APW7104A Просмотр технического описания (PDF) - Anpec Electronics

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APW7104A

1.5MHz, 1A Synchronous Buck Regulator

Anpec Electronics 

APW7104A

Function Description

Main Control Loop

The APW7104A 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.

Under-Voltage Lockout

An under-voltage lockout function prevents the device from

operating if the input voltage on VIN is lower than approxi-

mately 1.8V. The device automatically enters the shut-

down mode if the voltage on VIN drops below approxi-

mately 1.8V. This under-voltage lockout function is imple-

mented in order to prevent the malfunctioning of the

converter.

internally 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 APW7104A uses

a special scheme that counteracts this compensating

ramp, which allows the maximum inductor peak current

to remain unaffected throughout all duty cycles.

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.

Dropout Operation (Cont.)

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 APW7104A is used at 100% duty cycle with

low input voltage.

Soft-Start

The APW7104A has a built-in soft-start to control the out-

put voltage rise during start-up. During soft-start, an in-

ternal ramp voltage, connected to the one of the positive

inputs of the error amplifier, raises up to replace the ref-

erence voltage (0.6V typical) until the ramp voltage

reaches the reference voltage. Then, the voltage on FB

regulated at reference voltage.

Enable/Shutdown

Driving RUN to the ground places the APW7104A 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.

Slope Compensation and Inductor Peak Current

The APW7104A is a peak current mode PWM step down

converter. To prevent sub-harmonic oscillations, the

APW7104A sense the peak current and add slope com-

pensation to stable the converter. It is accomplished

Over-Temperature Protection (OTP)

The over-temperature circuit limits the junction tempera-

ture of the APW7104A. When the junction temperature

exceeds 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

(T ) during continuous thermal overload conditions, in-

J

creasing the lifetime of the device.

Short-Circuit Protection

When the output is shortened to the ground, the frequency

of the oscillator is reduced to about 210kHz, 1/7 of the

nominal frequency. This frequency foldback ensures that

the inductor current has more time to decay, thereby pre-

venting runaway. The oscillator’s frequency will progres-

sively increase to 1.5MHz when VFB or VOUT rises above

0V.

Copyright © ANPEC Electronics Corp.

8

Rev. A.2 - Nov., 2012

www.anpec.com.tw

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