Rainbow Electronics MAX1842 User Manual

MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
________________________________________________________________ Maxim Integrated Products 1
19-1760; Rev 1; 2/02
General Description
The MAX1742/MAX1842 constant-off-time, pulse-width­modulated (PWM) step-down DC-DC converters are ideal for use in 5V and 3.3V to low-voltage conversion neces­sary in notebook and subnotebook computers. These devices feature internal synchronous rectification for high efficiency and reduced component count. They require no external Schottky diode. The internal 90mΩ PMOS power switch and 70mNMOS synchronous-rectifier switch easily deliver continuous load currents up to 1A. The MAX1742/MAX1842 produce a preset 2.5V, 1.8V, or
1.5V output voltage or an adjustable output from 1.1V to VIN. They achieve efficiencies as high as 95%.
The MAX1742/MAX1842 use a unique current-mode, constant-off-time, PWM control scheme, which includes Idle Mode™ to maintain high efficiency during light-load operation. The programmable constant-off-time architec­ture sets switching frequencies up to 1MHz, allowing the user to optimize performance trade-offs between effi­ciency, output switching noise, component size, and cost. Both devices are designed for continuous output currents up to 1A. The MAX1742 uses a peak current limit of 1.3A (min) and is suitable for applications requir­ing small external component size and high efficiency. The MAX1842 has a higher current limit of 3.1A (min) and is intended for applications requiring an occasional burst of output current up to 2.7A. Both devices also fea­ture an adjustable soft-start to limit surge currents during startup, a 100% duty cycle mode for low-dropout opera­tion, and a low-power shutdown mode that disconnects the input from the output and reduces supply current below 1µA. The MAX1742/MAX1842 are available in 16­pin QSOP packages.
For similar devices that provide continuous output cur­rents up to 2A and 3A, refer to the MAX1644 and MAX1623 data sheets.
Applications
5V or 3.3V to Low-Voltage Conversion
CPU I/O Ring
Chipset Supplies
Notebook and Subnotebook Computers
Features
±1% Output Accuracy
95% Efficiency
Internal PMOS and NMOS Switches
90mOn-Resistance at V
IN
= 4.5V
110mOn-Resistance at VIN= 3V
Output Voltage
2.5V, 1.8V, or 1.5V Pin Selectable
1.1V to V
IN
Adjustable
3V to 5.5V Input Voltage Range
600µA (max) Operating Supply Current
<1µA Shutdown Supply Current
Programmable Constant-Off-Time Operation
1MHz (max) Switching Frequency
Idle-Mode Operation at Light Loads
Thermal Shutdown
Adjustable Soft-Start Inrush Current Limiting
100% Duty Cycle During Low-Dropout Operation
Output Short-Circuit Protection
16-Pin QSOP Package
PART
MAX1742EEE
-40°C to +85°C
TEMP RANGE PIN-PACKAGE
16 QSOP
Ordering Information
Idle Mode is a trademark of Maxim Integrated Products.
Typical Configuration
MAX1842EEE
-40°C to +85°C 16 QSOP
Pin Configuration appears 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.
EVALUATION KIT
AVAILABLE
INPUT
3V TO
5.5V
2.2µF
10
470pF
IN
V
CC
SHDN COMP TOFF
MAX1742 MAX1842
LX
FB
PGND
GND
FBSEL
REF
SS
0.01µF
OUTPUT
1.1V TO V
IN
1µF
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN= VCC= 3.3V, FBSEL = GND, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
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.
VCC, IN to GND ........................................................-0.3V to +6V
IN to V
CC
.............................................................................±0.3V
GND to PGND.....................................................................±0.3V
All Other Pins to GND.................................-0.3V to (V
CC
+ 0.3V)
LX Current (Note 1).............................................................±4.7A
REF Short Circuit to GND Duration ............................Continuous
ESD Protection .....................................................................±2kV
Continuous Power Dissipation (T
A
= +70°C) SSOP (derate 16.7mW/°C above +70°C; part mounted on 1 in.
2
of 1oz. copper)...............................1W
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) ................................ +300°C
Note 1: LX has internal clamp diodes to PGND and IN. Applications that forward-bias these diodes should take care not to exceed
the ICs package power dissipation limits.
Input Voltage V
Preset Output Voltage V
Adjustable Output Voltage Range
AC Load Regulation Error 2%
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
IN
, V
OUT
3.0 5.5 V
CC
TA = +25°C
FBSEL = V
CC
VIN = 3V to
5.5V, I
= 0
LOAD
to 1A for MAX1742,
= 0
I
LOAD
to 2.5A for MAX1842, V
= V
FB
VIN = VCC = 3V to 5.5V, I FBSEL = GND
FBSEL = unconnected
FBSEL = REF
OUT
FBSEL = GND
LOAD
to +85°C
TA = +0°C to +85°C
TA = +25°C to +85°C
= +0°C
T
A
to +85°C
TA = +25°C to +85°C
T
= +0°C
A
to +85°C
TA = +25°C to +85°C
T
= +0°C
A
to +85°C
= 0,
DC Load Regulation Error 0.4 % Dropout Voltage V
Reference Voltage V
Reference Load Regulation V PMOS Switch
On-Resistance
NMOS Switch
On-Resistance
R
R
DO
REF
ON, P ILX
ON, N ILX
V
= VCC = 3V, I
IN
TA = +25°C to +85°C 1.089 1.100 1.111
TA = +0°C to +85°C 1.084 1.100 1.117
REF IREF
= -1µA to +10µA 0.5 2 mV
= 0.5A
= 0.5A
= 1A 250 mV
LOAD
V
= 4.5V 90 200
IN
V
= 3V 110 250
IN
V
= 4.5V 70 150
IN
V
= 3V 80 200
IN
2.500 2.525 2.550
2.487 2.525 2.563
1.500 1.515 1.530
1.492 1.515 1.538
1.800 1.818 1.836
1.791 1.818 1.845
1.089 1.100 1.111
1.084 1.100 1.117
V
REF
V
IN
V
V
V
m
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VIN= VCC= 3.3V, FBSEL = GND, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
MAX1742 1.3 1.5 1.7
Current-Limit Threshold I
LIMIT
MAX1842 3.1 3.6 4.1
A
RMS LX Output Current 3.1 A
MAX1742 0.1 0.3 0.5
Idle Mode Current Threshold I
IM
MAX1842 0.3 0.6 0.9
A
Switching Frequency f (Note 2) 1
MHz
No-Load Supply Current
VFB = 1.2V
µA
Shutdown Supply Current
SHDN = GND <1 5 µA
PMOS Switch Off-Leakage
Current
I
IN
SHDN = GND 15 µA
Thermal Shutdown Threshold
Hysteresis = 15°C
°C
Undervoltage Lockout Threshold
V
IN
falling, hysteresis = 90mV 2.5 2.6 2.7 V
FB Input Bias Current I
FB
V
FB
= 1.2V 0 60
nA
R
TOFF
= 110k 0.9
1.1
R
TOFF
= 30.1k
Off-Time Default Period t
OFF
R
TOFF
= 499k 3.8 4.5 5.2
µs
Off-Time Startup Period t
OFF
FB = GND 4
t
OFF
µs
On-Time Period t
ON
(Note 2) 0.4 µs
SS Source Current I
SS
4 5 6 µA
SS Sink Current I
SS
V
SS
= 1V
µA
SHDN Input Current
V
SHDN
= 0 to V
CC
-1 1 µA
SHDN Input Low Threshold V
IL
0.8 V
SHDN Input High Threshold V
IH
2.0 V
FBSEL Input Current -4 +4 µA
FBSEL = GND 0.2
FBSEL = REF 0.9 1.3
FBSEL = unconnected
- 0.2
+0.2
FBSEL Logic Thresholds
FBSEL = V
CC
V
CC
V
IIN + I
I
CC
T V
CC
(SHDN)
SHDN
UVLO
350 600
160
1.00
0.24 0.30 0.37
I
SHDN
100
0.7 ✕ VCC
- 0.2
0.7 ✕ VCC
250
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS
(VIN= VCC= 3.3V, FBSEL = GND, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.) (Note 3)
Note 2: Recommended operating frequency, not production tested. Note 3: Specifications from 0°C to -40°C are guaranteed by design, not production tested.
PARAMETER
CONDITIONS
UNITS
Input Voltage V
IN
3.0 5.5 V
VIN = 3V to 5.5V, FBSEL = V
CC
FBSEL = REF
Preset Output Voltage V
OUT
I
LOAD
= 0 to 1A for MAX1742, I
LOAD
= 0 to 2.5A for MAX1842, V
FB
= V
OUT
FBSEL = GND
V
Adjustable Output Voltage Range
VIN = VCC = 3V to 5.5V, I
LOAD
= 0,
FBSEL = GND
V
IN
V
Reference Voltage V
REF
V
V
IN
= 4.5V
PMOS Switch
On-Resistance
I
LX
= 0.5A
V
IN
= 3V
V
IN
= 4.5V
NMOS Switch
On-Resistance
I
LX
= 0.5A
V
IN
= 3V
m
MAX1742 1.2 1.8
Current-Limit Threshold I
LIMIT
MAX1842 2.9 4.3 MAX1742
Idle Mode Current Threshold I
IM
MAX1842 0.2 1.0
A
No-Load Supply Current
VFB = 1.2V
µA
FB Input Bias Current I
FB
VFB = 1.2V 0
nA
Off-Time Default Period t
OFF
R
TOFF
= 110k
µs
SYMBOL
MIN MAX
2.475 2.575
FBSEL = unconnected 1.485 1.545
1.782 1.854
1.078 1.122
V
REF
1.078
R
ON, P
R
ON, N
0.05
IIN + I
CC
0.85 1.15
1.122 200 250 150 200
0.55
600
300
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
_______________________________________________________________________________________ 5
MAX1742
EFFICIENCY vs. OUTPUT CURRENT
= 5.0V, L = 6.0µH)
(V
100
95
90
85
80
75
70
EFFICIENCY (%)
65
60
55
50
0.001 0.01 1
V
V
= 1.8V, R
OUT
f = 833kHz
V
IN
= 2.5V, R
OUT
OUT
TOFF
= 75k,
TOFF
= 1.5V, R
TOFF
OUTPUT CURRENT (A)
= 47k, f = 926kHz
= 100k, f = 692kHz
0.1
MAX1742 toc01
EFFICIENCY (%)
100
V
95
90
85
80
75
70
65
60
55
50
0.001 0.01 1
MAX1742
NORMALIZED OUTPUT ERROR
vs. OUTPUT CURRENT
0.5
0.4
0.3
VIN = 5V, V
0.2
0.1
0
-0.1
-0.2
-0.3
NORMALIZED OUTPUT ERROR (%)
-0.4
-0.5
0.001 0.01 1
= 1.5V, L = 6µH
OUT
VIN = 3.3V, V
OUTPUT CURRENT (A)
OUT
0.1
= 1.5V
MAX1742 toc04
EFFICIENCY vs. OUTPUT CURRENT
= 2.5V, R
OUT
V
OUT
MAX1742
= 3.3V, L = 3.9µH)
(V
IN
= 36k, f = 456kHz
TOFF
V
= 1.8V, R
OUT
= 1.5V, R
TOFF
= 56k, f = 833kHz
TOFF
OUTPUT CURRENT (A)
= 43k, f = 869kHz
0.1
1100
1000
FREQUENCY (kHz)
EFFICIENCY vs.OUTPUT CURRENT
(f
PWM
VIN = 5V, V
µH, R
L = 15
OUTPUT CURRENT (A)
MAX1742 toc02
100
95
90
85
80
75
70
EFFICIENCY (%)
65
60
55
50
0.001 0.01 1
MAX1742
SWITCHING FREQUENCY
vs. OUTPUT CURRENT
VIN = 5V, V
900
800
700
600
500
400
300
200
100
0
0 0.2 0.4 0.6 0.8 1.0
= 2.5V, L = 6µH
OUT
VIN = 5V, V
OUT
VIN = 3.3V, V
OUTPUT CURRENT (A)
= 1.5V, L = 3.9µH
OUT
= 1.5V, L = 6µH
MAX1742
= 270kHz)
= 1.8V,
OUT
= 240k
TOFF
VIN = 3.3V, V
µH, R
L = 10
OUT
TOFF
0.1
MAX1742 toc03
= 1.8V, = 160k
MAX1742 toc05
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
0
1.0
0.5
2.0
1.5
3.0
2.5
3.5
4.5
4.0
5.0
0 100 150 20050 250 300 350 450400 500
OFF-TIME vs. R
TOFF
MAX1742 toc10
R
TOFF
(k)
t
OFF
(µs)
MAX1742
STARTUP AND SHUTDOWN
1ms/div
MAX1742 toc06
MAX1742
LOAD-TRANSIENT RESPONSE
I
INPUT
0A
1A/div
V
SHDN
0V
5V/div
V
OUTPUT
0V
1V/div
V
0V
SS
2V/div
10µs/div
MAX1742 toc07
0V
MAX1742
LINE-TRANSIENT RESPONSE
OUT
20µs/div
= 1.5V, R
TOFF
I
OUT
= 1A, V
MAX1742 toc08
= 100k, L = 6µH
V
INPUT
2V/div
0V
V
OUTPUT
20mV/div AC-COUPLED
500
450
(µA)
CC
400
+ I
IN
350
300
250
200
150
100
NO-LOAD SUPPLY CURRENT, I
50
0
021 3456
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
NO LOAD
SHUTDOWN
VIN (V)
MAX1742 toc09
V
OUTPUT
AC-COUPLED, 50mV/div
I
L
0.5A/div
100
(nA)
90
CC
80
+ I
IN
70
60
50
40
30
20
10
SHUTDOWN SUPPLY CURRENT, I
0
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
_______________________________________________________________________________________ 7
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
0.001 0.01 0.1 1 10
MAX1842
EFFICIENCY vs. OUTPUT CURRENT
(V
IN
= 5.0V, L = 2.5µH)
MAX1842 toc11
OUTPUT CURRENT (A)
V
OUT
= 1.5V, R
TOFF
= 1OOk,
f
PWM
= 770kHz
V
OUT
= 1.8V, R
TOFF
= 75k,
f
PWM
= 910kHz
V
OUT
= 2.5V, R
TOFF
= 47k,
f
PWM
= 1070kHz
100
95
90
85
80
70
65
60
55
50
75
EFFICIENCY (%)
0.001 0.01 0.1 1 10
MAX1842
EFFICIENCY vs. OUTPUT CURRENT
(V
IN
= 3.3V, L = 1.5µH)
MAX1842 toc12
OUTPUT CURRENT (A)
V
OUT
= 2.5V, R
TOFF
= 56k,
f
PWM
= 1000kHz
V
OUT
= 2.5V, R
TOFF
= 39k,
f
PWM
= 610kHz
V
OUT
= 1.8V, R
TOFF
= 43k,
f
PWM
= 1050kHz
100
95
90
85
80
70
65
60
55
50
75
EFFICIENCY (%)
0.001 0.01 0.1 1 10
MAX1842
EFFICIENCY vs. OUTPUT CURRENT
(f
PWM
= 270kHz)
MAX1842 toc13
I
OUT
(A)
VIN = 3.3V, V
OUT
= 1.8V,
L = 4.7µH, R
TOFF
= 160k
100
95
90
85
80
70
65
60
55
50
75
EFFICIENCY (%)
VIN = 5V, V
OUT
= 1.8V, L = 5.6µH,
R
TOFF
= 240k
MAX1842
NORMALIZED OUTPUT ERROR
vs. OUTPUT CURRENT
MAX1842 toc14
0.001 0.01 0.1 1 10 OUTPUT CURRENT (A)
VIN = 3.3V, V
OUT
= 1.5V, L = 1.5µH
VIN = 5V, V
OUT
= 1.5V, L = 2.5µH
0.1
0
-0.1
-0.2
-0.3
-0.4
NORMALIZED OUTPUT ERROR (%)
0
400
200
800
600
1000
1200
0 1.0 1.50.5 2.0 2.5 3.0
MAX1842
SWITCHING FREQUENCY
vs. OUTPUT CURRENT
MAX1842 toc15
OUTPUT CURRENT (A)
FREQUENCY (kHz)
VIN = 5V, V
OUT
= 2.5V, L = 2.5µH
VIN = 3.3V, V
OUT
= 1.5V, L = 1.5µH
VIN = 5V, V
OUT
= 1.5V, L = 2.5µH
0
0
0
0
MAX1842
STARTUP AND SHUTDOWN
V
SS
2V/div
MAX1842 toc16
V
SHDN
5V/div
R
OUT
= 0.5, R
TOFF
= 56k
V
IN
= 3.3V, V
OUT
= 1.5V
I
INPUT
1A/div
V
OUTPUT
1V/div
1ms/div
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
8 _______________________________________________________________________________________
NAME FUNCTION
1
SHDN
Shutdown Control Input. Drive SHDN low to disable the reference, control circuitry, and internal MOSFETs. Drive high or connect to V
CC
for normal operation.
PIN
Pin Description
MAX1842
LOAD-TRANSIENT RESPONSE
V
OUTPUT
50mV/div
I
L
2A/div
MAX1842 toc17
10µs/div
0
MAX1842
LINE-TRANSIENT RESPONSE
V
INPUT
2V/div
V
OUTPUT
20mV/div AC-COUPLED
MAX1842 toc18
20µs/div
I
OUT
= 2.5A, V
OUT
= 1.5V, R
TOFF
= 100k, L = 2.2µH
2, 4 IN Supply Voltage Inputfor the internal PMOS power switch.
3, 14, 16 LX
Connection for the drains of the PMOS power switch and NMOS synchronous-rectifier switch. Connect the inductor from this node to the output filter capacitor and load.
5 SS Soft-Start. Connect a capacitor from SS to GND to limit inrush current during startup.
6 COMP
Integrator Compensation. Connect a capacitor from COMP to VCCfor integrator compensation. See Integrator Amplifier section.
7 TOFF
Off-Time Select Input. Sets the PMOS power switch off-time during constant-off-time operation. Connect a resistor from TOFF to GND to adjust the PMOS switch off-time.
8 FB
Feedback Inputfor both preset-output and adjustable-output operating modes. Connect directly to output for fixed-voltage operation or to a resistive divider for adjustable operating modes.
9 GND Analog Ground
10 REF Reference Output. Bypass REF to GND with a 1µF capacitor.
11 FBSEL Feedback Select Input. Selects output voltage. See Table 3 for programming instructions.
12 V
CC
Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass VCCwith a 10and 2.2µF low­pass filter. See Figure 1.
13, 15 PGND Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch.
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
_______________________________________________________________________________________ 9
_______________Detailed Description
The MAX1742/MAX1842 synchronous, current-mode, constant-off-time, PWM DC-DC converters step down input voltages of 3V to 5.5V to a preset output voltage of
2.5V, 1.8V, or 1.5V, or to an adjustable output voltage from 1.1V to VIN. Both devices deliver up to 1A of contin­uous output current; the MAX1842 delivers bursts of out­put current up to 2.7A (see the Extended Current Limit section). Internal switches composed of a 0.09Ω PMOS power switch and a 0.07NMOS synchronous rectifier switch improve efficiency, reduce component count, and eliminate the need for an external Schottky diode.
The MAX1742/MAX1842 optimize efficiency by operat­ing in constant-off-time mode under heavy loads and in Maxims proprietary Idle Mode under light loads. A sin­gle resistor-programmable constant-off-time control sets switching frequencies up to 1MHz, allowing the user to optimize performance trade-offs in efficiency, switching noise, component size, and cost. Under low­dropout conditions, the device operates in a 100% duty-cycle mode, where the PMOS switch remains con­tinuously on. Idle Mode enhances light-load efficiency by skipping cycles, thus reducing transition and gate­charge losses.
When power is drawn from a regulated supply, constant­off-time PWM architecture essentially provides constant­frequency operation. This architecture has the inherent advantage of quick response to line and load transients.
The MAX1742/MAX1842s current-mode, constant-off­time PWM architecture regulates the output voltage by changing the PMOS switch on-time relative to the con­stant off-time. Increasing the on-time increases the peak inductor current and the amount of energy trans­ferred to the load per pulse.
Modes of Operation
The current through the PMOS switch determines the mode of operation: constant-off-time mode (for load currents greater than half the Idle Mode threshold), or Idle Mode (for load currents less than half the Idle Mode threshold). Current sense is achieved through a proprietary architecture that eliminates current-sensing I2R losses.
Constant-Off-Time Mode
Constant-off-time operation occurs when the current through the PMOS switch is greater than the Idle Mode threshold current (which corresponds to a load current of half the Idle Mode threshold). In this mode, the regu­lation comparator turns the PMOS switch on at the end of each off-time, keeping the device in continuous-con­duction mode. The PMOS switch remains on until the
output is in regulation or the current limit is reached. When the PMOS switch turns off, it remains off for the programmed off-time (t
OFF
). To control the current under short-circuit conditions, the PMOS switch remains off for approximately 4 x t
OFF
when V
OUT
<
V
OUT(NOM)
/ 4.
Idle Mode
Under light loads, the devices improve efficiency by switching to a pulse-skipping Idle Mode. Idle Mode operation occurs when the current through the PMOS switch is less than the Idle Mode threshold current. Idle Mode forces the PMOS to remain on until the current through the switch reaches the Idle Mode threshold, thus minimizing the unnecessary switching that degrades efficiency under light loads. In Idle Mode, the device operates in discontinuous conduction. Current­sense circuitry monitors the current through the NMOS synchronous switch, turning it off before the current reverses. This prevents current from being pulled from the output filter through the inductor and NMOS switch to ground. As the device switches between operating modes, no major shift in circuit behavior occurs.
100% Duty-Cycle Operation
When the input voltage drops near the output voltage, the duty cycle increases until the PMOS MOSFET is on continuously. The dropout voltage in 100% duty cycle is the output current multiplied by the on-resistance of the internal PMOS switch and parasitic resistance in the inductor. The PMOS switch remains on continuously as long as the current limit is not reached.
Shutdown
Drive SHDN to a logic-level low to place the MAX1742/MAX1842 in low-power shutdown mode and reduce supply current to less than 1µA. In shutdown, all circuitry and internal MOSFETs turn off, and the LX node becomes high impedance. Drive SHDN to a logic-level high or connect to VCCfor normal operation.
Summing Comparator
Three signals are added together at the input of the summing comparator (Figure 2): an output voltage error signal relative to the reference voltage, an integrated output voltage error correction signal, and the sensed PMOS switch current. The integrated error signal is pro­vided by a transconductance amplifier with an external capacitor at COMP. This integrator provides high DC accuracy without the need for a high-gain amplifier. Connecting a capacitor at COMP modifies the overall loop response (see the Integrator Amplifier section).
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
10 ______________________________________________________________________________________
Synchronous Rectification
In a step-down regulator without synchronous rectifica­tion, an external Schottky diode provides a path for cur­rent to flow when the inductor is discharging. Replacing the Schottky diode with a low-resistance NMOS syn-
chronous switch reduces conduction losses and improves efficiency.
The NMOS synchronous-rectifier switch turns on follow­ing a short delay after the PMOS power switch turns off, thus preventing cross conduction or shoot through. In
Figure 2. Functional Diagram
Figure 1. Typical Circuit
INPUT
= 10µF (MAX1742)
C
IN
= 33µF (MAX1842)
C
IN
10
2.2µF
470pF
R
TOFF
IN
V
CC
SHDN COMP
TOFF
MAX1742
LX
FB
PGND
GND
FBSEL
REF
SS
L
1µF
0.01µF
OUTPUT C
= 47µF (MAX1742)
OUT
= 150µF (MAX1842)
C
OUT
V
= 2.5V, FBSEL = V
OUT
V
= 1.8V, FBSEL = REF
OUT
= 1.5V, FBSEL = FLOATING
V
OUT
0.01µF
CC
FBSEL
FEEDBACK
COMP
V
SHDN
REF
REF
CC
G
m
470pF
10
V
IN
2.2µF
1µF
NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.
SELECTION
REF
REF
GND
MAX1742 MAX1842
SUMMING
COMPARATOR
TIMER
R
SS
CURRENT
SENSE
SKIP
PWM LOGIC
AND
DRIVERS
CURRENT
SENSE
PGNDTOFF
TOFF
FB
IN
10µF
LX
V
3.0V TO 5.5V
C
IN
OUT
V
OUT
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
______________________________________________________________________________________ 11
constant-off-time mode, the synchronous-rectifier switch turns off just prior to the PMOS power switch turning on. While both switches are off, inductor current flows through the internal body diode of the NMOS switch. The internal body diodes forward voltage is rel­atively high.
Thermal Resistance
Junction-to-ambient thermal resistance, θJA, is highly dependent on the amount of copper area immediately surrounding the IC leads. The MAX1742 evaluation kit has 0.5in2of copper area and a thermal resistance of 80°C/W with no forced airflow. Airflow over the board significantly reduces the junction-to-ambient thermal resistance. For heatsinking purposes, evenly distribute the copper area connected at the IC among the high­current pins.
Power Dissipation
Power dissipation in the MAX1742/MAX1842 is domi­nated by conduction losses in the two internal power switches. Power dissipation due to supply current in the control section and average current used to charge and discharge the gate capacitance of the internal switches (i.e., switching losses) is approximately:
P
DS
= C x V
IN
2
x f
PWM
where C = 2.5nF and f
PWM
is the switching frequen-
cy in PWM mode.
This number is reduced when the switching frequency decreases as the part enters Idle Mode. Combined con­duction losses in the two power switches are approxi­mated by:
P
D
= I
OUT
2
x R
PMOS
where R
PMOS
is the on-resistance of the PMOS switch.
The junction-to-ambient thermal resistance required to dissipate this amount of power is calculated by:
θ
JA
= (T
J,MAX
- T
A,MAX
) / P
D(TOT)
where: θJA= junction-to-ambient thermal resistance
T
J,MAX
= maximum junction temperature
T
A,MAX
= maximum ambient temperature
P
D(TOT)
= total losses
__________________Design Procedure
For typical applications, use the recommended compo­nent values in Tables 1 or 2. For other applications, take the following steps:
1) Select the desired PWM-mode switching frequency; 1MHz is a good starting point. See Figure 3 for maxi­mum operating frequency.
V
OUT
(V)
R
TOFF
(kΩ)
5.6 39
L
(µH)
5 3.3
5.6
V
IN
(V)
47
5.6 755 1.8
3.9 393.3 2.5
3.9 433.3 1.8
3.9 563.3 1.5
5 2.5
Table 1. MAX1742 Recommended Component Values (I
OUT
= 1A)
5.6 1005 1.5
f
PWM
(kHz)
850
910
610
1050
1000
770
1070
Table 2. MAX1842 Recommended Component Values (Continuous Output Current = 1A, Burst Output Current = 2.7A)
1180
715
940
985
570
850
800
f
PWM
(kHz)
1.55 1002.2
2.55
1.53.3 561.5
1.83.3 431.5
2.53.3 391.5
1.85 752.2
47
V
IN
(V)
2.2
3.35
L
(µH)
392.2
R
TOFF
(kΩ)
V
OUT
(V)
Figure 3. Maximum Recommended Operating Frequency vs. Input Voltage
OPERATING FREQUENCY vs. INPUT VOLTAGE
1400
1200
1000
800
600
400
OPERATING FREQUENCY (kHz)
200
0
2.6 3.6 4.13.1 4.6 5.1 5.6
MAXIMUM RECOMMENDED
V
= 1.5V
OUT
V
= 1.8V
OUT
V
= 2.5V
OUT
VIN (V)
V
OUT
= 3.3V
MAX1842 fig03
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
12 ______________________________________________________________________________________
2) Select the constant off-time as a function of input
voltage, output voltage, and switching frequency.
3) Select R
TOFF
as a function of off-time.
4) Select the inductor as a function of output voltage,
off-time, and peak-to-peak inductor current.
Setting the Output Voltage
The output of the MAX1742/MAX1842 is selectable between one of three preset output voltages: 2.5V,
1.8V, and 1.5V. For a preset output voltage, connect FB
to the output voltage and connect FBSEL as indicated in Table 3. For an adjustable output voltage, connect FBSEL to GND and connect FB to a resistive divider between the output voltage and ground (Figure 4). Regulation is maintained for adjustable output voltages when VFB= V
REF
. Use 50kfor R1. R2 is given by the
equation:
where V
REF
is typically 1.1V.
Programming the Switching Frequency
and Off-Time
The MAX1742/MAX1842 features a programmable PWM mode switching frequency, which is set by the input and output voltage and the value of R
TOFF
, con-
nected from TOFF to GND. R
TOFF
sets the PMOS power switch off-time in PWM mode. Use the following equation to select the off-time according to your desired switching frequency in PWM mode:
where: t
OFF
= the programmed off-time
V
IN
= the input voltage
V
OUT
= the output voltage
V
PMOS
= the voltage drop across the internal
PMOS power switch
V
NMOS
= the voltage drop across the internal
NMOS synchronous-rectifier switch
f
PWM
= switching frequency in PWM mode
Select R
TOFF
according to the formula:
R
TOFF
= (t
OFF
- 0.07µs) (110k/ 1.00µs)
Recommended values for R
TOFF
range from 36kΩ to
430kfor off-times of 0.4µs to 4µs.
Inductor Selection
The key inductor parameters must be specified: inductor value (L) and peak current (I
PEAK
). The following equa­tion includes a constant, denoted as LIR, which is the ratio of peak-to-peak inductor AC current (ripple current) to maximum DC load current. A higher value of LIR allows smaller inductance but results in higher losses and ripple. A good compromise between size and losses is found at approximately a 25% ripple-current to load-current ratio (LIR = 0.25), which corresponds to a peak inductor cur­rent 1.125 times the DC load current:
where: I
OUT
= maximum DC load current
LIR = ratio of peak-to-peak AC inductor current
to DC load current, typically 0.25
Figure 4. Adjustable Output Voltage
PIN
2.5
V
CC
Output voltage
1.5
1.8REF Output voltage
AdjustableGND
Resistive
divider
FB
OUTPUT
VOLTAGE
(V)
FBSEL
Unconnected Output voltage
Table 3. Output Voltage Programming
LX
MAX1742
R1 = 50k R2 = R1(V
= 1.1V
V
REF
MAX1842
/ V
OUT
REF
FB
- 1)
V
OUT
R2
R1
t
OFF
VV V
()
=
IN OUT PMOS
fVV V
PWM IN PMOS NMOS
−+
()
Vt
OUT OFF
L
=
×
I LIR
×
OUT
V
R2 R1
=−
OUT
V
REF
1
 
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
______________________________________________________________________________________ 13
The peak inductor current at full load is 1.125 x I
OUT
if the above equation is used; otherwise, the peak current is calculated by:
Choose an inductor with a saturation current at least as high as the peak inductor current. The inductor you select should exhibit low losses at your chosen operat­ing frequency.
Capacitor Selection
The input filter capacitor reduces peak currents and noise at the voltage source. Use a low-ESR and low­ESL capacitor located no further than 5mm from IN. Select the input capacitor according to the RMS input ripple-current requirements and voltage rating:
where I
RIPPLE
= input RMS current ripple.
The output filter capacitor affects the output voltage rip­ple, output load-transient response, and feedback loop stability. For stable operation, the MAX1742/MAX1842 requires a minimum output ripple voltage of V
RIPPLE
1% x V
OUT
.
The minimum ESR of the output capacitor should be:
Stable operation requires the correct output filter capaci­tor. When choosing the output capacitor, ensure that:
Integrator Amplifier
An internal transconductance amplifier fine tunes the output DC accuracy. A capacitor, C
COMP
, from COMP to VCCcompensates the transconductance amplifier. For stability, choose C
COMP
= 470pF.
A large capacitor value maintains a constant average output voltage but slows the loop response to changes in output voltage. A small capacitor value speeds up the loop response to changes in output voltage but
decreases stability. Choose the capacitor values that result in optimal performance.
Soft-Start
Soft-start allows a gradual increase of the internal cur­rent limit to reduce input surge currents at startup and at exit from shutdown. A timing capacitor, CSS, placed from SS to GND sets the rate at which the internal cur­rent limit is changed. Upon power-up, when the device comes out of undervoltage lockout (2.6V typ) or after the SHDN pin is pulled high, a 4µA constant-current source charges the soft-start capacitor and the voltage on SS increases. When the voltage on SS is less than approximately 0.7V, the current limit is set to zero. As the voltage increases from 0.7V to approximately 1.8V, the current limit is adjusted from 0 to the current-limit threshold (see the Electrical Characteristics).The volt­age across the soft-start capacitor changes with time according to the equation:
The soft-start current limit varies with the voltage on the soft-start pin, SS, according to the equation:
where I
LIMIT
is the current threshold from the Electrical
Characteristics.
Figure 5. Soft-Start Current Limit over Time
II
PEAK OUT
=+
Vt
×
OUT OFF
L
×
2
II
RIPPLE LOAD
VV V
OUT IN OUT
=−
()
V
IN
ESR
% 1
t
OFF
L
t
C
C
OUT
OUT
OFF
≥µµ / 33 1742
V
OUT
t
OFF
≥µµ / 79 1842
V
OUT
FV s for the MAX
FV s for the MAX
SHDN
0
V
(V)
SS
0
(A)
I
LIMIT
0
0.7V
V
SS
At
4µ
C
SS
SSI
LIMIT
VV
−× .
07
SS
=
.
11
V
I
LIMIT
1.8V
I
LIMIT
t
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
14 ______________________________________________________________________________________
The constant-current source stops charging once the voltage across the soft-start capacitor reaches 1.8V (Figure 5).
Extended Current Limit (MAX1842)
For applications requiring occasional short bursts of high output current (up to 2.7A), the MAX1842 provides a higher current-limit threshold. When using the MAX1842, choose external components capable of withstanding its higher peak current limit.
The MAX1842 is capable of delivering large output cur­rents for limited durations, and its thermal characteris­tics allow it to operate at continuously higher output currents. Figure 6 shows its maximum recommended continuous output current versus ambient temperature. Figure 7 shows the maximum recommended burst cur­rent versus the output current duty cycle at high tem­peratures.
Figure 7 assumes that the output current is a square wave with a 100Hz frequency. The duty cycle is defined as the duration of the burst current divided by the period of the square wave. This figure shows the limitations for continuous bursts of output current.
Note that if the thermal limitations of the MAX1842 are exceeded, it will enter thermal shutdown to prevent destructive failure.
Frequency Variation with Output Current
The operating frequency of the MAX1742/MAX1842 is determined primarily by t
OFF
(set by R
TOFF
), VIN, and
V
OUT
as shown in the following formula:
f
PWM
= (VIN- V
OUT
- V
PMOS
) / [t
OFF(VIN
- V
PMOS
+
V
NMOS
)]
However, as the output current increases, the voltage drop across the NMOS and PMOS switches increases and the voltage across the inductor decreases. This causes the frequency to drop. The change in frequency can be approximated with the following formula:
f
PWM
= -I
OUT
x R
PMOS
/ (VINx t
OFF
)
where R
PMOS
is the resistance of the internal MOSFETs
(90mtyp).
Circuit Layout and Grounding
Good layout is necessary to achieve the MAX1742/ MAX1842s intended output power level, high efficiency, and low noise. Good layout includes the use of a ground plane, careful component placement, and correct rout­ing of traces using appropriate trace widths. The follow­ing points are in order of decreasing importance:
1) Minimize switched-current and high-current ground loops. Connect the input capacitors ground, the out­put capacitors ground, and PGND. Connect the resulting island to GND at only one point.
2) Connect the input filter capacitor less than 5mm away from IN. The connecting copper trace carries large currents and must be at least 1mm wide, preferably 2.5mm.
Figure 6. MAX1842 Maximum Recommended Continuous Output Current vs. Temperature
Figure 7. MAX1842 Maximum Recommended Burst Current vs. Burst Current Duty Cycle
MAX1842
MAXIMUM RECOMMENDED CONTINUOUS
OUTPUT CURRENT vs. TEMPERATURE
2.70
2.65
2.60
2.55
2.50
2.45
OUTPUT CURRENT (A)
2.40
2.35
2.30 25 4535 55 65 75 85
TEMPERATURE (°C)
MAXIMUM RECOMMENDED BURST CURRENT
vs. BURST CURRENT DUTY CYCLE
2.7
TA = +85°C
2.6
2.5
2.4
BURST CURRENT (A)
2.3
I
IS A 100Hz SQUARE WAVE
OUT
FROM 1A TO THE BURST CURRENT
2.2
04020 60 80 100
DUTY CYCLE (%)
TA = +55°C
MAX1842 fig06
MAX1842 fig07
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
______________________________________________________________________________________ 15
3) Place the LX node components as close together and as near to the device as possible. This reduces resistive and switching losses as well as noise.
4) A ground plane is essential for optimum perfor­mance. In most applications, the circuit is located on a multilayer board, and full use of the four or more layers is recommended. Use the top and bottom lay­ers for interconnections and the inner layers for an uninterrupted ground plane. Avoid large AC currents through the ground plane.
Chip Information
TRANSISTOR COUNT: 3662
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
SHDN LX
PGND
LX
PGND
V
CC
FBSEL
REF
GND
TOP VIEW
MAX1742 MAX1842
QSOP
IN
LX
COMP
IN
SS
TOFF
FB
Pin Configuration
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
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.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)
QSOP.EPS
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