MAX1920 Maxim, MAX1920 Datasheet - Page 5

• Manufacturer Part Number: MAX1920
• Description: PLASTIC ENCAPSULATED DEVICES
• Manufacturer: Maxim

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.
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
Figure 1. Typical Output Application Circuit (MAX1921)
PIN
2V TO 5.5V
1
2
3
4
5
6
INPUT
C
OFF
IN
ON
NAME
AGND
SHDN
PGND
OUT
FB
LX
1
2
3
IN
IN
AGND
SHDN
MAX1921
_______________________________________________________________________________________
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.
Analog Ground. Connect to PGND.
Active-Low Shutdown Input. Connect SHDN to IN for normal operation. In shutdown, LX becomes
high-impedance and quiescent current drops to 0.1µA.
MAX1921 Voltage Sense Input. OUT is connected to an internal voltage-divider.
MAX1920 Voltage Feedback Input. FB regulates to 1.25V nominal. Connect FB to an external resistive
voltage-divider between the output voltage and GND.
Power Ground. Connect to AGND.
Inductor Connection
PGND
Detailed Description
OUT
LX
6
5
4
R1
Control Scheme
C
L
FF
Low-Voltage, 400mA Step-Down
UP TO 400mA
OUTPUT
C
DC-DC Converters in SOT23
OUT
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.
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.
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.
FUNCTION
Pin Description
Shutdown Mode
Soft-Start
5