MAX1775 查看數據表（PDF） - Maxim Integrated

零件编号

产品描述 (功能)

比赛名单

MAX1775

Dual-Output Step-Down DC-DC Converter for PDA/Palmtop Computers

Maxim Integrated 

Dual-Output Step-Down

DC-DC Converter for PDA/Palmtop Computers

VMIN

CS+

CS- VVALLEY

FB

VREF

VCLM

TOFFMIN

TONMIN

PON

PON

SQ

R

NON

SQ

VIN

PSW

VO

NSW

R

VZERO

Figure 2. Simplified Control System Block Diagram

NONOVERLAP

PROTECTION

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 ≈ 2.5MHz ✕ D, where D is the duty cycle. If the

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

sets the frequency; and the frequency is approximately

f ≈ 2.5MHz ✕ (1 - D).

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

approximately VDROP ≈ VRIPPLE. At light output loads,

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

(IVALLEY) and a minimum current detector (IMIN). IVALLEY

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

when VOUT/L is small. IMIN is useful in ensuring that a

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

(because (VIN - VOUT) / L is small) to avoid multiple

______________________________________________________________________________________ 11

Share Link: