MAX1921EUT15+T ,Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23FeaturesThe MAX1920/MAX1921 step-down converters deliver♦ 400mA Guaranteed Output Currentover 400mA ..
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MAX488CSA+T ,Low-Power, Slew-Rate-Limited RS-485/RS-422 TransceiversFeaturesThe MAX481, MAX483, MAX485, MAX487–MAX491, and♦ For Fault-Tolerant
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MAX1920EUT-MAX1920EUT+T-MAX1921EUT15+-MAX1921EUT15+T-MAX1921EUT18+T-MAX1921EUT25+T-MAX1921EUT33
Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23
General DescriptionThe MAX1920/MAX1921 step-down converters deliver
over 400mA to outputs as low as 1.25V. These convert-
ers use a unique proprietary current-limited control
scheme that achieves over 90% efficiency. These
devices maintain extremely low quiescent supply current
(50µA), and their high 1.2MHz (max) operating frequency
permits small, low-cost external components. This combi-
nation makes the MAX1920/MAX1921 excellent high-
efficiency alternatives to linear regulators in space-
constrained applications.
Internal synchronous rectification greatly improves effi-
ciency and eliminates the external Schottky diode
required in conventional step-down converters. Both
devices also include internal digital soft-start to limit
input current upon startup and reduce input capacitor
requirements.
The MAX1920 provides an adjustable output voltage
(1.25V to 4V). The MAX1921 provides factory-preset
output voltages (see the Selector Guide). Both are
available in space-saving 6-pin SOT23 packages. The
MAX1920 is also available in a 6-pin TDFN package.
ApplicationsNext-Generation Wireless Handsets
PDAs, Palmtops, and Handy-Terminals
Battery-Powered Equipment
CDMA Power Amplifier Supply
Features400mA Guaranteed Output CurrentInternal Synchronous Rectifier for >90% EfficiencyTiny 6-Pin SOT23 PackageAvailable in 6-Pin TDFN Package (MAX1920)Up to 1.2MHz Switching Frequency for Small
External Components50µA Quiescent Supply Current0.1µA Logic-Controlled Shutdown2V to 5.5V Input RangeFixed 1.5V, 1.8V, 2.5V, 3V, and 3.3V Output
Voltages (MAX1921)Adjustable Output Voltage (MAX1920)±1.5% Initial AccuracySoft-Start Limits Startup Current
Note:The MAX1921 offers five preset output voltage options.
See the Selector Guide, and then insert the proper designator
into the blanks above to complete the part number.
+Denotes lead-free package.
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT2319-2296; Rev 3; 8/05
MAX1921
AGND
SHDN
PGND
OUT
OFF
4.75kΩ4.7μF
4.7μH
5600pF
OUTPUT
1.5V UP TO 400mA
INPUT
2V TO 5.5V
CIN
ypical Operating Circuit
PARTTEMP RANGEPIN-PACKAGEMAX1920EUT-T-40°C to +85°C6 SOT23-6
MAX1920EUT+T-40°C to +85°C6 SOT23-6
MAX1920ETT-T-40°C to +85°C6 TDFN
MAX1920ETT+T-40°C to +85°C6 TDFN
MAX1921EUT_ _-T-40°C to +85°C6 SOT23-6
MAX1921EUT_ _+T-40°C to +85°C6 SOT23-6
Ordering InformationAGND
AGND
OUT (FB)SHDNLXFBPGND
PGND
MAX1920
MAX1921MAX1920
SOT23-6TOP VIEW
( ) ARE FOR MAX1920 ONLY
A "+" sign will replace the first pin indicator on lead-free packages.
SHDN34
TDFN
Pin Configuration
MAX1920/MAX1921IN, FB, SHDNto AGND . . . . . . . . . . . . . . . . . . . . .-0.3V to +6V
OUT to AGND, LX to PGND . . . . . . . . . . . .-0.3V to (IN + 0.3V)
AGND to PGND . . . . . . . . . . . . . . . . . . . . . . . . . .-0.3V to +0.3V
OUT Short Circuit to GND . . . . . . . . . . . . . . . . . . . . . . . . . . .10s
Continuous Power Dissipation (TA = +70°C)
6-Pin SOT23-6 (derate 8.7mW/°C above +70°C) . . . .695mW
6-Pin TDFN (derate 18.2mW/°C above +70°C) . . .1454.5mW
Operating Temperature Range . . . . . . . . . . . . . .-40°C to +85°C
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . .+150°C
Storage Temperature . . . . . . . . . . . . . . . . . . . .-65°C to +150°C
Lead Temperature (soldering 10s) . . . . . . . . . . . . . . . . .+300°C
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
ABSOLUTE MAXIMUM RATINGSStresses 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.
ELECTRICAL CHARACTERISTICS(VIN= 3.6V, SHDN= IN, TA= 0°C to +85°C. Typical parameters are at TA= +25°C, unless otherwise noted.) (Note 1)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSI(LX) < 400mA2.55.5
Input Voltage RangeVINI(LX) < 200mA
(MAX1921EUT15, MAX1921EUT18)2.02.5V
Startup Voltage2.0V
VIN rising1.851.95UVLO ThresholdUVLOVIN falling1.501.65V
UVLO Hysteresis200mV
Quiescent Supply CurrentIINNo switching, no load5070µA
Quiescent Supply Current
DropoutIINSHDN = IN, OUT/FB = 0220300µA
Shutdown Supply CurrentISHDNSHDN = GND0.14.0µA
IOUT = 0, TA = +25°C-1.5+1.5
IOUT = 0 to 400mA, TA = -40°C to +85°C-3+3Output Voltage Accuracy
(MAX1921)
IOUT = 0 to 200mA, TA = -40°C to +85°C-3+3
SHDN = 01OUT BIAS CurrentIOUT
OUT at regulation voltage816µA
Output Voltage Range (MAX1920)Figure 4, IN = 4.5V1.254.00V
TA= 25°C1.2311.251.269
1.2201.251.280FB Feedback Threshold
(MAX1920)VFB
TA = -40°C to +85°C1.2101.280
FB Feedback Hysteresis
(MAX1920)VHYS5mV
FB Bias Current (MAX1920)IFBFB = 1.5V0.010.20µA
Load RegulationIOUT = 0 to 400mA0.005%/mA
Line RegulationVIN = 2.5V to 5.5V0.2%/V
SHDN Input Voltage HighVIH1.6V
SHDN Input Voltage LowVIL0.4V
SHDN Leakage CurrentISHDNSHDN = GND or IN0.0011.000µA
High-Side Current LimitILIMP525730950mA
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23EFFICIENCY vs. LOAD CURRENT
(VOUT = 3.3V)
MAX1920 toc01
LOAD CURRENT (mA)
EFFICIENCY (%)40
VIN = 3.6V
VIN = 4.2V
VIN = 5V
EFFICIENCY vs. LOAD CURRENT
(VOUT = 2.5V)
MAX1920 toc02
LOAD CURRENT (mA)
EFFICIENCY (%)40
VIN = 2.7V
VIN = 3.3VVIN = 5V
EFFICIENCY vs. LOAD CURRENT
(VOUT = 1.5V)
MAX1920 toc03
LOAD CURRENT (mA)
EFFICIENCY (%)40VIN = 2.5V
VIN = 3.3V
VIN = 5V
OUTPUT VOLTAGE ACCURACY vs. LOAD
(VOUT = 3.3V)
MAX1920 toc04
LOAD (mA)
OUTPUT VOLTAGE
VIN = 3.6V
VIN = 5V
VIN = 4.2V
OUTPUT VOLTAGE ACCURACY vs. LOAD
(VOUT = 2.5V)
MAX1920 toc05
LOAD (mA)
OUTPUT VOLTAGE
VIN = 3V
VIN = 5V
OUTPUT VOLTAGE ACCURACY vs. LOAD
(VOUT = 1.5V)
MAX1920 toc06
LOAD (mA)
OUTPUT VOLTAGE
VIN = 5V
VIN = 3.3V
VIN = 2.5V
Typical Operating Characteristics(CIN = 2.2µF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS (continued)(VIN= 3.6V, SHDN= IN, TA= 0°C to +85°C. Typical parameters are at TA= +25°C, unless otherwise noted.) (Note 1)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSLow-Side Current LimitILIMN350550800mA
High-Side On-ResistanceRONHSILX = -40mA, VIN = 3V0.61.1Ω
Rectifier On-ResistanceRONSRILX = 40mA, VIN = 3V0.50.9Ω
Rectifier Off-Current ThresholdILXOFF60mA
LX Leakage CurrentILXLEAKIN = SHDN = 5.5V, LX = 0 to IN0.15.0µA
LX Reverse Leakage CurrentILXLKRIN unconnected, VLX = 5.5V, SHDN = GND0.15.0µA
Minimum On-TimetON(MIN)0.280.40.5µs
Minimum Off-TimetOFF(MIN)0.280.40.5µs
Note 1:All devices are 100% production tested at TA= +25°C. Limits over the operating temperature range are guaranteed
by design.
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Typical Operating Characteristics (continued)(CIN = 2.2µF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
10,000
SWITCHING FREQUENCY vs. LOAD
(VOUT = 1.8V)
MAX1920 toc07
LOAD (mA)
SWITCHING FREQUENCY (kHz)
VIN = 3.3
10,000
SWITCHING FREQUENCY vs. LOAD
(VOUT = 1.5V)
MAX1920 toc08
LOAD (mA)
SWITCHING FREQUENCY (kHz)
VIN = 3.3
NO LOAD SUPPLY CURRENT
vs. SUPPLY VOLTAGEMAX1920 toc09
SUPPLY VOLTAGE (V)
NO-LOAD SUPPLY CURRENT (
10,000
VOUT = 3.3V
VOUT = 2.5V
VOUT = 1.5V
LIGHT-LOAD SWITCHING WAVEFORMMAX1920 toc10
VOUT
AC-COUPLED
5mV/div
VLX
2V/div
1μs/div
VIN = 3.3V, VOUT = 1.5V,
ILOAD = 40mA
MEDIUM-LOAD SWITCHING WAVEFORMMAX1920 toc11
VOUT
AC-COUPLED
5mV/div
VLX
2V/div
1μs/div
VIN = 3.3V, VOUT = 1.5V,
ILOAD = 250mA
SOFT-START AND SHUTDOWN RESPONSEMAX1920 toc12
VOUT
1V/div
IIN
100mA/div
VSHDN
5V/div
200μs/div
VIN = 3.3V, VOUT = 1.5V,
RLOAD = 6Ω
MEDIUM-LOAD
LINE-TRANSIENT RESPONSEMAX1920 toc13
VIN
AC-COUPLED
200mV/div
VOUT
AC-COUPLED
5mV/div
4μs/div
VIN = 3.8V to 4.2V,
VOUT = 1.5V, ILOAD = 250mA
LIGHT-LOAD
LINE-TRANSIENT RESPONSEMAX1920 toc14
VIN
AC-COUPLED
200mV/div
VOUT
AC-COUPLED
5mV/div
4μs/div
VIN = 3.8V to 4.2V,
VOUT = 1.5V, ILOAD = 20mA
LOAD-TRANSIENT RESPONSEMAX1920 toc15
VOUT
AC-COUPLED
100mV/div
ILOAD
200mA/div
200mA/div
40μs/div
VIN = 3.3V, VOUT = 1.5V,
ILOAD = 20mA TO 320mA
Detailed DescriptionThe MAX1920/MAX1921 step-down DC-DC converters
deliver over 400mA to outputs as low as 1.25V. They use
a unique proprietary current-limited control scheme that
maintains extremely low quiescent supply current (50µA),
and their high 1.2MHz (max) operating frequency permits
small, low-cost external components.
Control SchemeThe MAX1920/MAX1921 use a proprietary, current-limited
control scheme to ensure high-efficiency, fast transient
response, and physically small external components. This
control scheme is simple: when the output voltage is out
of regulation, the error comparator begins a switching
cycle by turning on the high-side switch. This switch
remains on until the minimum on-time of 400ns expires
and the output voltage regulates or the current-limit
threshold is exceeded. Once off, the high-side switch
remains off until the minimum off-time of 400ns expires
and the output voltage falls out of regulation. During this
period, the low-side synchronous rectifier turns on and
remains on until either the high-side switch turns on again
or the inductor current approaches zero. The internal syn-
chronous rectifier eliminates the need for an external
Schottky diode.
This control scheme allows the MAX1920/MAX1921 to
provide excellent performance throughout the entire
load-current range. When delivering light loads, the
high-side switch turns off after the minimum on-time to
reduce peak inductor current, resulting in increased effi-
ciency and reduced output voltage ripple. When deliver-
ing medium and higher output currents, the
MAX1920/MAX1921 extend either the on-time or the off-
time, as necessary to maintain regulation, resulting in
nearly constant frequency operation with high-efficiency
and low-output voltage ripple.
Shutdown ModeConnecting SHDNto GND places the MAX1920/
MAX1921 in shutdown mode and reduces supply cur-
rent to 0.1µA. In shutdown, the control circuitry, internal
switching MOSFET, and synchronous rectifier turn off
and LX becomes high impedance. Connect SHDNto
IN for normal operation.
Soft-StartThe MAX1920/MAX1921 have internal soft-start circuitry
that limits current draw at startup, reducing transients
on the input source. Soft-start is particularly useful for
higher impedance input sources, such as Li+ and alka-
line cells. Soft-start is implemented by starting with the
current limit at 25% of its full current value and gradual-
ly increasing it in 25% steps until the full current limit is
reached. See Soft-Start and Shutdown Response in the
Typical Operating Characteristics.
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Pin Description
PIND F N * SONAMEFUNCTION1IN
Supply voltage input for MAX1921EUT15 and MAX1921EUT18 is 2V to 5.5V. Supply voltage input for
MAX1920 and other voltage versions of MAX1921 is 2.5V to 5.5V. Bypass IN to GND with a 2.2µF
ceramic capacitor as close to IN as possible.AGNDAnalog Ground. Connect to PGND.SHDNActive-Low Shutdown Input. Connect SHDN to IN for normal operation. In shutdown, LX becomes
high-impedance and quiescent current drops to 0.1µA.OUTMAX1921 Voltage Sense Input. OUT is connected to an internal voltage-divider.FBMAX1920 Voltage Feedback Input. FB regulates to 1.25V nominal. Connect FB to an external
resistive voltage-divider between the output voltage and GND.PGNDPower Ground. Connect to AGND.LXInductor Connection
MAX1921
AGND
SHDN
PGND
OUT
OFFCOUT
CFF
OUTPUT
UP TO 400mA
INPUT
2V TO 5.5V
CIN4
Figure 1. Typical Output Application Circuit (MAX1921)
*MAX1920 only.
