PWM Controller

The heart of the MAX668/MAX669 current-mode PWM

controller is a BiCMOS multi-input comparator that

simultaneously processes the output-error signal, the

current-sense signal, and a slope-compensation ramp

(Figure 1). The main PWM comparator is direct sum-

ming, lacking a traditional error amplifier and its associ-

ated phase shift. The direct summing configuration

approaches ideal cycle-by-cycle control over the out-

put voltage since there is no conventional error amp in

the feedback path.

In PWM mode, the controller uses fixed-frequency, cur-

rent-mode operation where the duty ratio is set by the

in the boost configuration). The current-mode feedback

loop regulates peak inductor current as a function of

the output error signal.

At light loads the controller enters Idle Mode. During

Idle Mode, switching pulses are provided only as need-

ed to service the load, and operating current is mini-

mized to provide best light-load efficiency. The

minimum-current comparator threshold is 15mV, or 15%

troller is synchronized to an external clock, Idle Mode

occurs only at very light loads.

Bootstrapped/Non-Bootstrapped Operation

Low-Dropout Regulator (LDO)

Several IC biasing options, including bootstrapped and

non-bootstrapped operation, are made possible by an

on-chip, low-dropout 5V regulator. The regulator input is

functions, including EXT, are internally powered from

as low as 3V (as long as LDO exceeds 2.7V), but with

reduced amplitude FET drive at EXT. The maximum

LDO can supply up to 12mA to power the IC, supply

gate charge through EXT to the external FET, and sup-

ply small external loads. When driving particularly large

FETs at high switching rates, little or no LDO current

may be available for external loads. For example, when

switched at 500kHz, a large FET with 20nC gate charge

requires 20nC x 500kHz, or 10mA.

optimize efficiency, circuit quiescent current, and full-

load start-up behavior for different input and output

voltage ranges. Connections are shown in Figures 2, 3,

4, and 5. The characteristics of each are outlined in

Table 1.

1.8V to 28V Input, PWM Step-Up

Controllers in µMAX

_______________________________________________________________________________________

9

ANTISAT

MUX

LOW-VOLTAGE

START-UP

OSCILLATOR

(MAX669 ONLY)

+A

-A X6

+C

-C X1

+S

-S

X1

SLOPE COMPENSATION

SQ

BIAS

OSC

OSC

FREQ

SYNC/SHDN

0

1

LDO

PGND

1.25V

REF

EXT

UVLO

R1

552k

R2

276k

R3

276k

100mV

15mV

MAIN PWM

COMPARATOR

1.25V

FB

CURRENT SENSE

CS+

MAX668

MAX669

LDO

MAX669 ONLY

R

Figure 1. MAX668/MAX669 Functional Diagram