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MAX8650EEG+T
4.5V to 28V Input Current-Mode Step-Down Controller with Adjustable Frequency
General DescriptionThe MAX8650 synchronous PWM buck controller oper-
ates from a 4.5V to 28V input and generates an
adjustable 0.7V to 5.5V output voltage at loads up to 25A.
The MAX8650 uses a peak current-mode control archi-
tecture with an adjustable (200kHz to 1.2MHz) constant
switching frequency and is externally synchronizable. The
IC’s current limit uses the inductor’s DC resistance to
improve efficiency or an external sense resistor for high
accuracy. The current-limit threshold is adjusted with an
external resistor. Foldback-type current limit can be
implemented to reduce the power dissipation in overload
or short-circuit conditions. Short-circuit protection is
provided based on sensing the current in the low-side
MOSFET. A reference input is provided for use with a
high-accuracy external reference or for double-data-rate
(DDR)-tracking applications.
Monotonic prebiased startup is available for a safe-start
in applications where the output capacitor may have an
initial charge. This feature prevents the output from
pulling low during startup, which is a common charac-
teristic of conventional buck regulators.
A 180°out-of–phase synchronization output is available
for synchronizing with another converter.
FeaturesOperates from 4.5V to 28V Supply1% FB Voltage Accuracy Over TemperatureAdjustable Output Voltage Down to 0.7V or REFINAdjustable Switching Frequency or External
Synchronization from 200kHz to 1.2MHz180°Phase-Shifted Clock OutputAdjustable Overcurrent LimitAdjustable Foldback Current LimitAdjustable Slope CompensationSelectable Current-Limit Mode: Latch-Off or
Automatic RecoveryMonotonic Output-Voltage Rise at StartupOutput Sources and Sinks CurrentEnable InputPower-OK (POK) Output Adjustable Soft-StartIndependently Adjustable Overvoltage Protection
MAX8650SYNCON
FSYNC
POK
SYNCO
SCOMP
ILIM2
REFIN
ILIM1
COMP
OVP
MODE
BST
PGND
AVL
CS+
CS-
GNDR9
VOUT
0.7V TO 5.5V
VIN
7V TO 28V
C10
C11
OFF
POK
SYNCO
MAX8650EEG
Typical Operating Circuit19-3973; Rev 1; 8/08
Pin Configuration appears at end of data sheet.
EVALUATION KIT
AVAILABLE4.5V to 28V Input Current-Mode Step-
Down Controller with Adjustable Frequency
ApplicationsBase StationsDDR
Network and TelecomPower Modules
StorageIBA Applications
Servers
Ordering Information
PARTTEMP RANGEPIN-PACKAGEMAX8650EEG+-40°C to +85°C24 QSOP
+Denotes a lead-free/RoHS-compliant package.
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VIN= 12V, VBST- VLX= 6.5V, TA= -40°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (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.
IN, EN to GND........................................................-0.3V to +30V
BST to LX...............................................................-0.3V to +7.5V
DH to LX....................................................-0.3V to (VBST + 0.3V)
LX to GND....................-1V (-2.5V for < 50ns transient) to +30V
DL to PGND.................................................-0.3V to (VVL+ 0.3V)
ILIM2, ILIM1, SYNCO, FSYNC, OVP,
SCOMP to GND .....................................-0.3V to (VAVL+ 0.3V)
VL to PGND...........................................................-0.3V to +7.5V
AVL, FB, POK, COMP, SS, MODE, REFIN to GND.....-0.3V to +6V
CS+, CS- to GND.....................................................-0.3V to +6V
PGND to GND.......................................................-0.3V to +0.3V
Continuous Power Dissipation (TA= +70°C)
24-Pin QSOP (derate 9.5mW/°C above +70°C)..........762mW
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
PARAMETER CONDITIONS MIN TYP MAX UNITSOperating Input Voltage Range VL = IN for VIN < 7V 4.5 28.0 VQuiescent Supply Current VFB = 0.75V, no switching 2 3 mAEN = GND, VIN ≤ 28V 10 Shutdown Supply Current
IIN + IVL + IAVL EN = GND, VAVL = VVL = VIN = 5V 32 µAAVL Undervoltage-Lockout Trip
Level VAVL rising, 3% typ hysteresis 3.90 4.15 4.40 VOutput Voltage Adjust Range Minimum output voltage is limited by minimum duty cycle
and external components 0.7 5.5 VVL Regulation Voltage 7V < VIN < 28V, 1mA < ILOAD < 40mA 6.0 6.5 7.0 VVL Output Current 40 mAAVL Regulation Voltage 5.5V < VVL < 7V, 1mA < ILOAD < 10mA 4.900 4.975 5.050 VAVL Output Current 10 mA
SOFT-STARTSS Shutdown Resistance From SS to GND, VEN = 0V 20 100 ΩSS Soft-Start Current VSS = 0.625V 18 23 28 µA
REFIN INPUTREFIN Dual Mode™ Threshold VAVL -
1.0V VAVL VREFIN Input Bias Current VREFIN = 0.7V to 1.5V -250 +250 nAREFIN Input Voltage Range 0 1.5 V
Dual Mode is a trademark of Maxim Integrated Products, Inc.
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
ELECTRICAL CHARACTERISTICS (continued)(VIN= 12V, VBST- VLX= 6.5V, TA= -40°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)
PARAMETER CONDITIONS MIN TYP MAX UNITSERROR AMPLIFIERREFIN = AVL 0.693 0.7 0.707FB Regulation Voltage VREFIN = 0.7V to 1.5V VREFIN
0.00375 VREFIN VREFIN
0.00375VTransconductance 70 110 160 µSCOMP Shutdown Resistance From COMP to GND, VEN = 0V 20 100 ΩFB Input Leakage Current VFB = 0.7V 5 50 nAFB Input Common-Mode Range -0.1 +1.5 V
CURRENT-SENSE AMPLIFIERVoltage GainVOUT = 0 to 5.5V, VCS+ - VCS- = 30mV12V/V
CURRENT LIMITRILIM1 = 24kΩ 27.2 32.0 36.8 Peak Current-Limit
Threshold (VCS+ - VCS-) ILIM1 = AVL 68.0 80.0 92.0 mVRILIM2 = 50kΩ -42.5 -50.0 -57.5 Valley Current-Limit Threshold
(VLX - VPGND) RILIM2 = 200kΩ -170 -200 -230 mVNegative Current-Limit Threshold % of (typ) positive direction current limit (VLX - VPGND) -90 -120 -150 %CS+, CS- Input Current VCS+ = VCS- = 0V or 5.5V -25 +25 µACS+, CS- Input Common-Mode
Range 0 5.5 V
SLOPE COMPENSATIONVSCOMP = 2.5V231.25250.00268.75VSCOMP = 1.25V113.77123.00132.23SCOMP = AVL231.25250.00268.75SCOMP = GND, TA = 0°C to +85°C113.77123.00132.23Slope Compensation at Maximum
Duty CycleTA = -40°C to +85°C110.70123.00132.23mVSCOMP High Threshold VAVL - 0.5 VSCOMP Low Threshold 0.5 VSCOMP Adjustment Range 1.25 2.5 VSCOMP Input Leakage Current VSCOMP = 1.25V to 2.5V 5 200 nA
OSCILLATORRFSYNC = 21.0kΩ 800 1000 1200 Switching Frequency RFSYNC = 143kΩ 160 200 240 kHzMinimum Off-Time Measured at DH 235 nsMinimum On-Time Measured at DH 75 100 ns
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
ELECTRICAL CHARACTERISTICS (continued)(VIN= 12V, VBST- VLX= 6.5V, TA= -40°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)
PARAMETER CONDITIONS MIN TYP MAX UNITSFSYNC Synchronization Range 160 1200 kHzFSYNC Input-High Pulse Width 100 nsFSYNC Input-Low Pulse Width 100 nsFSYNC Rise/Fall Time 100 nsSYNCO Phase Shift 180 D eg r eesSYNCO Output Low Level ISYNCO = 5mA 0.4 VSYNCO Output High Level ISYNCO = 5mA VAVL - VFSYNC Pin Threshold
for SYNC Mode 1.7 2.5 VFSYNC Input Low 0.4 VFSYNC Input High 2.5 V
FET DRIVERSVBST - VLX = 6.5V 1.13 1.8 DH On-Resistance, High State VBST - VLX = 5V 1.4 2.2 ΩVBST - VLX = 6.5V 1.0 2 DH On-Resistance, Low State VBST - VLX = 5V 1.3 2.2 ΩVVL = 6.5V 1.6 2.5 DL On-Resistance, High State VVL = 5V 1.7 2.8 ΩVVL = 6.5V 0.8 1.5 DL On-Resistance, Low State VVL = 5V 0.85 1.5 ΩBreak-Before-Make Dead Time Low side off to high side on, high side off to low side on 20 30 nsLX, BST Leakage Current VBST = 35V, VLX = 28V, VIN = 28V 5 µA
THERMAL PROTECTIONThermal Shutdown Rising temperature +160 °CThermal-Shutdown Hysteresis 15 °C
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
ELECTRICAL CHARACTERISTICS (continued)(VIN= 12V, VBST- VLX= 6.5V, TA= -40°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)
PARAMETER CONDITIONS MIN TYP MAX UNITSPOKREFIN = AVL, VFB rising, typical hysteresis is 3% 629.0 650.0 671.0 mV Power-OK Threshold VREFIN = 0.7V to 1.5V, VFB rising, typical hysteresis is 3% 88.7 91.7 94.7 % ofPOK Output Voltage, Low VFB = 0.6V, IPOK = 2mA 25 200 mVPOK Leakage Current, High VPOK = 5.5V 1 µA
OVPREFIN = AVL 770 800 840 mVOVP Threshold Voltage VREFIN = 0.7V to 1.5V 110 115 120 % of
VREFINOVP Leakage Current, High VOVP = 0.8V 500 nA
MODE CONTROLMODE Logic-Level Low 4.5V ≤ VAVL ≤ 5.5V 0.4 VMODE Logic-Level High 4.5V ≤ VAVL ≤ 5.5V 1.8 VMODE Input Current VMODE = 0 to VAVL -1 +1 µA
SHUTDOWN CONTROLEN Logic-Level Low 4.5V ≤ VAVL ≤ 5.5V 0.45 VEN Logic-Level High 4.5V ≤ VAVL ≤ 5.5V 2 VVEN = 0V -1 +1 EN Input Current VEN = 28V 1.5 6.0 µA
Note 1:Specifications are 100% production tested at TA= +85°C. Limits over the operating temperature range are guaranteed
by design.
Typical Operating Characteristics(Circuit of Figure 3, 500kHz switching, VIN= 17V, VOUT= 3.3V, TA= +25°C, unless otherwise noted.)
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
EFFICIENCY vs. LOAD CURRENTLOAD CURRENT (A)
EFFICIENCY (%)
MAX8650 toc01
12V INPUT, 3.3V OUTPUT
12V INPUT, 2.5V OUTPUT
12V INPUT, 1.8V OUTPUT
24V INPUT, 3.3V OUTPUT
EFFICIENCY vs. LOAD CURRENT
(CIRCUIT OF FIGURE 4)LOAD CURRENT (A)
EFFICIENCY (%)
MAX8650 toc02
12V INPUT, 1.25V OUTPUT
12V INPUT, 0.9V OUTPUT
LOAD REGULATIONLOAD CURRENT (A)
OUTPUT VOLTAGE (V)
MAX8650 toc0351015
LINE REGULATION
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
MAX8650 toc04101418222630
NO LOAD
15A LOAD
RILIM1 vs. PEAK CURRENT LIMITPEAK CURRENT LIMIT VCS+ - VCS- (V)
ILIM1
(k
MAX8650 toc05
OSCILLATOR FREQUENCY
vs. INPUT VOLTAGE
INPUT VOLTAGE (V)
OSCILLATOR FREQUENCY (kHz)
MAX8650 toc06101418222630
TA = +85°C
TA = +25°CTA = -40°C
IOUT
VOUT
5A/div
100mV/div
(AC-COUPLED)
40μs/div
STEP-LOAD RESPONSE
7.5A TO 15A TO 7.5AMAX8650 toc07
IOUT
VOUT
5A/div
50mV/div
(AC-COUPLED)
100μs/div
STEP-LOAD RESPONSE
-8A TO 8A TO -8A (CIRCUIT OF FIGURE 4)MAX8650 toc08
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable FrequencyVOUT
VPOK
VIN
5A/div
5V/div
5V/div
1V/div
2ms/div
POWER-UP WAVEFORMSMAX8650 toc09
VOUT
VPOK
VIN
5A/div
5V/div
5V/div
1V/div
200μs/div
POWER-DOWN WAVEFORMSMAX8650 toc10
VOUT
VPOK
VEN
10A/div
5V/div
5V/div
2V/div
2ms/div
ENABLE/SHUTDOWN WAVEFORMSMAX8650 toc11
VDH
VFSYNC
VSYNCO
5V/div
5V/div
10V/div
1μs/div
SYNCHRONIZATION WAVEFORMSMAX8650 toc12
IIN
VOUT
2V/div
10A/div
10A/div
200μs/div
SHORT CIRCUIT AND RECOVERYMAX8650 toc13
VDL
VOUT
VDH
1V/div
10V/div
5V/div
40μs/div
OVERVOLTAGE PROTECTIONMAX8650 toc14
7.5A LOAD
PHASE
0dB
10dB/div
30°/div1M100k10k
CLOSED-LOOP BODE PLOT WITH NO LOADGAIN
MAX8650 toc15
FREQUENCY (Hz)
PHASE
0dB
10dB/div
30°/div1M100k10k
CLOSED-LOOP BODE PLOT WITH 15A LOADGAIN
MAX8650 toc16
FREQUENCY (Hz)
Typical Operating Characteristics (continued)(Circuit of Figure 3, 500kHz switching, VIN= 17V, VOUT= 3.3V, TA= +25°C, unless otherwise noted.)
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
Pin Description
PINNAMEFUNCTIONFSYNC
Frequency Set and Synchronization. Connect a resistor from FSYNC to GND to set the switching
frequency, or drive with an external clock signal between 160kHz and 1.2MHz. See the Switching
Frequency and Synchronization section.MODECurrent-Limit Operating-Mode Selection. Connect MODE to AVL for latch-off current limit or connect
MODE to GND for automatic-recovery current limit.SYNCOSynchronization Output. Provides a clock output that is 180° out-of-phase with the internal oscillator
for synchronizing another MAX8650.BSTBoost Capacitor Connection. Connect a 0.1µF ceramic capacitor from BST to LX.
5DHHigh-Side n-Channel MOSFET Gate-Driver Output. Connect DH to the gate of the high-side MOSFET.
DH is internally pulled low in shutdown.LXExternal Inductor Connection
7DLLow-Side n-Channel MOSFET Gate-Driver Output. Connect DL to the gate of the low-side MOSFET
(synchronous rectifier). DL is internally pulled low in shutdown.PGNDPower Ground. Connect PGND to the power ground plane and to the source of the low-side external
MOSFET. The return path for both gate drivers is through PGND.
9VLInter nal 6.5V Li near - Reg ul ator Outp ut. C onnect a 1µF to 10µF cer am i c cap aci tor fr om V L to g r ound . For IN < 7V , connect V L d i r ectl y to IN . V L p ow er s b oth g ate d r i ver s. V L i s the i np ut to the AV L l i near r eg ul ator .INInput Supply Voltage. IN is the input to the VL linear regulator. Connect VL to IN for VIN < 7V.ENEnable. Apply logic-high to enable the output, or logic-low to put the controller in low-power shutdown
mode. Connect EN to IN for always-on operation.AVLInternal 5V Linear-Regulator Output. Connect a 1µF ceramic capacitor from AVL to ground. AVL
powers the MAX8650’s internal circuits.GNDGround. Connect GND to the analog ground plane. Connect the analog ground and power ground
planes at a single point near the IC. Low-current signals return to GND.CS+Positive Differential Current-Sense InputCS-Negative Differential Current-Sense InputILIM1
Programmable Current-Limit Input for Inductor Current. Connect a resistor from ILIM1 to GND to set
the peak current-limit threshold. ILIM1 sources 10µA through the resistor, and the voltage at ILIM1 is
attenuated 7.5:1 to set the final current limit. For example, a 60kΩ resistor results in 600mV at ILIM1.
This results in a current-limit threshold (VCS+ - VCS-) of 80mV. The ILIM1 resistor range is 24kΩ to
60kΩ. Connect ILIM1 to AVL to set the default current-limit threshold of 80mV.
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
Pin Description (continued)
PINNAMEFUNCTIONOVP
Output Voltage Sensing for Overvoltage Protection. Connect OVP to the center of a resistor-divider
from OUT to GND to set the FB independent output overvoltage trip point. Connect OVP to FB if this
independence is not desired. The OVP threshold is 115% of the nominal FB regulation voltage.FBFeedback Input. Connect FB to the center of a resistor voltage-divider between the output and GND
to set the output voltage. The FB threshold regulates at 0.7V or VREFIN.COMPLoop Compensation. Connect COMP to an external RC network to compensate the loop. COMP is
internally pulled to GND through 20Ω during shutdown.SSSoft-Start. Connect a 0.1µF to 1µF ceramic capacitor from SS to GND. This capacitor sets the soft-
start period during startup. SS is internally pulled to GND through 20Ω during shutdown.REFINExternal Reference Input. Connect REFIN to AVL to use the internal 0.7V reference for the feedback
threshold.ILIM2r og r am m ab l e C ur r ent- Li m i t Inp ut for the Low - S i d e M OS FE T ( LX - P G N D ) . C onnect a r esi stor fr om ILIM 2
to GN D to set the val l ey cur r ent- l i m i t thr eshol d . ILIM 2 sour ces 5µA thr oug h the r esi stor , and the vol tag e
at ILIM 2 i s attenuated 5:1 to set the fi nal cur r ent l i m i t. For exam p l e, a 50kΩ r esi stor r esul ts i n 250m V at
ILIM 2. Thi s r esul ts i n a cur r ent- l i m i t thr eshol d ( V L X - V P GN D ) of 50m V . V I LI M 2 m ust not exceed 1V .SCOMP
Programmable Slope-Compensation Input. The slope-compensation voltage rate is the voltage at
SCOMP times 0.1 divided by the oscillator period (T). Connect SCOMP to AVL or GND to set to the
default of 250mV/T or 125mV/T, respectively.POK
Open-Drain Output that Is High Impedance when the Output Voltage Rises Above 92% of the
Nominal Regulation Value. POK pulls low during shutdown and when the output drops below 88% of
the nominal regulation value.
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency6.5V LDO
REGULATOR
5V AVL
LDO
PWM
CONTROL
LOGIC
LEVEL
SHIFT
BST
SYNCO
FSYNC
MODE
CS+
CS-
ILIM1
GND
ILIM2
POK
CURRENT-
LIMIT
CONTROL LOGIC
UVLO
DIVIDE BY 7.5
5μA
10μA
OVP
SCOMP
THERMAL
SHDN
COMPPWM
COMPARATOR
REF
SELECT
LOGIC
REFIN
SOFT-START
CIRCUITRY
AVL
VOLTAGE
REFERENCE
REF
PGND
MAX8650
COMP
CLAMP
OSCILLATOR
SLOPE
COMP
ERROR
AMP
OVP
1.15V REF
CSA
CURRENT-
LIMIT COMPX1
LEVEL
SHIFT
0.92VREF
DIVIDE BY 5
Figure 1. Functional Diagram
Detailed Description
DC-DC Converter Control ArchitectureThe MAX8650 step-down controller uses a PWM, cur-
rent-mode control scheme. An internal transconduc-
tance amplifier establishes an integrated error voltage.
The heart of the PWM controller is an open-loop com-
parator that compares the integrated voltage-feedback
signal against the amplified current-sense signal plus
the adjustable slope-compensation ramp, which are
summed into the main PWM comparator to preserve
inner-loop stability. 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, storing energy in a magnetic field and
sourcing current to the output. The current-mode feed-
back system regulates the peak inductor current as a
function of the output-voltage error signal. The circuit
acts as a switch-mode transconductance amplifier and
pushes the output LC filter pole normally found in a
voltage-mode PWM to a higher frequency.
During the second half of the cycle, the high-side
MOSFET turns off and the low-side MOSFET turns on.
The inductor releases the stored energy as the current
ramps down, providing current to the output. The output
capacitor stores charge when the inductor current
exceeds the required load current and discharges when
the inductor current is lower, smoothing the voltage
across the load. Under soft-overload conditions, when
the peak inductor current exceeds the selected current
limit (see the Current-Limit Circuitsection), the high-side
MOSFET is turned off immediately and the low-side
MOSFET is turned on and remains on to let the inductor
current ramp down until the next clock cycle. Under
heavy-overload or short-circuit conditions, the valley
foldback current limit is enabled to reduce power dissi-
pation of external components.
The MAX8650 operates in a forced-PWM mode. As a
result, the controller maintains a constant switching fre-
quency, regardless of load, to allow for easier filtering
of the switching noise.
Internal Linear RegulatorsThe MAX8650 contains two internal LDO regulators. The
AVL regulator provides 5V for the IC’s internal circuitry,
and the VL regulator provides 6.5V for the MOSFET gate
drivers. Connect a 4.7µF ceramic capacitor from VL to
PGND, and connect a 1µF ceramic capacitor from AVL
to GND. For applications where the input voltage is
between 4.5V and 7V, connect VL directly to IN and con-
nect a 10Ωresistor from VL to AVL.
Undervoltage LockoutWhen AVL drops below 4.03V, the MAX8650 assumes
that the supply voltage is too low for proper operation,
so the undervoltage-lockout (UVLO) circuitry inhibits
switching and forces the DL and DH gate drivers low.
When AVL rises above 4.15V, the controller enters the
startup sequence and then resumes normal operation.
Startup and Soft-StartThe internal soft-start circuitry gradually ramps up the
reference voltage to control the rate of rise of the step-
down controller’s output and reduce input surge cur-
rents during startup. The soft-start period is determined
by the value of the capacitor from SS to GND. The soft-
start time is approximately (30.4ms/µF) x CSS. The
MAX8650 also features monotonic output-voltage rise;
therefore, both external power MOSFETs are kept off if
the voltage at FB is higher than the voltage at SS. This
allows the MAX8650 to start up into a prebiased output
without pulling the output voltage down.
Before the MAX8650 can begin the soft-start and power-
up sequence, the following conditions must be met:
•VAVLexceeds the 4.15VUVLO threshold.EN is at logic-high.The thermal limit is not exceeded.
Enable (EN)The MAX8650 features a low-power shutdown mode. A
logic-low at EN shuts down the controller. During shut-
down, the output is high impedance, and both DH and
DL are low. Shutdown reduces the quiescent current
(IQ) to less than 10µA. A logic-high at EN enables the
controller.
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
MAX8650 also uses the synchronous rectifier to ensure
proper startup of the boost gate-driver circuit and to
provide the current-limit signal. The low-side gate driver
(DL) swings from 0 to the 6.5V provided from VL. The
DL waveform is always the complement of the DH high-
side gate-drive waveform (with controlled dead time to
prevent cross-conduction or shoot-through). An adap-
tive dead-time circuit monitors the DL voltage and pre-
vents the high-side MOSFET 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 MAX8650 can interpret the
MOSFET gate as off when gate charge actually
remains. Use very short, wide traces, approximately 10
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
MAX8650to 20 squares (50 mils to 100 mils wide if the MOSFET
is 1in from the device) for the gate drive. The dead time
at the other edge (DH turning off) also has an adaptive
dead-time circuit operating in a similar manner. For
both edges, there is an additional 20ns fixed dead time
after the adaptive dead time expires.
High-Side Gate-Drive Supply (BST)A flying capacitor boost circuit (Figure 2) generates the
gate-drive voltage for the high-side n-channel MOSFET.
The capacitor between BST and LX is charged from VL
to 6.5V minus the diode forward-voltage drop while the
low-side MOSFET is on. When the low-side MOSFET is
switched off, the stored voltage of the capacitor is
stacked above LX to provide the necessary turn-on
voltage (VGS) for the high-side MOSFET. The controller
then closes an internal switch between BST and DH to
turn the high-side MOSFET on.
Current-Sense AmplifierThe current-sense circuit amplifies the differential cur-
rent-sense voltage (VCS+- VCS-). This amplified cur-
rent-sense signal and the internal slope-compensation
signal are summed (VSUM) together and fed into the
PWM comparator’s inverting input. The PWM compara-
tor shuts off the high-side MOSFET when VSUM
exceeds the integrated feedback voltage (VCOMP).
The differential current sense is also used to provide
peak inductor current limiting. This current limit is more
accurate than the valley current limit, which is mea-
sured across the low-side MOSFET’s on-resistance.
Current-Limit CircuitThe MAX8650 uses both foldback and peak current
limiting (Figure 5). The valley foldback current limit is
used to reduce power dissipation of external compo-
nents, mainly inductor and power MOSFETs, and
upstream power source, when output is severely over-
loaded or short circuited and POK is low. Thus, the cir-
cuit can withstand short-circuit conditions continuously
without causing overheating of any component. The
peak constant-current limit sets the current-limit point
more accurately since it does not have to suffer the wide
variation of the low-side power MOSFET’s on-resistance
due to tolerance and temperature.
The valley current is sensed across the on-resistance of
the low-side MOSFET (VPGND- VLX). The valley current
limit trips when the sensed voltage exceeds the valley
current-limit threshold. The valley current limit recovers
when the sensed voltage drops below the valley current-
limit threshold (except when using the latch-off option).
Set the minimum valley current-limit threshold, when the
output voltage is at the nominal regulated value, higher
than the maximum peak current-limit setting. With this
method, the current-limit point accuracy is controlled by
the peak current limit and is not interfered with by the
wide variation of MOSFET on-resistance. See the Setting
the Current Limitsection for how to set these limits.
The MAX8650 can be configured for either an
adjustable valley current-limit threshold with adjustable
foldback ratio, or a fixed valley current limit that latches
the converter off. When latch-off is used (MODE is con-
nected to AVL), set the current-limit threshold by only
one resistor from ILIM2 to GND and make sure this
threshold is higher than the maximum output current
required by at least a 20% margin. Cycle EN or input
power to reset the current-limit latch.
The peak current limit is used to sense the inductor cur-
rent, and is more accurate than the valley current limit
since it does not depend upon the on-resistance of the
low-side MOSFET. The peak current can be measured
across the resistance of the inductor for the highest effi-
ciency, or alternatively, a current-sense resistor can be
used for more accurate current sensing. A resistor con-
nected from ILIM1 to GND sets the peak current-limit
threshold.
For more information on the current limit, see the
Setting the Current Limitsection.
Switching Frequency and SynchronizationThe MAX8650 has an adjustable internal oscillator that
can be set to any frequency from 200kHz to 1.2MHz.
To set the switching frequency, connect a resistor from
FSYNC to GND. Calculate the resistor value from the
following equation:
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable FrequencyBST
MAX8650
Figure 2. DH Boost Circuit
The MAX8650 can also be synchronized to an external
clock by connecting the clock signal to FSYNC. In
addition, SYNCO is provided to synchronize a second
MAX8650 controller 180°out-of-phase with the first by
connecting SYNCO of the first controller to FSYNC of
the second. When the first controller is synchronized to
an external clock, the external clock is inverted to gen-
erate SYNCO. Therefore, to get 180°out-of-phase oper-
ation, the clock input to the first controller should have
a 50% duty cycle.
Power-Good Signal (POK)POK is an open-drain output on the MAX8650 that mon-
itors the output voltage. When the output is above 92%
of its nominal regulation voltage, POK is high imped-
ance. When the output drops below 89% of its nominal
regulation voltage, POK is internally pulled low. POK is
also internally pulled low when the MAX8650 is shut
down. To use POK as a logic-level signal, connect a
pullup resistor from POK to the logic supply rail.
Thermal-Overload ProtectionThermal-overload protection limits total power dissipa-
tion in the MAX8650. When the junction temperature
exceeds +160°C, an internal thermal sensor shuts
down the device, allowing the IC to cool. The thermal
sensor turns the IC on again after the junction tempera-
ture cools by 15°C, resulting in a pulsed output during
continuous thermal-overload conditions.fnskFSYNC=−⎛⎜⎞⎟⎛⎜⎞⎟1162134
MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable FrequencySYNCONC13
R14
C11
FSYNC
POK
SYNCO
SCOMP
ILIM2
REFIN
ILIM1
COMP
OVP
MODE
BST
PGND
AVL
CS+
CS-
GND
C9A
C9B
C14R15
VOUT
3.3V/15A
VIN
10V TO 24V
C15
C10
C12
R13
R10
OFF
POK
SYNCO
MAX8650EEG
R11
R12
Figure 3. Applications Circuit with 500kHz Switching, 10V to 24V Input, and 3.3V/15A Output
