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MAX8537EEI+T ,Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power SuppliesMAX8537/MAX8538/MAX853919-3141; Rev 1; 6/05Dual-Synchronous Buck Controllers for Point-of-Load, Tra ..
MAX8538 ,Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power SuppliesFeatures♦ MAX8537/MAX8539: Complete DDR SuppliesThe MAX8537/MAX8539 controllers provide a completep ..
MAX8538EEI ,Dual-Synchronous Buck Controllers for Point-of- Load / Tracking / and DDR Memory Power SuppliesFeatures♦ MAX8537/MAX8539: Complete DDR SuppliesThe MAX8537/MAX8539 controllers provide a completep ..
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MAX8538EEI+T ,Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power SuppliesELECTRICAL CHARACTERISTICS(V+ = 12V, EN_ = VL, BST_ = 6V, LX_ = 1V, VL load = 0mA, C = 10µF (cerami ..
MAX8538EEI+T ,Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power Suppliesfeaturesenhance flexibility. Out-of-phaseMAX8538EEI -40°C to +85°C 28 QSOPnontracking
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MAX8537EEI+T-MAX8538EEI+-MAX8538EEI+T
Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power Supplies
General Description
The MAX8537/MAX8539 controllers provide a complete
power-management solution for both double-data-rate
(DDR) and combiner supplies. The MAX8537 and
MAX8539 are configured for out-of-phase and in-phase
DDR power-supply operations, respectively, and gener-
ate three outputs: the main memory voltage (VDDQ), the
tracking sinking/sourcing termination voltage (VTT), and
the termination reference voltage (VTTR). The MAX8538
is configured as a dual out-of-phase controller for point-
of-load supplies. Each buck controller can source or
sink up to 25A of current, while the termination refer-
ence can supply up to 15mA output.
The MAX8537/MAX8538/MAX8539 use constant-
frequency voltage-mode architecture with operating
frequencies of 200kHz to 1.4MHz. An internal high-
bandwidth (25MHz) operational amplifier is used as an
error amplifier to regulate the output voltage. This
allows fast transient response, reducing the number of
output capacitors. An all-N-FET design optimizes effi-
ciency and cost. The MAX8537/MAX8538/MAX8539
have a 1% accurate reference. The second synchro-
nous buck controller in the MAX8537/MAX8539 and the
VTTR amplifier generate 1/2 VDDQvoltage for VTTand
VTTR, and track the VDDQwithin ±1%.
This family of controllers uses a high-side current-sense
architecture for current limiting. ILIM pins allow the set-
ting of an adjustable, lossless current limit for different
combinations of load current and RDSON. Alternately,
more accurate overcurrent limit is achieved by using
a sense resistor in series with the high-side FET.
Overvoltage protection is achieved by latching off the
high-side MOSFET and latching on the low-side MOSFET
when the output voltage exceeds 17% of its set output.
Independent enable, power-good, and soft-start features
enhance flexibility.
Applications
DDR Memory Power Supplies
Notebooks and Desknotes
Servers and Storage Systems
Broadband Routers
XDSL Modems and Routers
Power DSP Core Supplies
Power Combiner in Advanced VGA Cards
Networking Systems
RAMBUS Memory Power Supplies
FeaturesMAX8537/MAX8539: Complete DDR SuppliesMAX8538: Dual Nontracking ControllerOut-of-Phase (MAX8537/MAX8538) or In-Phase
(MAX8539) Operation4.5V to 23V Wide Input Range (Operate Down to
1.8V Input with External 5V Supply) Tracking Supply Maintains VTT= VTTR= 1/2 VDDQAdjustable Output from 0.8V to 3.6V with 1%
AccuracyVTTR Reference Sources and Sinks Up to 15mA200kHz to 1.4MHz Adjustable Switching
FrequencyAll-Ceramic Design Achievable>90% EfficiencyIndependent POK_ and EN_Adjustable Soft-Start and Soft-Stop for Each
OutputLossless Adjustable-Hiccup Current LimitOutput Overvoltage Protection28-Pin QSOP Package
MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
19-3141; Rev 1; 6/05
EVALUATION KIT
AVAILABLE
Ordering Information
PARTTEMP RANGEPIN-
PACKAGEOPERATION
MAX8537EEI-40°C to +85°C28 QSOPOut-of-phase
trackingAX 8537E E I+ -40°C to +85°C28 QSOPOut-of-phase
tracking
MAX8538EEI-40°C to +85°C28 QSOPOut-of-phase
nontrackingAX 8538E E I+ -40°C to +85°C28 QSOPOut-of-phase
nontracking
MAX8539EEI-40°C to +85°C28 QSOPIn-phase
trackingAX 8539E E I+ -40°C to +85°C28 QSOPIn-phase
tracking
Pin Configurations appear at end of data sheet.
+Denotes lead-free package.
MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
ABSOLUTE MAXIMUM RATINGS
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.
V+ to GND..............................................................-0.3V to +25V
AVL, VL to GND........................................................-0.3V to +6V
PGND to GND.......................................................-0.3V to +0.3V
FB_, EN_, POK_ to GND...........................................-0.3V to +6V
REFIN, VTTR, FREQ, SS_, COMP_ to GND....-0.3Vto (AVL + 0.3V)
BST_, ILIM_ to GND...............................................-0.3V to +30V
DH1 to LX1...............................................-0.3V to (BST1 + 0.3V)
DH2 to LX2...............................................-0.3V to (BST2 + 0.3V)
LX_ to BST_..............................................................-6V to +0.3V
LX_ to GND................................................................-2V to +25V
DL_ to PGND................................................-0.3V to (VL + 0.3V)
Continuous Power Dissipation (TA= +70°C)
28-Pin QSOP (derate 10.8mW/°C above +70°C)........860mW
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
ELECTRICAL CHARACTERISTICS
(V+ = 12V, EN_ = VL, BST_ = 6V, LX_ = 1V, VL load = 0mA, CVL= 10µF (ceramic), REFIN = 1.25V, PGND = AGND = FB_ = ILIM_ =
0V, CSS= 10nF, CVTTR= 1µF, RFREQ= 20kΩ, DH_ = open, DL_ = open, POK_ = open, circuit of Figure 1, TA= 0°C to +85°C,unless
otherwise noted.)PARAMETER CONDITIONS MIN TYP MAX UNITSGENERALV+ Operating Range VL regulator drops out below 5.5V (Note 1) 4.5 23.0 VV+/ VL Operating Range VL is externally generated (Note 1) 4.5 5.5 VV+ Operating Supply Current IL(VL) = 0, FB_ forced 50mV above threshold 3.5 7 mAV+ Standby Supply Current IL(VL) = 0, BST_ = VL, EN = LX_ = FB_ = 0V 350 700 µAVL REGULATOROutput Voltage 5.5V < V+ < 23V, 1mA < ILOAD < 70mA 4.75 5 5.25 VVL Undervoltage-Lockout Trip
LevelRising edge, hysteresis = 550mV (typ) (trip level is
typically 85% of VL) 4.18 4.3 4.42 VOutput Current This is for gate current of DL_ /DH_ drivers, C(VL) =
1µF/10mA ceramic capacitor 70 mAThermal Shutdown Rising temperature, typical hysteresis = 10°C +160 °CCURRENT-LIMIT THRESHOLD (all current limits are tested at V+ = VL = 4.5V and 5.5V)ILIM Sink Current ILIM_ = LX - 200mV, 1.8V < LX < 23V, BST = LX +5V 180 200 220 µASOFT-STARTSoft-Start Source Current SS_ = 100mV -7 -5 -3 µASoft-Start Sink Current SS_ = 0.8 or REFIN 3 5 7 µASoft-Start Full-Scale Voltage 0.8 or
REFIN VFREQUENCY
Low End of Range RFREQ = 100kΩ, V+ = VL = 5V 160 200 240 kHz
Intermediate Range RFREQ = 20kΩ, V+ = VL = 5V 800 1000 1200 kHz
High End of Range RFREQ = 14.3kΩ, V+ = VL = 5V 1120 1400 1680 kHzRFREQ = 100kΩ 95 RFREQ = 20kΩ 80 Maximum Duty CycleRFREQ = 14.3kΩ 72 %
MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 12V, EN_ = VL, BST_ = 6V, LX_ = 1V, VL load = 0mA, CVL= 10µF (ceramic), REFIN = 1.25V, PGND = AGND = FB_ = ILIM_ =
0V, CSS= 10nF, CVTTR= 1µF, RFREQ= 20kΩ, DH_ = open, DL_ = open, POK_ = open, circuit of Figure 1, TA= 0°C to +85°C,unless
otherwise noted.)PARAMETER CONDITIONS MIN TYP MAX UNITSRFREQ = 100kΩ 2.4 4RFREQ = 20kΩ 12 18Minimum Duty CycleRFREQ = 14.3kΩ 16 25%
DH_ Minimum Off-Time 140 200 ns
DH_ Minimum On-Time 120 nsERROR AMPLIFIERFB_ Input Bias Current VFB_ = 0.8V 250 nAFB1 Input-Voltage Set Point Over line and load 0.792 0.800 0.808 VMAX8538 0.792 0.800 0.808 FB2 Input-Voltage Set Point MAX8537/MAX8539, REFIN = 0.9V 0.894 0.900 0.906 V
Op-Amp Open-Loop Voltage
GainCOMP_ = 1.3V to 2.3V72>80dB
Op-Amp Gain Bandwidth25MHz
Op-Amp Output-Voltage Slew
Rate15V/µsDRIVERSBreak-Before-Make Time 30 nsDH1, DH2 On-Resistance in Low
State 0.9 2.5 ΩDH1, DH2 On-Resistance in High
State 1.3 2.5 ΩDL1, DL2 On-Resistance in Low
State 0.7 1.5 ΩDL1, DL2 On-Resistance in High
State 1.6 2.8 ΩLOGIC INPUTS (EN_)Input Low Level 4.5V < VL < 5.5V 0.8 VInput High Level 4.5V < VL < 5.5V 2.4 VInput Bias Current 0V to 5.5V -1 +0.1 +1 µAVTTRVTTR Output Voltage Range Source or sink 15mA 0.5 2.5 VVTTR Output Accuracy -15mA ≤ IVTTR ≤ +15mA, REFIN = 0.9V or 1.25V -1.0 REFIN +1.0 %REFINREFIN Input Bias Current REFIN = 0.9V or 1.25V -250 +250 nA
MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 12V, EN_ = VL, BST_ = 6V, LX_ = 1V, VL load = 0mA, CVL= 10µF (ceramic), REFIN = 1.25V, PGND = AGND = FB_ = ILIM_ =
0V, CSS= 10nF, CVTTR= 1µF, RFREQ= 20kΩ, DH_ = open, DL_ = open, POK_ = open, circuit of Figure 1, TA= 0°C to +85°C,unless
otherwise noted.)PARAMETER CONDITIONS MIN TYP MAX UNITSREFIN Input Voltage Range 0.5 2.5 VREFIN Undervoltage-Lockout Trip
Level Rising and falling edge, hysteresis = 15mV 0.4 0.45 0.5 V
OUTPUT-VOLTAGE FAULT COMPARATORS
Upper FB2 Fault ThresholdRising voltage, hysteresis = 15mV115117120% of
REFIN
Lower FB2 Fault ThresholdFalling voltage, hysteresis = 15mV687072% of
REFIN
Upper FB1 Fault ThresholdRising voltage, hysteresis = 15mV115117120% of
0.8V
Lower FB1 Fault ThresholdFalling voltage, hysteresis = 15mV687072% of
0.8V
POWER-OK OUTPUT (POK_)
POK_ Delay64Clock
cycles
Upper FB2 POK_ ThresholdRising voltage, hysteresis = 20mV110112114% of
REFIN
Lower FB2 POK_ ThresholdFalling voltage, hysteresis = 20mV868890% of
REFIN
Upper FB1 POK_ ThresholdRising voltage, hysteresis = 20mV110112114% of
0.8V
Lower FB1 POK_ ThresholdFalling voltage, hysteresis = 20mV868890% of
0.8V
POK_ Output Low LevelISINK = 2mA0.4V
POK_ Output High LeakagePOK_ = 5.5V1µA
ELECTRICAL CHARACTERISTICS (Note 2)
(V+ = 12V, EN_ = VL, BST_ = 6V, LX_ = 1V, VL load = 0mA, CVL= 10µF (ceramic), REFIN = 1.25V, PGND = AGND = FB_ = ILIM_ =
0V, CSS= 10nF, CVTTR= 1µF, RFREQ= 20kΩ, DH_ = open, DL_ = open, POK_ = open, circuit of Figure 1, TA= -40°C to +85°C,
unless otherwise noted.)PARAMETER CONDITIONS MIN TYP MAX UNITSV+ Operating Range VL regulator drops out below 5.5V (Note 1) 4.75 23.00 VFB_ Input-Voltage Set Point Over line and load 0.788 0.800 0.812 VFB2 Input-Voltage Set Point MAX8537/MAX8539, REFIN = 0.9V 0.891 0.900 0.909 VVTTR Output Accuracy -15mA < IVTTR ≤ +15mA, REFIN = 0.9V or 1.25V -1 REFIN +1 %
Note 1:Operating supply range is guaranteed by the VL line-regulation test. User must short V+ to VL if a fixed 5V supply is used
(i.e., if V+ is less than 5.5V).
Note 2:Specifications to -40°C are guaranteed by design, not production tested.
MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
VDDQ EFFICIENCY vs. LOAD CURRENT
MAX8537 toc01
LOAD CURRENT (A)
EFFICIENCY (%)
VDDQ = 2.5V
VDDQ = 1.8V
SENSE RESISTOR = 0Ω
VIN = 12V
VDDQ EFFICIENCY vs. LOAD CURRENT
MAX8537 toc02
LOAD CURRENT (A)
EFFICIENCY (%)
VDDQ = 2.5V
VDDQ = 1.8V
SENSE RESISTOR = 3mΩ
VIN = 12V
VDDQ vs. LOAD CURRENT
MAX8537 toc03
LOAD CURRENT (A)
DDQ15105
VIN = 12V
VTT vs. LOAD CURRENT
MAX8537 toc04
LOAD CURRENT (A)963
VIN = 12V
VTTR vs. LOAD CURRENT
MAX8537 toc05
LOAD CURRENT (mA)
TTR15105
VIN = 12V
OUTPUT VOLTAGE vs. INPUT VOLTAGE
MAX8537 toc06
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
VDDQ
VTT AND VTTR
IOUT_VDDQ = 20A
IOUT_VTT = 12A
IOUT_VTTR = 15mA
POWER-UP
MAX8537 toc07
VDDQ
1V/div
VTTR
8.5V/div
VTT
8.5V/div
5V/div
4ms/div
POWER-DOWN
MAX8537 toc08
4ms/div
2V/div
2V/div
2V/div
5V/divVIN
VDDQ
VTT
VVTTR
STARTUP AND SHUTDOWN
MAX8537 toc09
VDDQ
2V/div
VTTR
1V/div
VTT
1V/div
EN1/EN2
5V/div
1ms/div
Typical Operating Characteristics
(Circuit of Figure 1, TA= +25°C, 400kHz switching frequency, VIN= 12V, unless otherwise noted.)
MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
POWER-OK
MAX8537 toc10
POK1
5V/div
POK2
5V/div
VTT
1V/div
VDDQ
2V/div
2ms/div
IOUT_VDDQ = 20A
IOUT_VTT = 8A
VTT STARTUP AND SHUTDOWN
MAX8537 toc11
VTT
1V/div
VDDQ
2V/div
VTTR
1V/div
EN2
5V/div
2ms/div
VDDQ LOAD TRANSIENT
AND VTT TRACKING
MAX8537 toc12
VTT
50mV/div
VDDQ_IOUT
10A/div
VTTR
50mV/div
VDDQ
100mV/div
200μs/div
VTT_IOUT = 12A
VTTR = 15mA
di/dt = 5A/μs
20A
10A
VTT LOAD-TRANSIENT RESPONSE
MAX8537 toc13
VTTR
AC-COUPLED
50mV/div
VTT_IOUT
10A/div
VDDQ
AC-COUPLED
50mV/div
VTT
AC-COUPLED
50mV/div
200μs/div
IOUT_VDDQ = 20A
IOUT_VTTR = 15mA
di/dt = 1A/μs
-8A
VDDQ = 2.5V AT 20A BODE PLOT,
VIN = 12V
MAX8537 toc14
kHz
dB (DEGREES)
100k10k1k
1001M
VTT = 1.25V AT 12A BODE PLOT,
VIN = 12V
MAX8537 toc15
dB (DEGREES)
SHORT CIRCUIT AND RECOVERY
MAX8537 toc16
10ms/div
VOUT1
VOUT2
IL1
IIN
1V/div
1V/div
10V/div
5A/divypical Operating Characteristics (continued)
(Circuit of Figure 1, TA= +25°C, 400kHz switching frequency, VIN= 12V, unless otherwise noted.)
MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
Pin Description
PIN
NAME
(MAX8537/
MAX8539)
NAME
(MAX8538)FUNCTIONBST2BST2Bootstrap Input to Power Internal High-Side Driver for Step-Down 2. Connect to an
external capacitor and diode according to Figure 1.DH2DH2High-Side Gate-Driver Output for Step-Down 2. Swings from LX2 to BST2.LX2LX2
External Inductor Input for Step-Down 2. Connect to the switched side of the inductor.
LX2 serves as the lower supply-voltage rail for the DH2 high-side gate driver and the
current-limit circuitry.ILIM2ILIM2
Output Current-Limit Setting for Step-Down 2. Connect a resistor from ILIM2 to the
drain of the step-down 2 high-side MOSFET, or to the junction of the source of the
high-side MOSFET and the current-sense resistor to set the current-limit threshold.
See the Current-Limit Setting section.POK1POK1Open-Drain Output. High impedance when step-down 1 is within 12% of its regulation
voltage. POK1 is pulled low in shutdown.DL2DL2Low-Side Gate-Driver Output for Step-Down 2. Swings from PGND to VL.POK2POK2Open-Drain Output. High impedance when step-down 2 is within 12% of its regulation
voltage. POK2 is pulled low in shutdown or if REFIN is undervoltage.EN2EN2Enable Input for Step-Down 2 (also for VTTR for the MAX8537 and MAX8539)EN1EN1Enable Input for Step-Down 1FREQFREQ
Frequency Adjust. Connect a resistor from this pin to ground to set the frequency. The
range of the FREQ resistor is 163kΩ, 20kΩ, and 100kΩ (corresponding to 1.4MHz,
1.0MHz, and 200kHz).COMP2COMP2Compensation Pin for Step-Down 2. Connect to compensation networks.FB2FB2Feedback Input for Step-Down 2 with VREFIN as the Threshold. User must have
impedance <40kΩ.SS2SS2Soft-Start for Step-Down 2. Connect a capacitor to GND to set the soft-start time.
REFIN—
Reference Input for VTT and VTTR. Connect it to a resistor-divider from VDDQ. REFIN
common-mode voltage range is 0.5V to 2.5V. Current through the divider-resistors
must be ≥100µA.14N.C.For the MAX8538, connect pin 14 to GND.GNDGNDAnalog Ground for Internal CircuitrySS1SS1Soft-Start for Step-Down 1. Connect a capacitor to GND to set the soft-start time.FB1FB1Feedback Input for Step-Down 1 with 0.8V Threshold. User must have impedance
<40kΩ.COMP1COMP1Compensation Pin for Step-Down 1. Connect to compensation networks.
VTTR—VTTR Output Capable of Sourcing and Sinking Up to 15mA. Always bypass with a 1µF
ceramic capacitor (or larger) to GND.19GNDAnalog Ground for Internal CircuitryAVLAVLAnalog VL Input Pin. Connect to VL through a 4.7Ω resistor. Bypass with a 0.1µF (or
larger) ceramic capacitor to GND.V+V+Input Supply Voltage
MAX8537/MAX8538/MAX8539Detailed Description
The MAX8537/MAX8539 controllers provide a complete
power-management solution for both DDR and combin-
er supplies. The MAX8537 and MAX8539 are config-
ured for out-of-phase and in-phase DDR power-supply
operations, respectively. In addition to the dual-syn-
chronous buck controllers, they also contain an addi-
tional amplifier to generate a total of three outputs: the
main memory voltage (VDDQ), the tracking
sinking/sourcing termination voltage (VTT),and the ter-
mination reference voltage (VTTR). The MAX8538 is
configured as a dual out-of-phase controller for point-
of-load supplies. Each buck controller can source or
sink up to 25A of current, while the termination refer-
ence can supply up to 15mA output.
The MAX8537/MAX8539 have a 1% accurate refer-
ence. The first buck controller generates VDDQusing
external resistor-dividers. The second synchronous
buck controller and the amplifier generate 1/2 VDDQ
voltage for VTTand VTTR. The VTTand VTTRvoltages
are maintained within 1% of 1/2 VDDQ.
The MAX8537/MAX8538/MAX8539 use a constant-fre-
quency voltage-mode architecture with operating fre-
quencies of 200kHz to 1.4MHz to allow flexible design.
An internal high-bandwidth (25MHz) operational ampli-
fier is used as an error amplifier to regulate the output
voltage. This allows fast transient response, reducing
the number of output capacitors. Synchronous rectifica-
tion ensures high efficiency and balanced current
sourcing and sinking capability for VTT. An all-N-FET
design optimizes efficiency and cost. The two convert-
ers can be operated in-phase or out-of-phase to mini-
mize capacitance and optimize performance for all
VIN/VOUTcombinations.
Both channels have independent enable and power-
good functions. They also have high-side current-sense
architectures. ILIM pins allow the setting of an
adjustable, lossless current limit for different combina-
tions of load current and RDS(ON). Additionally, accu-
rate overcurrent protection is achieved by using a
sensing resistor in series with the high-side FET. The
positive current-limit threshold is programmable
through an external resistor. Overvoltage protection is
achieved by latching off the high-side MOSFET and
latching on the low-side MOSFET when the output volt-
age exceeds 17% of its set output.
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
Pin Description (continued)
PIN
NAME
(MAX8537/
MAX8539)
NAME
(MAX8538)FUNCTIONVLVL
Internal 5V Linear Regulator to Power the IC. VL is always on. Bypass with a ceramic
capacitor with 1µF/10mA of load current. The internal VL regulator can be disabled by
connecting VL and V+ to an externally generated 5V. VL output current can be
boosted with an external PNP transistor.DL1DL1Low-Side Gate-Driver Output for Step-Down 1. Swings from PGND to VL.PGNDPGNDPower Ground for Gate-Driver CircuitsILIM1ILIM1
Output Current-Limit Setting for Step-Down 1. Connect a resistor from ILIM1 to the
drain of the step-down 1 high-side MOSFET, or to the junction of the source of the
high-side MOSFET and the current-sense resistor to set the current-limit threshold.
See the Current-Limit Setting section.LX1LX1
External Inductor Input for Step-Down 1. Connect to the switched side of the inductor.
LX1 serves as the lower supply-voltage rail for the DH1 high-side gate driver and
current-limit circuitry.DH1DH1High-Side Gate-Driver Step-Down 1. Swings from LX1 to BST1.BST1BST1Bootstrap Input to Power Internal High-Side Driver for Step-Down 1. Connect to an
external capacitor and diode according to Figure 1.
DC-DC Controller
The MAX8537/MAX8538/MAX8539 step-down DC-DC
converters use a PWM voltage-mode control scheme.
An internal high-bandwidth (25MHz) operational amplifi-
er is used as an error amplifier to regulate the output
voltage. The output voltage is sensed and compared
with an internal 0.8V reference or REFIN to generate an
error signal. The error signal is then compared with a
fixed-frequency ramp by a PWM comparator to give the
appropriate duty cycle to maintain output voltage regula-
tion. At the rising edge of the internal clock, and with DL
(the low-side MOSFET gate drive) at 0V, the high-side
MOSFET turns on. When the ramp voltage reaches the
error-amplifier output voltage, the high-side MOSFET
latches off until the next clock pulse. During the high-
side MOSFET on-time, current flows from the input,
through the inductor, and to the output capacitor and
load. At the moment the high-side MOSFET turns off,
the energy stored in the inductor during the on-time is
released to support the load as the inductor current
ramps down by commutation through the low-side
MOSFET body diode. After a fixed delay, the low-side
MOSFET turns on to shunt the current from its body
diode for lower voltage drop and increased efficiency.
The low-side MOSFET turns off at the rising edge of the
next clock pulse, and when its gate voltage discharges
to zero, the high-side MOSFET turns on and another
cycle starts.
The controllers sense peak inductor current and pro-
vide hiccup-mode overload and short-circuit protection
(see the Current Limit section).
The MAX8537/MAX8538/MAX8539 operate in forced-
PWM mode where the inductor current is always contin-
uous, so even under light load the controller maintains
a constant switching frequency to minimize noise and
possible interference with system circuitry.
Synchronous-Rectifier Driver (DL)
Synchronous rectification reduces the conduction loss
in the rectifier by replacing the normal Schottky catch
diode with a low-resistance MOSFET switch. The
MAX8537/MAX8538/MAX8539 controllers also use the
synchronous rectifier to ensure proper startup of the
boost gate-drive circuit.
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 in parallel to
the high-side MOSFET’s gate-source terminals.
On startup, the synchronous rectifier (low-side
MOSFET) forces LX to ground and charges the boost
capacitor to VL. On the second half-cycle, the switch-
mode 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 input voltage.
Internal 5V Linear Regulator
All MAX8537/MAX8538/MAX8539 functions are pow-
ered from the on-chip low-dropout 5V regulator with the
input connected to V+. Bypass the regulator’s output
(VL) with a 1µF/10mA or greater ceramic capacitor. The
V+ to VL dropout voltage is typically 500mV, so when
V+ is less than 5.5V, VL is typically (V+ - 500mV).
The internal linear regulator can source up to 70mA to
supply the IC, power the low-side gate drivers, and
charge the external boost capacitors. The current
required to drive the external MOSFETs is calculated as
the total gate charge of the MOSFETs at 5V multiplied
by the switching frequency. At higher frequency, the
MOSFET drive current may exceed the capability of the
internal linear regulator. The output current at VL can
be supplemented with an external PNP transistor as
shown in Figures 4 and 5, which also moves most of
the power dissipation off the IC. The external PNP can
increase the output current at VL to over 200mA. The
dropout voltage increases to 1V (typ).
Undervoltage Lockout (UVLO)
If VL drops below 3.75V, the MAX8537/MAX8538/
MAX8539 assume that the supply voltage is too low to
make valid decisions, so UVLO circuitry inhibits switch-
ing and forces POK and DH low and DL high. After VL
rises above 4.3V, the controller powers up the outputs
(see the Startupsection).
Startup
Externally, the MAX8537/MAX8538/MAX8539 start
switching when VL rises above the 4.3V UVLO thresh-
old. However, the controller does not start unless all
four of the following conditions are met: 1) EN_ is high,
2) VL > 4.3V, 3) the internal reference exceeds 80% of
its nominal value (VREF> 0.64V), and 4) the thermal
limit is not exceeded. Once the MAX8537/MAX8538/
MAX8539 assert the internal enable signal, the con-
troller starts switching and enables soft-start.
MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
MAX8537
DL2COMP2
GND
POK2EN1
VTTR
FB2SS2
FB1
ILIM2
PGND
LX2
ILIM1
FREQVL
DL1LX1
DH1
BST1
BST2DH2EN2REFIN
SS1
COMP1
POK1234567891112131416171819202122232425262728
C17
3.9nF
C161
C28
C29
47pF0.4747pF
C11, C30,
C31, C32
C12, C36
C1310
C141
C151
C19
8.2nF
C18
15pF
C2039pF
C21
3.9nF
C22
820pF
C24
C25
220pF
R19
100k
R20
100k
100k
100k
51.1k
R10
10.0k
R11
51k
R12
R13
1.2k
R15
21.5k
R17
10.0k
R18
10.0k
0.8uH
POK1POK2
EN1EN2
VIN (10.8V TO 13.2V)
VOUT21.25V/±
12A
VOUT1
2.5V/20A
VTTR
R16
10.0k
C23
R14
22k
C26
C27
R24
C41
47nF
C42
R25
Figure 1. Typical Application Circuit: MAX8537 DDR Memory Application (400kHz Switching)
MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
MAX8539
DL2COMP2
GND
POK2EN1
VTTR
FB2SS2
FB1
ILIM2
PGND
LX2
ILIM1
FREQVL
DL1LX1
DH1
BST1
BST2DH2EN2REFIN
SS1
COMP1
POK1234567891112131416171819202122232425262728
C17
1.8nF
C161
VOUT1
C28
C29
C11, C30,
C31, C32
680uF
C12, C36, C37
C1310
C141
C402.2nF
C151
C19
2.2nF
C18
10pF
C2056pF
C21
15nF
C22
2.7nF
C24
C25
220pF
R19
100k
R20
100k
100k
100k
51.1k
R10
10.0k
R11
82k
R12
2.2k
R13
2.2k
R15
12.7k
R17
10.0k
R18
10.0k
0.5uH
C391.0nF
POK1POK2
EN1EN2
VIN (10.8V TO 13.2V)
VOUT20.9V/±
VOUT1
1.8V/15A
VOUT1
VTTR
R16
10.0k
C23
R14
12k
R21
R22
1.0k
C10
R251
R24
Figure 2. MAX8539 DDR Memory Application (400kHz Switching)
MAX8537/MAX8538/MAX8539
Dual-Synchronous Buck Controllers for Point-of-
Load, Tracking, and DDR Memory Power Supplies
MAX8538
DL2COMP2
GND
POK2EN1
GND
FB2SS2
FB1
ILIM2
PGND
LX2
ILIM1
FREQVL
DL1LX1
DH1
BST1
BST2DH2EN2N.C.
SS1
COMP1
POK1234567891112131416171819202122232425262728
C17
6.8nF
C28
C29
C12, C26
C1310
C141
C151
C19
1.8nF
C18
10pF
C2047pF
C21
C22
2.2nF
C24
C23
R19
100k
R20
100k
100k
100k
51.1k
R10
21.5k
R11
21.5k
R12
1.0k
R13
2.2k
R15
12.7k
C391.0nF
POK1POK2
EN1EN2
VIN (10.8V TO 13.2V)
VOUT22.5V/5A
VOUT1
1.8V/15A
R16
10.0k
R14
14k
R21
R22
1.0k
C10
C11470
R23
10.0k
C402.2nF
R24
R25
Figure 3. MAX8538 PowerPC™ Application (400kHz Switching)

Partno	Mfg	Dc	Qty	Available	Descript
MAX8537EEI+T	MAXIM	N/a	2122	avai	Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power Supplies
MAX8538EEI+ \|MAX8538EEI	MAX	N/a	1600	avai	Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power Supplies
MAX8538EEI+T \|MAX8538EEIT	MAXIM	N/a	606	avai	Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power Supplies
MAX8538EEI+T \|MAX8538EEIT	MAIMX	N/a	517	avai	Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power Supplies