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

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.

Low-Voltage, 400mA Step-Down

DC-DC Converters in SOT23

_______________________________________________________________________________________

5

Pin Description

PIN

NAME

FUNCTION

1IN

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.

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

MAX1921

IN

AGND

SHDN

LX

PGND

OUT

OFF

ON

R1

L

OUTPUT

UP TO 400mA

INPUT

2V TO 5.5V

1

2

3

4

5

6

Figure 1. Typical Output Application Circuit (MAX1921)