APW7068 datasheet(13/26 Pages) ANPEC

13 / 26 page

Rev. A.6 - Aug., 2009

APW7068

www.anpec.com.tw

Function Description

Figure 2. The Controlled Stepped FB and FBL Voltage

During Soft-Start

Figure 1. Soft-Start Interval

2560

)

(

OSC

−

∆

OSC

640

)

(

−

∆

Voltage(V)

20mV

32/F

OSC

32/F

OSC

Time

FBL

Side)

DS(ON)

OCSET

LIMIT

−

Power-On-Reset (POR)

The Power-On-Reset (POR) function of APW7068 con-

tinually monitors the input supply voltage (V

CC12), ensures

the supply voltage exceed its rising POR threshold

voltage. The POR function initiates soft-start interval op-

eration while VCC12 voltage exceeds its POR threshold

and inhibits operation under disabled status.

Soft-Start

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

CC12 reaches

the rising POR threshold voltage, the internal reference

voltage is controlled to follow a voltage proportional to the

soft-start voltage. The soft-start interval is variable by the

oscillator frequency. The formulation 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 V

FB and VFBL, and the period of one step

is about 32/FOSC. This method provides a controlled

voltage rise and prevents the large peak current to charge

the output capacitors. The FB voltage compares the FBL

voltage to shift to an earlier time the establishment as

Figure2. The voltage estabilishment time difference for

FB and VFBL is variable by the oscillator. The formulation

is given by:

Voltage (V)

Time

CC12

OUT1

POR

OUT2

Over-Current Protection

Connect a resistor (R

OCSET) from this pin to GND, an inter-

nal 40

µA 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 shutdown the IC directly, and the

converter's output is latched.

The APW7068 has a internal OCP voltage source, and

the value is around 0.25V. When the R

OCSET x IOCSET is

bigger than 0.25V or the OCSET PIN is floating (no R

OCSET

resistor), the over current threshold will be the internal

default value 0.25V.

The threshold of the over current limit is therefore given

by:

For the over-current is never occurred in the normal oper-

ating load range; the variation of all parameters in the

above equation should be determined.

- The MOSFET’s R

DS(ON) is varied by temperature and gate

to source voltage, the user should determine the maxi-

mum R

DS(ON) in manufacturer’

s datasheet.

- The minimum I

OCSET (36µA) and minimum ROCSET should

be used in the above equation.

- Note that the I

LIMIT is the current flow through the lower

MOSFET; I

LIMIT must be greater than maximum output

current add the half of inductor ripple current.

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