MAX1920EUT-T ,Low-Voltage / 400mA Step-Down DC-DC Converters in SOT23ELECTRICAL CHARACTERISTICS(V = 3.6V, SHDN = IN, T = 0°C to +85°C. Typical parameters are at T = +25 ..
MAX1921EUT15+ ,Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23applications.♦ 0.1µA Logic-Controlled ShutdownInternal synchronous rectification greatly improves e ..
MAX1921EUT15+T ,Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23FeaturesThe MAX1920/MAX1921 step-down converters deliver♦ 400mA Guaranteed Output Currentover 400mA ..
MAX1921EUT18+T ,Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23MAX1920/MAX192119-2296; Rev 3; 8/05Low-Voltage, 400mA Step-DownDC-DC Converters in SOT23
MAX1921EUT25+T ,Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23FeaturesThe MAX1920/MAX1921 step-down converters deliver♦ 400mA Guaranteed Output Currentover 400mA ..
MAX1921EUT25+T ,Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23ELECTRICAL CHARACTERISTICS(V = 3.6V, SHDN = IN, T = 0°C to +85°C. Typical parameters are at T = +25 ..
MAX488CPA ,Low-Power / Slew-Rate-Limited RS-485/RS-422 TransceiversMAX481/MAX483/MAX485/MAX487–MAX491/MAX148719-0122; Rev 5; 2/96Low-Power, Slew-Rate-LimitedRS-485/RS ..
MAX488CPA+ ,Low-Power, Slew-Rate-Limited RS-485/RS-422 TransceiversFeaturesThe MAX481, MAX483, MAX485, MAX487–MAX491, and♦ For Fault-Tolerant
MAX488CSA ,Low-Power / Slew-Rate-Limited RS-485/RS-422 TransceiversGeneral Description ________
MAX488CSA+ ,Low-Power, Slew-Rate-Limited RS-485/RS-422 TransceiversApplicationsIndustrial-Control Local Area Networks Ordering Information appears at end of data shee ..
MAX488CSA+T ,Low-Power, Slew-Rate-Limited RS-485/RS-422 TransceiversFeaturesThe MAX481, MAX483, MAX485, MAX487–MAX491, and♦ For Fault-Tolerant
MAX488CUA ,Low-Power / Slew-Rate-Limited RS-485/RS-422 TransceiversMAX481/MAX483/MAX485/MAX487–MAX491/MAX148719-0122; Rev 5; 2/96Low-Power, Slew-Rate-LimitedRS-485/RS ..
MAX1920EUT-T
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.
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 PackageUp 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.
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT2319-2296; Rev 1; 7/02
Typical Operating Circuit
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
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
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Typical Operating Characteristics(CIN = 2.2µF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS (continued)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.)
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 SOT23Figure 1. Typical Output Application Circuit (MAX1921)
MAX1920/MAX1921
Design ProcedureThe MAX1920/MAX1921 are optimized for small external
components and fast transient response. There are
several application circuits (Figures 1 through 4) to
allow the choice between ceramic or tantalum output
capacitor and internally or externally set output volt-
ages. The use of a small ceramic output capacitor is
preferred for higher reliability, improved voltage-posi-
tioning transient response, reduced output ripple, and
the smaller size and greater availability of ceramic versus
tantalum capacitors.
Voltage PositioningFigures 1 and 2 are the application circuits that utilize
small ceramic output capacitors. For stability, the circuit
obtains feedback from the LX node through R1, while
load transients are fed-forward through CFF. Because
there is no D.C. feedback from the output, the output volt-
age exhibits load regulation that is equal to the output
load current multiplied by the inductor’s series resistance.
This small amount of load regulation is similar to voltage
positioning as used by high-powered microprocessor
supplies intended for personal computers. For the
MAX1920/MAX1921, voltage positioning eliminates or
greatly reduces undershoot and overshoot during load
transients (see the Typical Operating Characteristics),
which effectively halves the peak-to-peak output voltage
excursions compared to traditional step-down converters.
For convenience, Table 1 lists the recommended external
component values for use with the MAX1921 application
circuit of Figure 1 with various input and output voltages.
Inductor SelectionIn order to calculate the smallest inductor, several cal-
culations are needed. First, calculate the maximum
duty cycle of the application as:
Second, calculate the critical voltage across the inductor as:
if DutyCycle(MAX) < 50%,
then VCRITICAL= (VIN(MIN) - VOUT),
else VCRITICAL= VOUT
Last, calculate the minimum inductor value as:
Select the next standard value larger than L(MIN). The
L(MIN) calculation already includes a margin for induc-
tance tolerance. Although values much larger than
L(MIN) work, transient performance, efficiency, and
inductor size suffer.
A 550mA rated inductor is enough to prevent saturation
for output currents up to 400mA. Saturation occurs
when the inductor’s magnetic flux density reaches the
maximum level the core can support and inductance
falls. Choose a low DC-resistance inductor to improve
efficiency. Tables 2 and 3 list some suggested inductors
and suppliers.
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
