Low-Noise, 14V Input, 1A, PWM

Step-Down Converters

8

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_______________Detailed Description

The MAX1684/MAX1685 step-down, pulse-width-modu-

lation (PWM) DC-DC converters provide an adjustable

output from 1.25V to the input voltage. They accept

inputs from 2.7V to 14V and deliver up to 1.6A. An inter-

nal MOSFET and synchronous rectifier reduce PC

board area while maintaining high efficiency. Operation

with up to 100% duty cycle minimizes dropout voltage.

Fixed-frequency PWM operation reduces interference

in sensitive communications and data-acquisition appli-

cations. A SYNC input allows synchronization to an

external clock. The MAX1684/MAX1685 can operate in

five modes. Setting the devices to operate in the appro-

priate mode for the intended application (Table 1)

achieves highest efficiency.

PWM Control

The MAX1684/MAX1685 use an oscillator-triggered min-

imum/maximum on-time current-mode control scheme

(Figure 2). The minimum on-time is typically 220ns

unless the regulator is in dropout. The maximum on-time

Current-mode feedback provides cycle-by-cycle current

limiting for superior load- and line-transient response.

At each falling edge of the internal oscillator, the inter-

nal P-channel MOSFET (main switch) turns on. This

allows current to ramp up through the inductor to the

load and stores energy in a magnetic field. The switch

remains on until either the current-limit comparator

trips, the maximum on-time expires, or the PWM com-

parator signals that the output is in regulation. When

the switch turns off during the second half of each

cycle, the inductor’s magnetic field collapses, releasing

the stored energy and forcing current through the out-

put diode to the output filter capacitor and load. The

output filter capacitor stores charge when the inductor

current is high and releases it when the inductor cur-

rent is low, smoothing the voltage across the load.

During normal operation, the MAX1684/MAX1685 regu-

late the output voltage by switching at a constant fre-

quency and modulating the power transferred to the

load on each cycle using the PWM comparator. A multi-

input comparator sums three weighted differential sig-

nals (the output voltage with respect to the reference,

the main switch current sense, and the slope-compen-

sation ramp) and changes states when a threshold is

reached. It modulates output power by adjusting the

MAX1685

LX

13, 14

L

10

µH*

MBRS

130LT3

16

5

12

7

IN

CVH

AIN

PGND

AGND

OUTPUT

3.3V AT 1A

*SUMIDA

CD54-100;

USE 22

µH FOR MAX1684

INPUT

14V MAX

BOOT

FB

SHDN

CVL

4

15

2

3

1

11

1

µF

0.1

µF

0.1

µF

22

µF

100

µF

0.1

µF

0.1

µF

0.01

µF

ON/OFF

9

STBY

SYNC/PWM

ILIM/SS

CC

6

8

10

REF

(OPTIONAL)

Figure 1. Standard Application Circuit

Table 1. Operating Modes

Fixed-Frequency PWM

H

PWM

MODE

FUNCTION

SYNC/PWM

H

STBY

H

SHDN

1.6A

TYPICAL

OUTPUT

CAPABILITY

H

H

1.6A

Fixed-Input Clock-Frequency PWM

Clocked

Sync PWM

H

H

1.6A

PFM at light loads (<150mA). Fixed-

frequency PWM at heavy loads (>150mA).

L

Normal

L

H

160mA

Low-Power or Standby Mode.

X

Low Power

X

L

0

Circuit Disabled

X

Shutdown