MAX1820 データシートの表示（PDF） - Maxim Integrated

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MAX1820

WCDMA Cellular Phone 600mA Buck Regulators

Maxim Integrated 

WCDMA Cellular Phone 600mA

Buck Regulators

MAX1820

UCSP

PIN

MAX1820 MAX1821

µMAX

UCSP

MAX1821

µMAX

—

—

A3

3

A4

4

—

—

—

—

A4

4

B4

5

B4

5

C4

6

C4

6

C3

7

C3

7

C2

8

C2

8

C1

9

C1

9

B1

10

B1

10

NAME

FB

REF

REF

GND

PGND

LX

BATT

SHDN

SYNC

Pin Description (continued)

FUNCTION

Output Feedback Sense Input. To set the output voltage,

connect FB to the center of an external resistive divider between

the output and GND. FB voltage regulates to 1.25V.

External Reference Input. Connect REF to the output of a D/A

converter for dynamic adjustment of the output voltage. REF-to-

OUT gain is 1.76.

Internal Reference Bypass. Connect a 0.047µF capacitor from

REF to GND.

Ground

Power Ground

Inductor Connection. LX connects to the drains of the internal

power MOSFETs. LX is high impedance in shutdown mode.

Supply Voltage Input. Connect BATT to a 2.6V to 5.5V source.

Bypass BATT to PGND with a low-ESR 10µF capacitor.

Active-Low, Shutdown Control Input

Clock Synchronization Input. Drive SYNC with a 13MHz

(MAX1820X, MAX1821X) or 19.8MHz (MAX1820Y) AC-coupled

sine-wave input to synchronize power switching at 1MHz.

MAX1820Z and MAX1821 do not have SYNC capability.

Connect SYNC to GND to use the internally generated,

free-running 1MHz clock. MAX1820Z and MAX1821 SYNC pin

must be connected to GND.

_______________Detailed Description

The MAX1820/MAX1821 PWM step-down DC-DC con-

verters are optimized for low-voltage, battery-powered

applications where high efficiency and small size are

priorities. The MAX1821 is a general-purpose device

that uses external feedback resistors to set the output

voltage from 1.25V to VBATT, and the MAX1820 is

specifically intended to power a linear PA in WCDMA

handsets. An analog control signal dynamically adjusts

the MAX1820’s output voltage from 0.4V to 3.4V with a

settling time <30µs.

The MAX1820/MAX1821 operate at a high 1MHz

switching frequency that reduces external component

size. Each device includes an internal synchronous rec-

tifier that provides for high efficiency and eliminates the

need for an external Schottky diode. The normal operat-

ing mode uses constant-frequency PWM switching at

medium and heavy loads, and automatically pulse

skips at light loads to reduce supply current and extend

battery life. An additional forced PWM mode (with

optional external synchronization) switches at a con-

stant frequency, regardless of load, to provide a well-

controlled spectrum in noise-sensitive applications.

Battery life is maximized by low-dropout operation at

100% duty-cycle and a 0.1µA (typ) logic-controlled

shutdown mode.

PWM Control

The MAX1820/MAX1821 use a slope-compensated,

current-mode PWM controller capable of achieving

100% duty cycle. The current-mode control design is

capable of minimum duty cycles of less than 10%,

ensuring a constant switching frequency with outputs

as low as 0.4V when powered from a single lithium-ion

(Li+) cell. Current-mode feedback provides stable

switching and cycle-by-cycle current limiting for superi-

or load and line response and protection of the internal

MOSFET and synchronous rectifier. The output voltage

is regulated by switching at a constant frequency and

then modulating the power transferred to the load dur-

_______________________________________________________________________________________ 9

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