APW717818QBI-TRG Просмотр технического описания (PDF) - Anpec Electronics

Номер в каталоге

Компоненты Описание

производитель

APW717818QBI-TRG

1.5MHz, 1A Synchronous Buck Regulator

Anpec Electronics 

APW7178

Function Description

Main Control Loop

The APW7178 is a constant frequency, synchronous rec-

tifier and current-mode switching regulator. In normal

operation, the internal P-channel power MOSFET is

turned on each cycle. The peak inductor current at which

ICMP turn off the P-FET is controlled by the voltage on the

COMP node, which is the output of the error amplifier

(EAMP). An external resistive divider connected between

VOUT and ground allows the EAMP to receive an output

feedback voltage VFB at FB pin. When the load current

increases, it causes a slightly decrease in VFB relative to

the 0.6V reference, which in turn causes the COMP volt-

age to increase until the average inductor current matches

the new load current.

Enable/Shutdown

Driving RUN to the ground places the APW7178 in shut-

down mode. When in shutdown, the internal power

MOSFETs turn off, all internal circuitry shuts down and

the quiescent supply current reduces to 0.5µA maximum.

Pulse Frequency Modulation Mode (PFM)

The APW7178 is a fixed frequency, peak current mode

PWM step-down converter. At light loads, the APW7178

will automatically enter in pulse frequency mode opera-

tion to reduce the dominant switching losses. In PFM

operation, the inductor current may reach zero or reverse

on each pulse. A zero current comparator turn off the N-

FET, forcing DCM operation at light load. These controls

get very low quiescent current, help to maintain high effi-

ciency over the complete load range.

Dropout Operation

As the input supply voltage decreases to a value ap-

proaching the output voltage, the duty cycle increases

toward the maximum on time. Further, reduction of the

supply voltage forces the main switch to remain on for

more than one cycle until it reaches 100% duty cycle. The

input voltage minus the voltage drop will determine the

output voltage across the P-FET and the inductor.

An important detail to remember is that on resistance of

P-FET switch will increase at low input supply voltage.

Therefore, the user should calculate the power dissipa-

tion when the APW7178 is used at 100% duty cycle with

low input voltage.

Over-Temperature Protection (OTP)

The over-temperature circuit limits the junction tempera-

ture of the APW7178. When the junction temperature ex-

ceeds 150oC, a thermal sensor turns off the both power

MOSFETs, allowing the devices to cool. The thermal sen-

sor allows the converters to start a soft-start process and

regulate the output voltage again after the junction tem-

perature cools by 30oC. The OTP is designed with a 30oC

hysteresis to lower the average Junction Temperature

(TJ) during continuous thermal overload conditions, in-

creasing the lifetime of the device.

Slope Compensation and Inductor Peak Current

The APW7178 is a peak current mode PWM step down

converter. To prevent sub-harmonic oscillations, the

APW7178 sense the peak current and add slope com-

pensation to stable the converter. It is accomplished in-

ternally by adding a compensating ramp to the inductor

current signal at duty cycles in excess of 40%. Normally,

this results in a reduction of maximum inductor peak cur-

rent for duty cycles > 40%. However, the APW7178 uses a

special scheme that counteracts this compensating

ramp, which allows the maximum inductor peak current

to remain unaffected throughout all duty cycles.

Copyright © ANPEC Electronics Corp.

9

Rev. A.2 - Jan., 2011

www.anpec.com.tw

Share Link: