Dual-Output Step-Down

DC-DC Converter for PDA/Palmtop Computers

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11

Regulation Control Scheme

The MAX1775 has a unique operating scheme that

allows PWM operation at medium and high current, with

automatic switching to pulse-skipping mode at lower

currents to improve light-load efficiency. Figure 2

shows a simplified block diagram.

Under medium- and heavy-load operation, the inductor

current is continuous and the part operates in PWM

mode. In this mode, the switching frequency is set by

either the minimum on-time or the minimum off-time,

depending on the duty cycle. The duty cycle is approx-

imately the output voltage divided by the input voltage.

If the duty cycle is less than 50%, the minimum on-time

controls the frequency; and the frequency is approxi-

mately f

duty cycle is greater than 50%, the minimum off-time

sets the frequency; and the frequency is approximately

f

In both cases, the voltage is regulated by the error

comparator. For low duty cycles (<50%), the P-channel

MOSFET turns on for the minimum on-time, causing

fixed-on-time operation. During the P-channel MOSFET

on-time, the output voltage rises. Once the P-channel

MOSFET turns off, the voltage drops to the regulation

threshold, at which time another cycle is initiated. For

high duty cycles (>50%), the P-channel MOSFET

remains off for the minimum off-time, causing fixed off-

time operation. In this case, the P-channel MOSFET

remains on until the output voltage rises to the regula-

tion threshold. Then the P-channel MOSFET turns off for

the minimum off-time, initiating another cycle.

By switching between fixed on-time and fixed off-time

operation, the MAX1775 can operate at high input-out-

put ratios, yet still operate up to 100% duty cycle for

low dropout. Note that when operating in fixed on-time,

the minimum output voltage is regulated; but in fixed

off-time operation, the maximum output voltage is regu-

lated. Thus, as the input voltage drops below approxi-

mately twice the output voltage, a decrease in line

regulation can be expected. The drop in voltage is

the inductor current is discontinuous, causing the

MAX1775 to operate at lower frequencies, reducing the

MOSFET gate drive and switching losses. In discontin-

uous mode, under most circumstances, the on-time will

be a fixed minimum of 400ns.

The MAX1775 features four separate current-limit

threshold detectors and a watchdog timer for each of

its step-down converters. In addition to the more com-

mon peak current detector and zero crossing detector,

each converter also provides a valley current detector

is used to force the inductor current to drop to a lower

level after hitting peak current before allowing the P-

channel MOSFET to turn on. This is a safeguard against

inductor current significantly overshooting above the

peak current when the inductor discharges too slowly

minimum current is built up in the inductor before turn-

ing off the P-channel MOSFET. This helps the inductor

to charge the output near dropout when dI/dt is small

PSW

NON

PON

NSW

NONOVERLAP

PROTECTION

Q

S

R

S

R

Q

TONMIN

TOFFMIN

FB

CS+

CS-

PON

Figure 2. Simplified Control System Block Diagram