero pair. The external inductor, high-side MOSFET, output capacitor, compensation resistor, and compen­sation capacitor determine the loop stability. The induc­tor and output capacitor are chosen based on performance, size, and cost. Additionally, the compen­sation resistor and capacitor are selected to optimize control-loop stability. The component values shown in the standard application circuits (Figures 1 and 6) yield stable operation over a broad range of input-to-output voltages.
The controller uses a current-mode control scheme that regulates the output voltage by forcing the required current through the external inductor, so the MAX1864/MAX1865 use the voltage across the high­side MOSFETs R
DS(ON)
to sense the inductor current. Using the current-sense amplifiers output signal and the amplified feedback voltage, the control loop deter­mines the peak inductor current by:
Figure 5. Reducing the Switching EMI
MAX1864 MAX1865
TO VL
BST
GND
R
GATE
(OPTIONAL)
DH
C
LX
DH
BST
R
GATE
(OPTIONAL)
N
H
L
N
L
VV V
RIPPLE RIPPLE ESR RIPPLE C
=+
() ()
V I ESR
RIPPLE ESR p p
V
RIPPLE C
I
=
pp
-
=
()
()
VVLV
IN OUTSWOUT
ƒ
=
-
2
C
OUT SW
-
I
pp
 
-
ƒ
V
IN
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
18 ______________________________________________________________________________________
where A
VCS
is the current-sense amplifiers gain (4.9
typ), A
VEA
is the DC gain of the error amplifier (2000
typ), and V
OUT(NOMINAL)
is the output voltage set by the feedback resistive-divider (internal or external). Since the output voltage is a function of the load cur­rent and load resistance, the total DC loop gain (A
V(DC)
) is approximately:
The compensation capacitor (C
COMP
) creates the dom­inant pole. Due to the current-mode control scheme, the output capacitor also creates a pole in the system that is a function of the load resistance. As the load resistance increases, the frequency of the output capacitors pole decreases. However, the DC loop gain increases with larger load resistance, so the unity gain bandwidth remains fixed. Additionally, the compensa­tion resistor and the output capacitors ESR both gener­ate zeros. Therefore, to achieve stable operation, use the following procedure to properly compensate the system:
1) First, select the desired crossover frequency. The
crossover frequency must be less than both 1/5th the switching frequency and 1/3rd the zero frequen­cy set by the output capacitors ESR:
2) Next, determine the pole set by the output capacitor
and the load resistor:
3) Determine the compensation resistor required to set
the desired crossover frequency:
where the error amplifiers transconductance (g
m
) is
100µS (see Electrical Characteristics).
4) Finally, select the compensation capacitor:
Boost-Supply Diode
A signal diode, such as the 1N4148, works well in most applications. If the input voltage goes below 6V, use a small 20mA Schottky diode for slightly improved effi­ciency and dropout characteristics. Do not use large power diodes, such as the 1N5817 or 1N4001, since high junction capacitance can charge up VL to exces­sive voltages.
Linear Regulator Controllers
Positive Output Voltage Selection
The MAX1864/MAX1865s positive linear regulator out­put voltages are set by connecting a voltage-divider from the output to FB_ to GND (Figure 6). Select R4 in the 5kto 50krange. Calculate R3 with the following equation:
where VFB= 1.24V, and V
OUT
may range from 1.24V to
30V.
Negative Output Voltage Selection (MAX1865)
The MAX1865s negative output voltage is set by con­necting a voltage-divider from the output to FB5 to a positive voltage reference (Figure 6). Select R6 in the 5kto 50krange. Calculate R5 with the following equation:
where V
REF
is the positive reference voltage used, and
V
OUT
may be set between 0 and -20V.
If the negative regulator is used, the OUT pin must be connected to a voltage supply between 2V and 5V that can source at least 25mA. Typically, the OUT pin is connected to the step-down converters output. However, if the step-down converters output voltage is set higher than 5V, OUT may be connected to one of the positive linear regulators with an output voltage between 2V and 5V.
VVA
I
=
PEAK
VRA
OUT REF VEA
OUT NOMINAL DS ON VCS
()()
I
A
VDC
()
PEAK
≈≈
I
LOAD
VR A
REF LOAD VEA
VRA
OUT NOMINAL DS ON VCS
()()
VR
×400
REF LOAD
VR
OUT NOMINAL DS ON()()
IJ
c
1
CR
65π
OUT ESR
and
ƒ
SW
ƒ= =
POLE OUT
()
1
CR
22ππ
OUT LOAD
I
LOAD MAX
()
CV
OUT OUT
׃
R
COMP
=
2000
gA
m V DC POLE OUT
() ( )
c
ƒ
C
COMP
R
2π
1
ƒ
COMP POLE OUT
()
RR
34 1=
 
 
V
OUT
V
-
 
FB
V
RR
56=
OUT
V
REF
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
______________________________________________________________________________________ 19
Figure 6. Standard MAX1865 Application Circuit
C
BE5
2200pF
C12
10µF
V
OUT5
-12V AT 50mA
R
BE5
220
TO LOGIC
Q4
TIP29
1µF
1µF
C1
C2
100k
C
COMP
8.2nF
C15 10nF
R10
470
R
POK
R
COMP
47k
C13 10µF
R8
120k
CENTRAL CMPSH-3
IN
VL
ILIM
POK
FB
COMP
MAX1865
B5
FB5
R7
50k
INPUT
FAIRCHILD FDS6912A
T1
1
C4 10µF
C6 10µF
C7 10µF
C
BE4
2200pF
R
BE4
220
R5
30k
R6
10k
9V TO 18V
C
IN
470µF
C3 10µF
V
OUT2
2.5V AT 500mA
V
OUT3
5.0V AT 100mA
Q3 TIP30
D2
NIHON
EP05Q03L
C5 470µF
C10 10µF
C
OUT
470µF
C9 10µF
1
V
OUT1
3.3V AT 1A
T1
NIHON
EP05Q03L
C8 470µF
V
OUT3
12V AT 100mA
T1
2
D3
R
SNUB
300
C
SNUB
100pF
D1
BST
R
NH
DH
LX
DL
OUT GND
B2
FB2
B3
FB3
B4
FB4
CONNECT TO V
OUT3
R
10
10
C
BST
0.1µF
NL
10k
R3
30k
10k
470
10k
4700pF
R4
C14
10nF
2200pF
R1
R2
C
BE3
R
BE3
220
R9
C
BE2
R 220
N
H
N
L
BE2
Q1 TIP30
Q2 2N3905
T1
C11 470µF
NIHON
EC10QS10
4
D4
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
20 ______________________________________________________________________________________
Transistor Selection
The pass transistors must meet specifications for cur­rent gain (hFE), input capacitance, collector-emitter sat­uration voltage, and power dissipation. The transistor’s current gain limits the guaranteed maximum output cur­rent to:
where I
DRV
is the minimum base-drive current, and R
BE
(220) is the pullup resistor connected between the transistors base and emitter. Furthermore, the transis­tors current gain increases the linear regulators DC loop gain (see Stability Requirements), so excessive gain will destabilize the output. Therefore, transistors with current gain over 100 at the maximum output cur­rent, such as Darlington transistors, are not recom­mended. The transistors input capacitance and input resistance also create a second pole, which could be low enough to destabilize the output when heavily loaded.
The transistors saturation voltage at the maximum out­put current determines the minimum input-to-output voltage differential that the linear regulator will support. Alternatively, the packages power dissipation could limit the useable maximum input-to-output voltage dif­ferential. The maximum power dissipation capability of the transistors package and mounting must exceed the actual power dissipation in the device. The power dissi­pated equals the maximum load current times the maxi­mum input-to-output voltage differential:
Stability Requirements
The MAX1864/MAX1865 linear regulators use an inter­nal transconductance amplifier to drive an external pass transistor. The transconductance amplifier, pass transistors specifications, the base-emitter resistor, and the output capacitor determine the loop stability. If the output capacitor and pass transistor are not proper­ly selected, the linear regulator will be unstable.
The transconductance amplifier regulates the output voltage by controlling the pass transistors base cur­rent. Since the output voltage is a function of the load current and load resistance, the total DC loop gain (A
V (LDO)
) is approximately:
where V
T
is 26mV, and I
BIAS
is the current through the
base-to-emitter resistor (R
BE
). This bias resistor is typical-
ly 220, providing approximately 3.2mA of bias current.
The output capacitor creates the dominant pole. However, the pass transistors input capacitance creates a second pole in the system. Additionally, the output capacitors ESR generates a zero, which may be used to cancel the second pole if necessary. Therefore, to achieve stable operation, use the following equations to verify that the linear regulator is properly compensated:
1) First, determine the dominant pole set by the linear regulators output capacitor and the load resistor:
2) Next, determine the second pole set by the base-to­emitter capacitance (including the transistors input capacitance), the transistors input resistance, and the base-to-emitter pullup resistor:
3) A third pole is set by the linear regulators feedback resistance and the capacitance between FB_ and GND, including 20pF stray capacitance:
4) If the second and third poles occur well after unity­gain crossover, the linear regulator will remain stable:
However, if the ESR zero occurs before unity-gain crossover, cancel the zero with ƒ
POLE(FB)
by changing
circuit components such that:
II
LOAD MAX DRV
PI V V I V
=
LOAD MAX LDOIN OUT LOAD MAX CE
() ()
=
()
-
 
-
V
BE
h
BE
FE MIN() ()
 
=
R
.
55
A
V LDO
ƒ= =
POLE CLDO
()
Unity GainCrossover A
ƒ=
POLE CBE
ƒ>ƒ
V
T
CR
22ππ
LDO LOAD
()
ƒ=
POLE FB
POLE CBE POLE CLDO V LDO
22π
RI Vh
BE LOAD T FE
=
()
() ( )()
2
Ih
BIAS FE
+
1
CR R
π
I
LOAD
1
V LDO POLE CLDO
() ( )
1
()
BE BE IN NPN
+
CRVh
BE BE T FE
1
FB
CRR
(|| )
212π
I
||
A
V
REF()
 
LOAD MAX
()
CV
LDO LDO
()
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
______________________________________________________________________________________ 21
Do not use output capacitors with more than 200mΩ of ESR. Typically, more output capacitance provides the best solution, since this also reduces the output voltage drop immediately after a load transient.
Linear Regulator Output Capacitors
Connect at least a 1µF capacitor between the linear regulators output and ground, as close to the MAX1864/MAX1865 and external pass transistors as possible. Depending on the selected pass transistor, larger capacitor values may be required for stability (see Stability Requirements). Furthermore, the output capacitors ESR affects stability, providing a zero that may be necessary to cancel the second pole. Use out­put capacitors with an ESR less than 200mto ensure stability and optimum transient response.
Once the minimum capacitor value for stability is deter­mined, verify that the linear regulators output does not contain excessive noise. Although adequate for stabili­ty, small capacitor values may provide too much band­width, making the linear regulator sensitive to noise. Larger capacitor values reduce the bandwidth, thereby reducing the regulators noise sensitivity.
If noise on the ground reference causes the design to be marginally stable for the negative linear regulator, bypass the negative output back to its reference volt­age (V
REF
, Figure 7). This technique reduces the differ-
ential noise on the output.
Base-Drive Noise Reduction
The high-impedance base driver is susceptible to sys­tem noise, especially when the linear regulator is lightly loaded. Capacitively coupled switching noise or induc­tively coupled EMI onto the base drive causes fluctua­tions in the base current, which appear as noise on the linear regulators output. Keep the base-drive traces away from the step-down converter and as short as possible to minimize noise coupling. Resistors in series with the gate drivers (DH and DL) reduce the LX switching noise generated by the step-down converter (Figure 5). Additionally, a bypass capacitor may be placed across the base-to-emitter resistor (Figure 7). This bypass capacitor, in addition to the transistor’s input capacitance, could bring in a second pole that will destabilize the linear regulator (see Stability Requirements). Therefore, the stability requirements determine the maximum base-to-emitter capacitance:
where C
IN(Q)
is the transistors input capacitance, and
f
POLE(CBE)
is the second pole required for stability.
Transformer Selection
In systems where the step-down controllers output is not the highest voltage, a transformer may be used to provide additional postregulated, high-voltage outputs. The transformer generates unregulated, high-voltage supplies that power the positive and negative linear regulators. These unregulated supply voltages must be high enough to keep the pass transistors from saturat­ing. For positive output voltages, connect the trans­former as shown in figure 6 where the minimum turns ratio (N) is determined by:
where V
SAT
is the pass transistors saturation voltage
under full load. For negative output voltages (MAX1865
Figure 7. Base-Drive Noise Reduction
ƒ≈
POLE FB
()
1
2π
CR
OUT ESR
V
SUP
C
C
B_
FB_
BF5
B5
1
()
BE
R1
R2
R4
R3
C
BE
RI Vh
BE LOAD T FE
 
MAX1864 MAX1865
a) POSITIVE OUTPUT VOLTAGE
MAX1865
b) NEGATIVE OUTPUT VOLTAGE (MAX1865 ONLY)
C
BE
ƒ
2π
POLE CBE
R
BE
Q
C
LDO
V
REF
Q
R
BE
+
RVh
BE T FE
PASS
PASS
BYP
V
POS
V
NEG
C
NEG
V
SUP
C
BYP
C
-
IN Q
()
 
VVV
N
POS
LDO POS SAT DIODE
 
++
()
V
OUT
-1
 
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
22 ______________________________________________________________________________________
only), connect the transformer as shown in Figure 6, where the minimum turns ratio is determined by:
Since power transfer occurs when the low-side MOS­FET is on (DL = high), the transformer cannot support heavy loads with high duty cycles.
Snubber Design
The MAX1864/MAX1865 use a current-mode control scheme that senses the current across the high-side MOSFET (NH). Immediately after the high-side MOSFET has turned on, the MAX1864/MAX1865 use a 60ns cur­rent-sense blanking period to minimize noise sensitivity. When the MOSFET turns on, however, the transformer’s secondary inductance and the diodes parasitic capac­itance form a resonant circuit that causes ringing. Reflected back through the transformer to the primary side, these oscillations across the high-side MOSFET may last longer than the blanking period. A series RC snubber circuit at the diode (Figure 6) increases the damping factor, allowing the ringing to settle quickly. Applications with multiple transformer windings require only one snubber circuit on the highest output voltage. Applications with low turn ratios (1:1), such as the MAX1864 typical application circuit (Figure 1), may not require a snubber curcuit.
The diodes parasitic capacitance can be estimated using the diodes reverse voltage rating (V
RRM
), current capability (IO), and recovery time (TRR). A rough approximation is:
For the EC10QS10 Nihon diode used in figure 6, the capacitance is roughly 15pF. The output snubber must only dampen the ringing, so the initial turn-on spike that occurs during the blanking period remains preset. A 100pF capacitor works well in most applications; larger capacitance values require more charge, thereby increasing the power dissipation.
The snubbers time constant (t
SNUB
) must be smaller than the 100ns blanking time. A typical RC time con­stant of approximately 30ns was chosen for Figure 6:
Minimum Load Requirements (Linear Regulators)
Under no-load conditions, leakage currents from the pass transistors supply the output capacitor, even when the transistor is off. Generally, this is not a prob­lem since the feedback resistors current drains the excess charge. However, charge may build up on the output capacitor over temperature, making V
LDO
rise above its set point. Care must be taken to ensure that the feedback resistors current exceeds the pass tran­sistors leakage current over the entire temperature range.
Applications Information
PC Board Layout Guidelines
Careful PC board layout is critical to achieve low switching losses and clean, stable operation. The switching power stage requires particular attention. Follow these guidelines for good PC board layout:
1) Place the power components first, with ground ter-
minals adjacent (NLsource, CIN, C
OUT
). If possible, make all these connections on the top layer with wide, copper-filled areas. Keep these high-current paths short, especially at ground terminals.
2) Mount the MAX1864/MAX1865 adjacent to the switching MOSFETs to keep IN-LX current-sense lines, LX-GND current-limit sense lines, and the dri­ver lines (DL and DH) short and wide. The current­sense amplifier inputs are connected between IN and LX, so these pins must be connected as close as possible to the high-side MOSFET. The current­limit comparator inputs are connected between LX and GND, but accuracy is not as important, so give priority to the high-side MOSFET connections. The IN, LX, and GND connections to the MOSFETs must be made using Kelvin sense connections to guaran­tee current-sense and current-limit accuracy.
3) Group the gate-drive components (BST diode and capacitor, IN bypass capacitor) together near the MAX1864/MAX1865.
4) All analog grounding must be done to a separate solid copper ground plane, which connects to the MAX1864/MAX1865 at the GND pin. This includes the VL bypass capacitor, feedback resistors, com­pensation components (R
COMP
, C
COMP
), and adjustable current-limit threshold resistors connect­ed to ILIM.
VVV
||
N
NEG
R
SNUB
LDO NEG SAT DIODE
 
C
DIODE
t
SNUB
==
C
SNUB SNUB
++
()
V
It
×
ORR
=
V
RRM
C
OUT
ns
30
 
5) Ensure all feedback connections are short and direct. Place the feedback resistors as close to the MAX1864/MAX1865 as possible.
6) When trade-offs in trace lengths must be made, it’s preferable to allow the inductor charging path to be made longer than the discharge path. For example, it is better to allow some extra distance between the input capacitors and the high-side MOSFET than to allow distance between the inductor and low-side MOSFET or between the inductor and output filter capacitor.
7) Route high-speed switching nodes away from sensi­tive analog areas (B_, FB_, COMP, ILIM).
Regulating High Voltage
The linear regulator controllers can be configured to regulate high output voltages by adding a cascode transistor to buffer the base-drive output. For example, to generate an output voltage between 30V and 60V, add a 2N5550 high-voltage NPN transistor as shown in Figure 8a where V
BIAS
is a DC voltage between 3V and
20V that can source at least 1mA. R
DROP
protects the cascode transistor by decreasing the voltage across the transistor when the pass transistor saturates. Similarly, to regulate a negative output voltage between
-20V and -120V, add a 2N5401 high-voltage PNP tran­sistor as shown in Figure 8b.
Chip Information
TRANSISTOR COUNT: 1617
PROCESS: BiCMOS
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
______________________________________________________________________________________ 23
Pin Configurations (continued)
TOP VIEW
1
POK
2
COMP
3
OUT
4
5
B2
6
FB2
7
B3
8
9
B4
10
20 QSOP
20
IN
19
VL
18
BST
17
DHFB
MAX1865
16
LX
15
DL
14
GND
13
ILIMFB3
12
FB5
11
B5FB4
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
24 ______________________________________________________________________________________
Table 1. Component Suppliers
Figure 8. High-Voltage Linear Regulation
V
V
BIAS
C
DROP
MAX1864 MAX1865
a) POSITIVE OUTPUT VOLTAGE WITH CASCODED BASE DRIVE
FB_
B_
Q
CASCODE
R1
R2
R
DROP
R
BE
Q
PASS
C
POS
SUP
C
BYP
V
POS
MAX1865
b) NEGATIVE OUTPUT VOLTAGE (MAX1865 ONLY)
WITH CASCODED BASE DRIVE
FB5
R4
R3
Q
CASCODE
B5
V
REF
V
R
DROP
C
DROP
Q
PASS
R
BE
NEG
C
NEG
V
SUP
C
BYP
SUPPLIER PHONE FAX INTERNET
INDUCTORS AND TRANSFORMERS
Coilcraft 847-639-6400 847-639-1469 http://www.coilcraft.com
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Toko 847-297-0070 847-699-1194 http://www.toko.co.jp
CAPACITORS
AVX 803-946-0690 803-626-3123 http://www.avxcorp.com
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DIODES
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International
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310-322-3331 310-322-3332 http://www.irf.com
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 25
© 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information
QSOP.EPS
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