MAX1920(2002) Ver la hoja de datos (PDF) - Maxim Integrated
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MAX1920 Datasheet PDF : 9 Pages
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Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Pin Description
PIN NAME
FUNCTION
Supply voltage input for MAX1921EUT15 and MAX1921EUT18 is 2V to 5.5V. Supply voltage input for
1
IN 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.
2
AGND Analog Ground. Connect to PGND.
3
SHDN Active-Low Shutdown Input. Connect SHDN to IN for normal operation. In shutdown, LX becomes
high-impedance and quiescent current drops to 0.1µA.
OUT MAX1921 Voltage Sense Input. OUT is connected to an internal voltage-divider.
4
FB
MAX1920 Voltage Feedback Input. FB regulates to 1.25V nominal. Connect FB to an external resistive
voltage-divider between the output voltage and GND.
5
PGND Power Ground. Connect to AGND.
6
LX Inductor Connection
Detailed Description
The 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 Scheme
The 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
INPUT
2V TO 5.5V
CIN
1
IN
6
LX
MAX1921
2
5
AGND
PGND
ON
OFF
3
SHDN
4
OUT
L
R1
CFF
OUTPUT
UP TO 400mA
COUT
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 Mode
Connecting SHDN to 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 SHDN to
IN for normal operation.
Soft-Start
The 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.
Figure 1. Typical Output Application Circuit (MAX1921)
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