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MAX1623EAP/T-MAX1623EAP+-MAX1623EAP+T-MAX1623EAP-T
3A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches
MAX1623
3A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
EVALUATION KIT MANUALAVAILABLE
General DescriptionThe MAX1623 switch-mode buck regulator with syn-
chronous rectification provides local CPU and bus-ter-
mination power in notebook and desktop computers.
An internal 55mΩ(typ), 3A PMOS power switch and
60mΩ (typ), 3A NMOS synchronous-rectifier switch
deliver continuous load currents up to 3A from a 5V
supply with 95% typical efficiency. Output accuracy is
±1%, including line and load regulation.
The MAX1623 features constant-off-time, current-mode
pulse-width-modulation (PWM) control with switching
frequencies as high as 350kHz. An external resistor at
the TOFF pin sets the off-time, allowing optimum design
flexibility in terms of switching frequency, output switch-
ing noise, and inductor size. This device is available in
a space-saving 20-pin SSOP package.
________________________Applications5V to 3.3V Conversion
Notebook Computer CPU I/O Supply
Desktop Computer Bus-Termination Supply
CPU Daughter Card Supply
DSP Supply
____________________________Features±1% Output Accuracy, Including Line and Load
Regulation94% EfficiencyInternal Switches
55mΩPMOS Power Switch
60mΩNMOS Synchronous-Rectifier SwitchGuaranteed 3A Load CapabilityMinimal External ComponentsPin-Selectable Fixed 3.3V, 2.5V, or
Adjustable (1.1Vto 3.8V) Output Voltage4.5V to 5.5V Input Voltage Range400µA (typ) Supply Current <1µA Shutdown Supply CurrentConstant-Off-Time PWM OperationSwitching Frequencies Up to 350kHzIdle Mode™Operation at Light LoadsThermal Shutdown ProtectionAvailable in 20-Pin SSOPPGND
PGNDIN
TOP VIEW
PGND
VCC
COMPFBSEL
REF
GNDFB
TOFF
MAX1623
SSOPSHDN
Pin ConfigurationINPUT
MAX1623
2.5V
OUTPUTLX
PGND
GND
REFCOMP
FBSEL
VCC
TOFF
SHDN
Typical Operating Circuit19-1436; Rev 2; 3/02
PARTMAX1623EAP -40°C to +85°C
TEMP RANGEPIN-PACKAGE20 SSOP
Ordering InformationIdle Mode is a trademark of Maxim Integrated Products.
MAX1623
3A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VIN= VCC= 5V, FBSEL unconnected, RTOFF= 110kΩ, TA
= 0°C to +85°C, unless otherwise noted. Typical values are at = +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.
IN to PGND.....................................................................0V to 6V
VCCto GND................................................................-0.3V to 6V
PGND to GND.....................................................................±0.5V
IN to VCC.............................................................................±0.5V
LX Current (Note 1).............................................................±5.5A
SHDNto GND.............................................................-0.3V to 6V
REF, FBSEL, COMP, FB, TOFF to GND.....-0.3V to (VCC+ 0.3V)
REF Short to GND......................................................Continuous
Continuous Power Dissipation (TA= +70°C) (with part mounted
on 1 sq. inch of one ounce copper)
20-Pin SSOP (derate 22mW/°C above +70°C)................1.3W
Operating Temperature Range...........................-40°C to +85°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
SHDN= GND or VCC
FBSEL = REF
FBSEL = GND
VIN= 4.5V to 5.5V,
ILOAD= 0 to 3A
(Note 2)
FBSEL = GND, adjustable output mode, VFB= 1.2V
VCCfalling, 100mV hysteresis
ILOAD≥1.5A (Note 2)
VIN= 4.5V
SHDN= GND
VIN= 4.5V
FBSEL = GND or REF (Note 2)
IREF= 0
Does not include switching losses
IREF= -1µA to 10µA
CONDITIONS0.8SHDNInput Low Voltage-10.031SHDNInput Current%1AC Output Load Regulation0.851.001.15Off-Time Default Period0.54Off-Time Adjustment Range
kHz500Error-Amplifier Gain Bandwidth -2525FB Input Bias Current4.14.24.3Undervoltage Lockout Threshold145Thermal Shutdown Threshold0.510Shutdown Supply Current400525No-Load Supply Current11.251.5Idle Mode Threshold (Note 3)
2.492.5252.550V
3.2963.3303.3664.55.5Input Voltage Range
Output Voltage
kHz350Maximum Switching Frequency60100NMOS Switch On-Resistance55100PMOS Switch On-Resistance3.654.65Current-Limit Threshold
1.0891.1001.110VREF3.80Output Adjustment Range1.0891.1001.110Reference Output Voltage1Reference Load Regulation
UNITSMINTYPMAXPARAMETERFBSEL = unconnected
FBSEL = VCC
FBSEL = GND or REF2SHDNInput High Voltage
VIN= 5.5V, VLX= 5.5V or 0µA±20LX Leakage Current3.65RMS LX Output Current
Note 1:LX has internal clamp diodes to PGND and IN. Applications that forward bias these diodes should take care not to exceed
the IC’s package power dissipation limits.
MAX1623
3A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
ELECTRICAL CHARACTERISTICS(VIN= VCC= 5V, FBSEL unconnected, RTOFF= 110kΩ, TA
= -40°C to +85°C, unless otherwise noted.) (Note 4)
Note 2:Guaranteed by design, not production tested.
Note 3:Idle Mode threshold is defined as the transition point in the load-current range between Idle Mode and constant-off-time
operation.
Note 4:Specifications to -40°C are guaranteed by design, not production tested.
SHDN= GND or VCC
VIN= 4.5V to 5.5V,
ILOAD= 0 to 3A
FBSEL = GND, adjustable output mode, VFB= 1.2V
VCCfalling, 100mV hysteresis
VIN= 4.5V
VIN= 5.5V, VLX= 5.5V or 0
SHDN= GND
VIN= 4.5V
FBSEL = GND or REF (Note 2)
IREF= 0
Does not include switching losses
CONDITIONS0.8
FBSEL = unconnected
SHDNInput Low Voltage-11
FBSEL = VCC
SHDNInput Current
FBSEL = GNDor REF
2.2SHDNInput High Voltage0.851.25Off-Time Default Period0.554Off-Time Adjustment Range-5050FB Input Bias Current4.04.3Undervoltage Lockout Threshold-2020LX Leakage Current10Shutdown Supply Current600No-Load Supply Current
2.4502.550V
3.2343.3664.55.5Input Voltage Range
Output Voltage0.1NMOS Switch On-Resistance0.1PMOS Switch On-Resistance3.54.75Current-Limit Threshold
1.0751.110VREF3.8Output Adjustment Range1.0751.110Reference Output Voltage
UNITSMINTYPMAXPARAMETER
MAX1623
3A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
__________________________________________Typical Operating Characteristics(Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
EFFICIENCY
vs. OUTPUT CURRENT
MAX1623 TOC01
OUTPUT CURRENT (A)
EFFICIENCY (%)40VOUT = 3.3V, RTOFF = 110kΩ
VOUT = 1.1V, RTOFF = 280kΩ
VOUT = 2.5V, RTOFF = 180kΩ200100300400500600
SWITCH OFF-TIME
vs. OFF-TIME RESISTANCEMAX1623 TOC02
RTOFF (kΩ)
tOFF
SUPPLY CURRENT
vs. INPUT VOLTAGE
MAX1623 TOC03
INPUT VOLTAGE (V)
SUPPLY CURRENT (
SHDN = IN
SHDN = GND
SWITCHING FREQUENCY
vs. LOAD CURRENT
MAX1623 TOC04
LOAD CURRENT (mA)
SWITCHING FREQUENCY (kHz)
VOUT = 2.5V, RTOFF = 180kΩ
VOUT = 3.3V, RTOFF = 110kΩ
VOUT = 1.1V, RTOFF = 280kΩ
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
MAX1623 TOC05
INPUT VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
VIN = 5V, VOUT = 3.3V,
RTOFF = 110kΩ
VIN = 5V, VOUT = 2.5V,
RTOFF = 180kΩ
LOAD REGULATION ERROR
vs. LOAD CURRENT
MAX1623 TOC07
LOAD CURRENT (A)
LOAD REGULATION ERROR (%)
VOUT = 2.5V, RTOFF = 180kΩ
VOUT = 3.3V, RTOFF = 110kΩ
REFERENCE LOAD REGULATION ERROR
vs. REFERENCE LOAD CURRENT
MAX1623 TOC06
REFERENCE LOAD CURRENT (μA)
REFERENCE LOAD REGULATION ERROR (%)
TA = +25°C
TA = -40°C
TA = +85°C
MAX1623
3A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches1ms/div
START-UP AND SHUTDOWN TRANSIENTMAX1623 TOC08
VIN = 5V, VOUT = 3.3V, ILOAD = 3A,
WAVEFORM AVERAGED
A: VOUT, 2V/div
B: IIN, 1A/div
C: VSHDN, 5V/div
LOAD-TRANSIENT RESPONSE
(FBSEL = REF)MAX1623 TOC10
ILOAD
0 to 3A
VOUT
50mV
AC-COUPLED
f = 300kHz
20μs/div
LINE-TRANSIENT RESPONSEMAX1623 TOC12
VIN = 4.5V
to 5.5V
AC-COUPLED
(1V/div)
VOUT = 3.3V
AC-COUPLED
IOUT = 1.5A
(20mV/div)
20μs/div
LOAD-TRANSIENT RESPONSE
(FBSEL = REF)MAX1623 TOC11
ILOAD
0 to 2A
VOUT
50mV
AC-COUPLED
f = 300kHz
20μs/div
____________________________Typical Operating Characteristics(continued)(Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
LINE-TRANSIENT RESPONSEMAX1623 TOC13VIN = 4.5V
to 5.5V
AC-COUPLED
(1V/div)
VOUT = 3.3V
AC-COUPLED
IOUT = 100mA
(20mV/div)
20μs/div
1ms/div
START-UP AND SHUTDOWN TRANSIENTMAX1623 TOC09
VIN = 5V, VOUT = 3.3V, ILOAD = 2A,
WAVEFORM AVERAGED
A: VOUT, 2V/div
B: IIN, 1A/div
C: VSHDN, 5V/div
MAX1623
3A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
General DescriptionThe MAX1623 current-mode, PWM, DC-DC regulator is
designed for 5V-input step-down applications. It fea-
tures a 55mΩ(typ) PMOS switch and a 60mΩ(typ)
NMOS synchronous-rectifier switch. Simple constant-off-
time control allows switching frequencies up to 350kHz.
Adjust the off-time with an external resistor RTOFF to
optimize performance trade-offs among efficiency, com-
ponent size, output switching noise, and cost. Idle Mode
operation enhances light-load efficiency by switching to
a pulse-skipping mode that reduces transition and gate-
charge losses. The power-switching circuit consists of
the IC and an LC output filter. The output voltage is the
average of the AC voltage at the switching node (LX).
The MAX1623 regulates the output voltage by changing
the PMOS switch on-time relative to the constant off-
time, thereby adjusting the duty cycle.
The MAX1623 contains six major circuit blocks (Figure 1):
a PWM comparator, a current-sense circuit, a PWM
logic block, an internal feedback mux, an off-time con-
trol block, and a 1.1V precision reference. The input
supply directly powers the internal blocks.
Modes of OperationThe load current determines the mode of operation:
Idle Mode (load currents less than 0.625A) or PWM
mode for inductor currents of 1.25A (which corre-
sponds to load currents greater than 0.625A). The
PWM current limit is continuously adjusted by the PWM
comparator and can vary from 0A to the maximum cur-
rent limit (4A). If the inductor current falls below the Idle
Mode threshold (1.25A), skip mode takes over.
Whenever the P-channel switch turns on, it stays on
until the sensed current reaches the active current limit.
The PWM current limit automatically adjusts with the
PMOS switch duty cycle required to generate the
desired output voltage. When the active current limit is
met, the PMOS switch turns off for the programmed
minimum off-time, and the N-channel synchronous rec-
tifier turns on. The synchronous rectifier stays on until
the P-channel switch turns back on or until the inductor
current reaches zero. At the end of the off-time, the P-
channel switch turns on again if the output voltage indi-
cates that energy is required at the output.
Pin Description
NAMEFUNCTION1, 3, 5,
16,18, 20LXConnection to the internal power switches.
2, 4, 6INPower Input. Internally connected to the PMOS switch source. Connect to 5V.
PINSHDNActive-Low Shutdown Input. Connect to VCCfor normal operation.FBSELFeedback Select Input. See Table 1.REFReference Output. Bypass with a minimum 0.1µF capacitor to GND. See the Internal Reference section.GNDAnalog Ground FBFeedback input for both fixed-output and adjustable operating modes. Connect to the output directly
for fixed-voltage operation or to a resistor-divider for adjustable operating modes.TOFF
Off-Time Select Input. Connect a resistor from TOFF to GND to adjust the switch off-time, and there-
fore the frequency: tOFF= . See the Typical Operating Characteristics.
15, 17, 19PGNDPower Ground. Internally connected to the NMOS synchronous rectifier source.VCCAnalog Supply-Voltage Input. Supplies internal analog circuitry. Connect to 5V. Bypass VCCwith 10Ω
and 4.7µF (Figure 2).COMPIntegrator Capacitor Connection. Connect a 470pF (470pF to 2000pF range) capacitor to GND to set
the typical integration time-constant. See the Integrator Amplifier section.
110ks)TOFF (μ
