MAX1921EUT18T Просмотр технического описания (PDF) - Maxim Integrated

Номер в каталоге

Компоненты Описание

производитель

MAX1921EUT18T

Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23

Maxim Integrated 

MAX1920/MAX1921

Low-Voltage, 400mA Step-Down

DC-DC Converters in SOT23

Pin Description

PIN

SOT

TDFN*

NAME

1

2

IN

2

6

3

1

4

—

4

5

5

3

6

4

*MAX1920 only.

AGND

SHDN

OUT

FB

PGND

LX

FUNCTION

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 as possible to IN.

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

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

INPUT

2V TO 5.5V

CIN

1

IN

6

LX

MAX1921

2 AGND

PGND 5

L

R1

CFF

OUTPUT

UP TO 400mA

COUT

ON

OFF

3 SHDN

4

OUT

Figure 1. Typical Output Application Circuit (MAX1921)

either the high-side switch turns on again or the inductor

current approaches zero. The internal synchronous recti-

fier 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 efficiency

and reduced output voltage ripple. When delivering medi-

um 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 alkaline cells.

Soft-start is implemented by starting with the current limit

at 25% of its full current value and gradually increas-

ing it in 25% steps until the full current limit is reached.

See Soft-Start and Shutdown Response in the Typical

Operating Characteristics.

www.maximintegrated.com

Maxim Integrated │  5

Share Link: