APW7064 Ver la hoja de datos (PDF) - Anpec Electronics

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APW7064

Synchronous Buck PWM Controller

Anpec Electronics 

APW7064

Function Description

Power-On-Reset (POR)

The Power-On-Reset (POR) function of APW7064 continually

monitors the input supply voltage (V ) and the COMP

CC

pin. The supply voltage (V ) must exceed its rising POR

CC

threshold voltage. The POR function initiates soft-start

operation after VCC and COMP voltages exceed their

POR thresholds. For operation with a single +12V power

source, VIN and VCC are equivalent and the +12V power

source must exceed the rising VCC threshold. The POR

function inhibits operation at disabled status (V is

COMP

less than 1.2V). With both input supplies above their

POR thresholds, the device initiates a soft-start interval.

Soft-Start

The APW7064 has a built-in digital soft-start to control

the output voltage rise and limit the current surge during

the start-up. In Figure 1, when V exceeds rising POR

CC

threshold voltage, it will delay 1024/Fosc seconds and

then begin soft-start. During soft-start, an internal ramp

connected to the one of the positive inputs of the Gm

amplifier rises up from 0V to 2V to replace the reference

voltage (1.2V) until the ramp voltage reaches the reference

voltage. The soft-start interval is decided by the oscillator

frequency (200kHz). The formulation is given by:

Tdelay = t2 − t1 = 1024/FOSC = 5.1ms

Tsoft−start = t3 − t2 = 1024/FOSC = 5.1ms

Figure 2. shows more detail of the FB voltage ramp. The

FB voltage soft-start ramp is formed with many small

steps of voltage. The voltage of one step is about 18.75mV

in VFB, and the period of one step is about 16/FOSC. This

method provides a controlled voltage rise and prevents

the large peak current to charge output capacitor.

Voltage (V)

Voltage (V)

VFB

18.75mV

16/FOSC

Time

Figure 2.The Controlled Stepped FB Voltage During Soft-

Start

Shutdown and Enable

Pulling the COMP voltage to GND by an open drain

transistor, shown in typical application circuit,

shutdown the APW7064 PWM controller. In shutdown

mode, the UGATE and LGATE turn off and pull to PHASE

and GND respectively.

Under Voltage Protection

The FB pin is monitored during converter operation by

the internal Under Voltage (UV) comparator. If the FB

voltage drops below 50% of the reference voltage (50%

of 1.2V = 0.6V), a fault signal is internally generated, and

the device turns off both high-side and low-side MOSFET

and the converter’s output is latched to be floating.

VCC

VOUT

t1

t2 t3

Figure 1.Soft-Start Interval

Copyright © ANPEC Electronics Corp.

Rev. A.3 - Aug., 2009

Time

10

www.anpec.com.tw

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