MAXIM MAX8550, MAX8551 Technical data
General Description
The MAX8550/MAX8551 integrate a synchronous-buck
PWM controller to generate V
DDQ
, a sourcing and sinking
LDO linear regulator to generate VTT, and a 10mA reference output buffer to generate V
TTR
. The buck controller
drives two external N-channel MOSFETs to generate output voltages down to 0.7V from a 2V to 28V input with output currents up to 15A. The LDO can sink or source up to
1.5A continuous and 3A peak current. Both the LDO output and the 10mA reference buffer output can be made
to track the REFIN voltage. These features make the
MAX8550/MAX8551 ideally suited for DDR memory applications in desktops, notebooks, and graphic cards.
The PWM controller in the MAX8550/MAX8551 utilizes
Maxim’s proprietary Quick-PWM™ architecture with programmable switching frequencies of up to 600kHz. This
control scheme handles wide input/output voltage ratios
with ease and provides 100ns response to load transients while maintaining high efficiency and a relatively
constant switching frequency. The MAX8550 offers fully
programmable UVP/OVP and skip-mode options ideal in
portable applications. Skip mode allows for improved
efficiency at lighter loads. The MAX8551, which is targeted towards desktop and graphic-card applications,
does not offer the pulse-skip feature.
The VTT and VTTR outputs track to within 1% of V
REFIN
/ 2.
The high bandwidth of this LDO regulator allows excellent transient response without the need for bulk capacitors, thus reducing cost and size.
The buck controller and LDO regulators are provided with
independent current limits. Adjustable lossless foldback
current limit for the buck regulator is achieved by monitoring the drain-to-source voltage drop of the low-side MOSFET. Additionally, overvoltage and undervoltage
protection mechanisms are built in. Once the overcurrent
condition is removed, the regulator is allowed to enter
soft-start again. This helps minimize power dissipation
during a short-circuit condition. The MAX8550/MAX8551
allow flexible sequencing and standby power management using the SHDNA, SHDNB, and STBY inputs.
Both the MAX8550 and MAX8551 are available in a
small 5mm × 5mm, 28-pin thin QFN package.
Applications
DDR I and DDR II Memory Power Supplies
Desktop Computers
Notebooks and Desknotes
Graphic Cards
Game Consoles
RAID
Networking
Features
Buck Controller
♦ Quick-PWM with 100ns Load-Step Response
♦ Up to 95% Efficiency
♦ 2V to 28V Input Voltage Range
♦ 1.8V/2.5V Fixed or 0.7V to 5.5V Adjustable Output
♦ Up to 600kHz Selectable Switching Frequency
♦ Programmable Current Limit with Foldback
Capability
♦ 1.7ms Digital Soft-Start and Independent
Shutdown
♦ Overvoltage/Undervoltage-Protection Option
♦ Power-Good Window Comparator
LDO Section
♦ Fully Integrated VTT and VTTR Capability
♦ VTT has ±3A Sourcing/Sinking Capability
♦ VTT and VTTR Outputs Track V
REFIN
/ 2
♦ All-Ceramic Output-Capacitor Designs
♦ 1.0V to 2.8V Input Voltage Range
♦ Power-Good Window Comparator
MAX8550/MAX8551
Integrated DDR Power-Supply Solutions for
Desktops, Notebooks, and Graphic Cards
________________________________________________________________ Maxim Integrated Products 1
PART
TEMP RANGE
PIN-PACKAGE
MAX8550ETI
28 5mm × 5mm TQFN
MAX8550ETI+
28 5mm × 5mm TQFN
MAX8551ETI
28 5mm × 5mm TQFN
Ordering Information
28
27
26
25
24
23
22
8
9
10
11
12
13
14
15
16
17
18
19
20
21
7
6
5
4
3
2
1
MAX8550
MAX8551
5mm x 5mm Thin QFN
TOP VIEW
OVP/UVP
(N.C. FOR
MAX8551)
TON
REF
ILIM
POK1
POK2
STBY
SHDNB
SHDNA
AV
DD
SKIP
(TP1 FOR
M
AX8551)
GND
PGND1
V
DD
DL
BST
LX
DH
V
IN
OUT
FB
REFIN
VTTI
VTT
PGND2
VTTR
VTTS
SS
Pin Configuration
19-3173; Rev 2; 9/04
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.
Typical Operating Circuit appears at end of data sheet.
Quick-PWM is a trademark of Maxim Integrated Products, Inc.
+Denotes lead-free package.
查询MAX8550ETI供应商查询MAX8550ETI供应商
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
PARAMETER
CONDITIONS
UNITS
MAIN PWM CONTROLLER
V
IN
228
Input Voltage Range
4.5 5.5
V
Output Adjust Range V
OUT
0.7 5.5 V
FB = OUT
0.7
FB = GND
2.5
Output Voltage Accuracy
(Note 2)
FB = V
DD
1.8
V
Soft-Start Ramp Time t
SS
Rising edge of SHDNA to full current limit 1.7 ms
TON = GND (600kHz)
219
TON = REF (450kHz)
273
389
On-Time t
ON
VIN = 15V,
V
OUT
= 1.5V
(Note 3)
571
ns
Minimum Off-Time
(Note 3)
450 ns
VIN Quiescent Supply Current I
IN
25 40 µA
VIN Shutdown Supply Current SHDNA = SHDNB = GND 1 5 µA
All on (PWM, VTT, and VTTR on) 2.5 5
SHDNA = GND (only VTT and VTTR on) 2 4
STBY = AVDD (only VTTR and PWM on) 1 2
AVDD Quiescent Supply Current I
AVDD
SHDNB = GND (only PWM on) 0.5 1
mA
AVDD + VDD Shutdown Supply
Current
SHDNA = SHDNB = GND 2 10 µA
Rising edge of V
IN
4.1
4.4 V
AVDD Undervoltage-Lockout
Threshold
Hysteresis 50 mV
VDD Quiescent Supply Current I
VDD
Set VFB = 0.8V 1 5 µA
MAX8550/MAX8551
Integrated DDR Power-Supply Solutions for
Desktops, Notebooks, and Graphic Cards
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN= +15V, VDD= AVDD= V
SHDNA
= V
SHDNB
= V
BST
= V
ILIM
= 5V, V
OUT
= V
REFIN
= V
VTTI
= 2.5V, UVP/OVP = STBY = FB = SKIP
= GND, PGND1 = PGND2 = LX = GND, TON = OPEN, V
VTTS
= V
VTT
, TA= -40°C to +85°C, unless otherwise noted. Typical values
are at T
A
= +25°C.) (Note 1)
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.
VINto GND .............................................................-0.3V to +30V
V
DD, AVDD
, VTTI to GND.........................................-0.3V to +6V
SHDNA, SHDNB, REFIN to GND..............................-0.3V to +6V
SS, POK1, POK2, SKIP, ILIM, FB to GND................-0.3V to +6V
STBY, TON, REF, UVP/OVP to GND........-0.3V to (AV
DD
+ 0.3V)
OUT, VTTR to GND..................................-0.3V to (AV
DD
+ 0.3V)
DL to PGND1..............................................-0.3V to (V
DD
+ 0.3V)
DH to LX....................................................-0.3V to (V
BST
+ 0.3V)
LX to BST..................................................................-6V to +0.3V
LX to GND .................................................................-2V to +30V
VTT to GND...............................................-0.3V to (V
VTTI
+ 0.3V)
VTTS to GND............................................-0.3V to (AV
DD
+ 0.3V)
PGND1, PGND2 to GND.......................................-0.3V to +0.3V
REF Short Circuit to GND...........................................Continuous
Continuous Power Dissipation (T
A
= +70°C)
28-Pin 5mm x 5mm TQFN (derate 35.7mW/°C
above +70°C).................................................................2.86W
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +165°C
Lead Temperature (soldering, 10s).................................+300°C
SYMBOL
MIN TYP MAX
VDD, AV
t
OFF_MIN
DD
0.693
2.47
1.78
TON = OPEN (300kHz) 316 352
TON = AVDD (200kHz) 461 516
170 194
213 243
200 300
4.25
0.707
2.53
1.82
MAX8550/MAX8551
Integrated DDR Power-Supply Solutions for
Desktops, Notebooks, and Graphic Cards
_______________________________________________________________________________________ 3
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
REFERENCE
Reference Voltage V
REF
AVDD = 4.5V to 5.5V; I
REF
= 0
2
V
Reference Load Regulation I
REF
= 0 to 50µA
V
V
REF
rising
V
REF Undervoltage Lockout
Hysteresis
mV
FAULT DETECTION
OVP Trip Threshold
UVP/OVP = AVDD (Note 4)
120 %
UVP Trip Threshold
(Referred to Nominal V
OUT
)
65 70 75 %
Lower level, falling edge, 1% hysteresis 87 90 93
POK1 Trip Threshold
(Referred to Nominal V
OUT
)
Upper level, rising edge, 1% hysteresis
113
%
Lower level, falling edge, 1% hysteresis
90
POK2 Trip Threshold
(Referred to Nominal V
VTTS
and V
VTTR
)
Upper level, rising edge, 1% hysteresis
%
UVP Blanking Time From rising edge of SHDNA 10 20 40 ms
OVP, UVP, POK_ Propagation
Delay
OVP not applicable in MAX8551 10 µs
POK_ Output Low Voltage I
SINK
= 4mA 0.3 V
POK_ Leakage Current V
POK_
= 5.5V, VFB = 0.8V, V
VTTS
= 1.3V 1 µA
ILIM Adjustment Range V
ILIM
V
ILIM Input Leakage Current 0.1 µA
Current-Limit Threshold (Fixed)
PGND1 to LX
45 50 55 mV
Current-Limit Threshold
(Adjustable) PGND1 to LX
V
ILIM
= 2V
235 mV
Current-Limit Threshold (Negative
Direction) PGND1 to LX
SKIP = AV
DD
(Note 4) -75 -60 -45 mV
Current-Limit Threshold (Negative
Direction) PGND1 to LX
SKIP = AV
DD
, V
ILIM
= 2V (Note 4)
mV
Zero-Crossing Detection
Threshold PGND1 to LX
3mV
Thermal-Shutdown Threshold
°C
Thermal-Shutdown Hysteresis 15 °C
ELECTRICAL CHARACTERISTICS (continued)
(VIN= +15V, VDD= AVDD= V
SHDNA
= V
SHDNB
= V
BST
= V
ILIM
= 5V, V
OUT
= V
REFIN
= V
VTTI
= 2.5V, UVP/OVP = STBY = FB = SKIP
= GND, PGND1 = PGND2 = LX = GND, TON = OPEN, V
VTTS
= V
VTT
, TA= -40°C to +85°C, unless otherwise noted. Typical values
are at T
A
= +25°C.) (Note 1)
(Referred to Nominal V
OUT
)
1.98
1.93
300
112 116
107 110
87.5
107.5 110 112.5
2.02
0.01
92.5
0.25 2.00
170 200
-250
+160
MAX8550/MAX8551
Integrated DDR Power-Supply Solutions for
Desktops, Notebooks, and Graphic Cards
4 _______________________________________________________________________________________
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
MOSFET DRIVERS
DH Gate-Driver On-Resistance V
BST
- VLX = 5V 1 4 Ω
DL Gate-Driver On-Resistance in
High State
14Ω
DL Gate-Driver On-Resistance in
Low State
0.5 3 Ω
DH falling to DL rising 30
Dead Time (Additional to
Adaptive Delay)
DL falling to DH rising 30
ns
INPUTS AND OUTPUTS
Rising edge
1.7
V
Logic Input Threshold
(SHDN_, STBY, SKIP (Note 4))
Hysteresis
mV
Logic Input Current
(SHDN_, STBY, SKIP (Note 4))
-1 +1 µA
Low (2.5V output)
Dual-Mode™ Input Logic
Levels (FB)
High (1.8V output) 2.1
V
Input Bias Current (FB)
µA
High
AV
DD
-
0.4
Floating
REF
Four-Level Input Logic Levels
(TON, OVP/UVP (Note 4))
Low 0.5
V
Logic Input Current
(TON, OVP/UVP (Note 4))
-3 +3 µA
FB = GND 90
350
FB = AV
DD
70
270OUT Input Resistance
FB adjustable mode
kΩ
OUT Discharge-Mode
On-Resistance
(Note 4) 10 25 Ω
DL Turn-On Level During
Discharge Mode
(Measured at OUT)
(Note 4) 0.3 V
ELECTRICAL CHARACTERISTICS (continued)
(VIN= +15V, VDD= AVDD= V
SHDNA
= V
SHDNB
= V
BST
= V
ILIM
= 5V, V
OUT
= V
REFIN
= V
VTTI
= 2.5V, UVP/OVP = STBY = FB = SKIP
= GND, PGND1 = PGND2 = LX = GND, TON = OPEN, V
VTTS
= V
VTT
, TA= -40°C to +85°C, unless otherwise noted. Typical values
are at T
A
= +25°C.) (Note 1)
Dual Mode is a trademark of Maxim Integrated Products, Inc.
1.20
225
-0.1 +0.1
3.15 3.85
1.65 2.35
175
135
400 800 1600
2.20
0.05
MAX8550/MAX8551
Integrated DDR Power-Supply Solutions for
Desktops, Notebooks, and Graphic Cards
_______________________________________________________________________________________ 5
Note 1: Specifications to -40°C are guaranteed by design, not production tested.
Note 2: When the inductor is in continuous conduction, the output voltage has a DC regulation level higher than the error-compara-
tor threshold by 50% of the ripple. In discontinuous conduction, the output voltage has a DC regulation level higher than the
trip level by approximately 1.5% due to slope compensation.
Note 3: On-time and off-time specifications are measured from 50% point to 50% point at the DH pin with LX = GND, V
BST
= 5V,
and a 250pF capacitor connected from DH to LX. Actual in-circuit times may differ due to MOSFET switching speeds.
Note 4: Not applicable to the MAX8551.
PARAMETER
CONDITIONS
UNITS
LINEAR REGULATORS (VTTR AND VTT)
VTTI Input Voltage Range V
VTTI
1 2.8 V
VTTI Supply Current I
VTTI
I
VTT
= I
VTTR
= 0
1mA
VTTI Shutdown Current SHDNA = SHDNB = GND 10 µA
REFIN Input Impedance V
REFIN
= 2.5V 12 20 30 kΩ
REFIN Range V
REFIN
1
V
V
REFIN
rising 0.7 0.9 V
REFIN Lockout Threshold
Hysteresis 75 mV
Soft-Start Charge Current I
SS
VSS = 0 4 µA
VTT Internal MOSFET High-Side
On-Resistance
I
VTT
= -100mA, V
VTTI
= 1.5V,
AV
DD
= 4.5V
0.3 Ω
VTT Internal MOSFET Low-Side
On-Resistance
I
VTT
= 100mA, AVDD = 4.5V 0.3 Ω
VTT Output Accuracy
(Referred to V
REFIN
/ 2)
V
REFIN
= 1.5V or 2.5V, I
VTT
= 1mA -1 +1 %
V
REFIN
= 2.5V, I
VTT
= 0 to ±1.5A 1
VTT Load Regulation
V
REFIN
= 1.5V, I
VTT
= 0 to ±1A 1
%
VTT Current Limit VTT = 0 or VTTI ±3 ±5
A
VTTS Input Current I
VTTS
V
VTTS
= 1.5V, VTT open
1µA
VTTR Output Error
(Referred to V
REFIN
/ 2)
V
REFIN
= 1.5V or 2.5V, I
VTTR
= 0 -1 +1 %
VTTR Current Limit V
VTTR
= 0 or V
VTTI
mA
ELECTRICAL CHARACTERISTICS (continued)
(VIN= +15V, VDD= AVDD= V
SHDNA
= V
SHDNB
= V
BST
= V
ILIM
= 5V, V
OUT
= V
REFIN
= V
VTTI
= 2.5V, UVP/OVP = STBY = FB = SKIP
= GND, PGND1 = PGND2 = LX = GND, TON = OPEN, V
VTTS
= V
VTT
, TA= -40°C to +85°C, unless otherwise noted. Typical values
are at T
A
= +25°C.) (Note 1)
SYMBOL
MIN TYP MAX
<0.1
2.8
±6.5
0.1
±23 ±40 ±60
MAX8550/MAX8551
Integrated DDR Power-Supply Solutions for
Desktops, Notebooks, and Graphic Cards
6 _______________________________________________________________________________________
Typical Operating Characteristics
(V
VIN
= 12V, V
OUT
= 2.5V, TON = GND, SKIP = AVDD, circuit of Figure 8, TA= +25°C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
(TON = GND)
MAX8550/51 toc01
I
LOAD
(A)
EFFICIENCY (%)
1010.1
60
70
80
90
100
50
10
20
30
40
0
0.01 100
fSW = 600kHz
V
OUT
= 2.5V
V
OUT
= 1.8V
V
OUT
= 1.5V
SKIP = GND
SKIP = AV
DD
EFFICIENCY vs. LOAD CURRENT
(TON = OPEN)
MAX8550/51 toc02
I
LOAD
(A)
EFFICIENCY (%)
1010.1
10
20
30
40
50
60
70
80
90
100
0
0.01 100
fSW = 300kHz
V
OUT
= 2.5V
V
OUT
= 1.8V
V
OUT
= 1.5V
SKIP = GND
SKIP = AV
DD
SWITCHING FREQUENCY vs. LOAD CURRENT
(TON = GND)
MAX8550/51 toc03
I
LOAD
(A)
FREQUENCY (kHz)
11108 92 3 4 5 6 71
50
100
150
200
250
300
350
400
450
500
550
600
650
700
0
012
SKIP = GND
SKIP = AV
DD
SWITCHING FREQUENCY vs. INPUT VOLTAGE
(TON = GND)
MAX8550/51 toc04
VIN (V)
FREQUENCY (kHz)
262420 228 10 12 14 16 186
420
440
460
480
500
540
520
560
580
600
620
640
660
680
700
400
428
I
LOAD
= 12A
I
LOAD
= 0A
SWITCHING FREQUENCY vs. TEMPERATURE
(TON = GND)
MAX8550/51 toc05
TEMPERATURE (°C)
FREQUENCY (kHz)
80655035205-10-25
650
660
670
680
690
700
600
640
630
620
610
-40
I
LOAD
= 12A
OUTPUT VOLTAGE
vs. LOAD CURRENT
MAX8550/51 toc06
I
LOAD
(A)
V
OUT
(V)
12106 842
2.495
2.500
2.505
2.510
2.515
2.520
2.525
2.530
2.535
2.540
2.490
014
VIN = 15V,
TON = GND
SKIP = GND
SKIP = AV
DD
VTT VOLTAGE
vs. VTT CURRENT
MAX8550/51 toc07
I
VTT
(A)
V
VTT
(V)
21-2 -1 0
1.21
1.22
1.23
1.24
1.25
1.26
1.27
1.28
1.20
-3 3
VTTR VOLTAGE
vs. VTTR CURRENT
MAX8550/51 toc08
I
VTTR
(mA)
V
VTTR
(V)
105-10 -5 0
1.21
1.22
1.23
1.24
1.25
1.26
1.27
1.28
1.20
-15 15
LINE REGULATION
(V
OUT
vs. VIN)
MAX8550/51 toc09
VIN (V)
V
OUT
(
V
)
262420 228 10 12 14 16 186
2.46
2.47
2.48
2.49
2.50
2.51
2.52
2.53
2.54
2.55
2.45
428
I
LOAD
= 0A
I
LOAD
= 12A
MAX8550/MAX8551
Integrated DDR Power-Supply Solutions for
Desktops, Notebooks, and Graphic Cards
_______________________________________________________________________________________ 7
LOAD TRANSIENT (BUCK)
MAX8550/51 toc10
20µs/div
12A
V
OUT
100mV/div
VTT
100mV/div
VTTR
100mV/div
I
LOAD
10A/div
0.1A
I
VTT
= 1.5A, I
VTTR
= 15mA
LOAD TRANSIENT VTT (-1.5A TO +1.5A)
MAX8550/51 toc11
40µs/div
V
OUT
50mV/div
VTT
50mV/div
VTTR
50mV/div
I
VTT
2A/div
0A
I
LOAD
= 12A, I
VTTR
= 15mA
LOAD TRANSIENT VTT (-3A TO +3A)
MAX8550/51 toc12
40µs/div
V
OUT
50mV/div
VTT
50mV/div
VTTR
50mV/div
I
VTT
5A/div
0A
I
LOAD
= 12A, I
VTTR
= 15mA
POWER-UP WAVEFORMS
MAX8550/51 toc13
200µs/div
OUT
1V/div
VTT
2V/div
VTTR
1V/div
V
IN
10V/div
0V
0V
0V
0V
VDD = 5V, I
LOAD
= 12A, I
VTT
= 1.5A, I
VTTR
= 15mA
POWER-DOWN WAVEFORMS
MAX8550/51 toc14
200µs/div
OUT
1V/div
VTT
2V/div
VTTR
1V/div
V
IN
10V/div
0V
0V
0V
0V
VDD = 5V, I
LOAD
= 12A, I
VTT
= 1.5A, I
VTTR
= 15mA
V
DDQ
STARTUP AND SHUTDOWN INTO
HEAVY LOAD, DISCHARGE DISABLED
MAX8550/51 toc15
1ms/div
V
OUT
2V/div
VTT
1V/div
SHDNA + SHDNB
5V/div
POK1
5V/div
0V
0V
0V
0V
I
LOAD
= 12A,
I
VTT
= 1.5A
Typical Operating Characteristics (continued)
(V
VIN
= 12V, V
OUT
= 2.5V, TON = GND, SKIP = AVDD, circuit of Figure 8, TA= +25°C, unless otherwise noted.)
V
DDQ
STARTUP AND SHUTDOWN INTO
LIGHT LOAD, DISCHARGE ENABLED
MAX8550/51 toc16
2ms/div
V
OUT
1V/div
VTT
1V/div
SHDNA + SHDNB
5V/div
POK1
5V/div
0V
0V
0V
0V
R
LOAD
= 10Ω,
R
VTT
= 20Ω
VTT, VTTR STARTUP AND SHUTDOWN
MAX8550/51 toc17
200µs/div
VTT
1V/div
VTTR
1V/div
SHDNB
5V/div
POK2
5V/div
0V
0V
0V
0V
I
VTT
= 1.5A, I
VTTR
= 15mA
MAX8550/MAX8551
Integrated DDR Power-Supply Solutions for
Desktops, Notebooks, and Graphic Cards
8 _______________________________________________________________________________________
OVERVOLTAGE AND TURN-OFF
OF BUCK OUTPUT
MAX8550/51 toc18
20µs/div
V
OUT
2V/div
I
L
25A/div
DL
5V/div
DH
20V/div
0V
0V
0V
0A
Typical Operating Characteristics (continued)
(V
VIN
= 12V, V
OUT
= 2.5V, TON = GND, SKIP = AVDD, circuit of Figure 8, TA= +25°C, unless otherwise noted.)
SHORT CIRCUIT AND
RECOVERY OF V
DDQ
MAX8550/51 toc19
400µs/div
V
OUT
2V/div
I
LOAD
10A/div
V
IN
10V/div
I
IN
2A/div
0A
0V
0A
0V
UVP DISABLED, FOLDBACK CURRENT LIMIT
SHORT CIRCUIT AND
RECOVERY OF V
DDQ
MAX8550/51 toc20
400µs/div
V
OUT
2V/div
I
LOAD
10A/div
V
IN
10V/div
I
IN
2A/div
0A
0V
0A
0V
UVP ENABLED
SHORT CIRCUIT OF VTT
MAX8550/51 toc21
400µs/div
VTT
1V/div
I
VTT
5A/div
0A
0V
MAX8550/MAX8551
Integrated DDR Power-Supply Solutions for
Desktops, Notebooks, and Graphic Cards
_______________________________________________________________________________________ 9
Pin Description
PIN NAME FUNCTION
1TON
On-Time Selection-Control Input. This four-level logic input sets the nominal DH on-time. Connect to
GND, REF, AV
DD
, or leave TON unconnected to select the following nominal switching frequencies:
TON = AV
DD
(200kHz)
TON = OPEN (300kHz)
TON = REF (450kHz)
TON = GND (600kHz)
OVP/
UVP
Overvoltage/Undervoltage-Protection Control Input. This four-level logic input enables or disables the
overvoltage and/or undervoltage protection. The overvoltage limit is 116% of the nominal output
voltage. The undervoltage limit is 70% of the nominal output voltage. Discharge mode is enabled when
OVP is also enabled. Connect the OVP/UVP pin to the following pins for the desired function:
OVP/UVP = AV
DD
(Enable OVP and discharge mode, enable UVP.)
OVP/UVP = OPEN (Enable OVP and discharge mode, disable UVP.)
OVP/UVP = REF (Disable OVP and discharge mode, enable UVP.)
OVP/UVP = GND (Disable OVP and discharge mode, disable UVP.)
2
N.C.
Do not connect; leave open.*
3 REF
+2.0V Reference Voltage Output. Bypass to GND with a 0.1µF (min) capacitor. REF can supply 50µA
for external loads. Can be used for setting voltage for ILIM. REF turns off when SHDNA, SHDNB, and
STBY are low.
4 ILIM
Valley Current-Limit Threshold Adjustment for Buck Regulator. The current-limit threshold across PGND
and LX is 0.1 times the voltage at ILIM. Connect ILIM to a resistive divider, typically from REF to GND,
to set the current-limit threshold between 25mV and 200mV. This corresponds to a 0.25V to 2V range at
ILIM. Connect ILIM to AV
DD
to select the 50mV default current-limit threshold. See the Setting the
Current Limit section.
5 POK1
Buck Power-Good Open-Drain Output. POK1 is low when the buck output voltage is more than 10%
above or below the normal regulation point or during soft-start. POK1 is high impedance when the
output is in regulation and the soft-start circuit has terminated. POK1 is low in shutdown.
6 POK2
LDO Power-Good Open-Drain Output. In normal mode, POK2 is low when either VTTR or VTTS is more
than 10% above or below the normal regulation point, which is typically REFIN / 2. In standby mode,
POK2 responds only to the VTTR input. POK2 is low in shutdown, and when V
REFIN
is less than 0.8V.
7 STBY
Standby. Connect to high for low-quiescent mode where the VTT output is disabled, but the VTTR
buffer is kept alive if SHDNB is high. POK2 takes input from only VTTR in this mode. PWM output can
be on or off, depending on the state of SHDNA.
8SS
Soft-Start Control for VTT and VTTR. Connect a capacitor (C9 in the Typical Applications Circuit) from
SS to ground (see the Soft-Start Capacitor Selection section). Leave SS open to disable soft-start.
SS discharges to ground when SHDNB is low. See the POR, UVLO, and Soft-Start section.
9 VTTS
Sensing Pin for Termination Supply Output. Normally connected to VTT pin to allow accurate regulation
to half the REFIN voltage. Connected to a resistive divider from VTT to GND to regulate VTT to higher
than half the REFIN voltage.
10 VTTR Termination Reference Voltage. VTTR tracks V
REFIN
/ 2.
*The MAX8551 has no OVP or discharge-mode feature. Only UVP is available.
(MAX8550)
(MAX8551)
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