APW7101 データシートの表示（PDF） - Anpec Electronics

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APW7101

1.5MHz, 600mA, Synchronous Buck Regulator

Anpec Electronics 

APW7101

Function Description

Main Control Loop

The APW7101 uses a constant frequency, current mode

step-down architecture. Both the main and synchro-

nous switches are internal to reduce the external

components. During normal operation, the internal

PMOSFET is turned on, but is turned off when the induc-

tor current at the input of ICOMP to reset the RS latch. The

load current increases, it causes a slight decrease in

the feedback voltage, which in turn, causes the EA’s out-

put voltage to increase until the average inductor current

matches the new load current. While the internal power

PMOSFET is off, the internal power NMOSFET is turned

on until the inductor current starts to reverse, as indicated

by the current reversal comparator IRCMP, or the beginning

of next cycle. When the NMOSFET is turned off by I , it

RCMP

operates in the discontinuous conduction mode.

Pulse Skipping Mode Operation

At light load with a relative small inductance, the in-

ductor current may reach zero. The internal power

NMOSFET is turned off by the current reversal comparator,

I , and the switching voltage will ring. This is discon-

RCMP

tinuous mode operation, and is normal behavior for the

switching regulator. At very light load, the APW7101 will

automatically skip some pulses in the pulse skipping

mode to maintain the output regulation. The skipping pro-

cess modulates smoothly depend on the load.

Short Circuit Protection

In the short circuit situation, the output voltage is al-

most zero volts. Output current is limited by the ICOMP to

prevent the damage of electrical circuit. In the normal

operation, the two straight line of the inductor current

ripple have the same height, it means the volts-sec-

onds product is the same. When the short circuit opera-

tion occurs, the output voltage down to zero leads to the

voltage across the inductor maximum in the on period and

the voltage across the inductor minimum in the off period.

In order to maintain the volts-seconds balance, the off-

time must be extended to prevent the inductor current run

away. Frequency decay will extend the switching period

to provide more times to the off-period, then the inductor

current has to restrict to protect the electrical circuit in the

short situation.

Dropout Operation

As the input supply voltage decreases to a value ap-

proaching the output voltage, the duty cycle increases to-

ward the maximum on time. Further reduction of the sup-

ply voltage forces the main switch to remain on for more

than one cycle until it reaches 100% duty cycle. The out-

put voltage will then be determined by the input volt-

age minus the voltage drop across the PMOSFET and the

inductor.

An important detail to remember is that on resistance of

PMOSFET switch will increase at low input supply voltage.

Therefore, the user should calculate the power dissipa-

tion when the APW7101 is used at 100% duty cycle with

low input voltage.

Slope Compensation

Slope compensation provides stability in constant fre-

quency current mode architecture by preventing sub-

harmonic oscillations at high duty cycle. It is accom-

plished internally by adding a compensating ramp to the

inductor current signal at duty cycle in excess of 40%.

Normally, this results in a reduction of maximum induc-

tor peak current for duty cycles greater than 40%. In the

APW7101, the reduction of inductor peak current recov-

ered by a special skill at high duty ratio. This allows the

maximum inductor peak current maintain a constant level

through all duty ratio.

Copyright © ANPEC Electronics Corp.

9

Rev. A.4 - Jun., 2008

www.anpec.com.tw

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