APW7068 datasheet(13/26 Pages) ANPEC

13 / 26 page

Rev. A.2 - Jun., 2006

APW7068

www.anpec.com.tw

Function Descriptions

Power On Reset (POR)

The Power-On Reset (POR) function of APW7068

continually monitors the input supply voltage (VCC12),

ensures the supply voltage exceed its rising POR

threshold voltage. The POR function initiates soft-start

interval operation while VCC12 voltages exceed their

POR threshold and inhibits operation under disabled

status.

Soft-Start

Figure 1. shows the soft-start interval. When VCC12

reaches the rising POR threshold voltage, the internal

reference voltage is controlled to follow a voltage pro-

portional to the soft-start voltage. The soft-start inter-

val is variable by the oscillator frequency. The formu-

lation is given by:

Figure 2. shows more detail of the FB and FBL voltage

ramps. The FB and FBL voltage soft-start ramps are

formed with many small steps of voltage. The voltage

of one step is about 20mV in FB and FBL, and the

period of one step is about 64/FOSC. This method pro-

vides a controlled voltage rise and prevents the large

peak current to charge output capacitor. The FB volt-

age compares the FBL voltage to shift to an earlier time

the establishment as Figure2. The voltage estabilishment

time difference for FB and FBL is variable by the

oscillator. The formulation is given by:

560

)

OSC

−

∆

Over-Current Protection

Connect a resistor (Rocset) from this pin to GND, an

internal 40uA current source will flow through this

resistor and create a voltage drop, which will be

compared with the voltage across the lower MOSFET.

When the voltage across the lower MOSFET exceeds

the voltage drop across the R

OCSET, an over-current

condition is detected and the controller will shut-

down the IC directly, and the converter's output is

latched.

Figure 2. The Controlled Stepped FB and FBL

Voltage during Soft-Start

OSC

)

−

∆

Figure 1. Soft-Start Interval

Voltage(V)

Time

FBL

20mV

32/Fosc

t 3

t 4

t 1

t 2

Voltage(V)

Time

VCC12

VOUT2

OUT1

t 0

POR

APW7068 Datasheet(PDF) 13 Page - Anpec Electronics Coropration

APW7068 Datasheet(HTML) 13 Page - Anpec Electronics Coropration

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Datasheet Download

Link URL