APW7067N 查看數據表（PDF） - Anpec Electronics

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APW7067N

Synchronous Buck PWM and Linear Controller

Anpec Electronics 

APW7067N

Function Description

Power-On-Reset (POR)

The Power-On-Reset (POR) function of APW7067N

continually monitors the input supply voltage (V ),

CC12

ensures the supply voltage exceed its rising POR

threshold voltage. The POR function initiates soft-start

interval operation while VCC12 voltage exceeds its 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 proportional to the

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

oscillator frequency. The formulation is given by:

TSS = ∆(t2 − t1) = 1 ×1280

FOSC

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 VFB 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 VFB and VFBL is variable by the

oscillator. The formulation is given by:

∆(t4 − t3) = 1 × 320 = 1 × TSS

FOSC

4

Voltage (V)

POR

VCC12

VOUT1

VOUT2

t0

t1

t2

Time

Figure 1. Soft-Start Interval

Voltage(V)

VFB

20mV

32/FOSC

VFBL

32/FOSC

20mV

t3

t4

Time

Figure 2. The Controlled Stepped FB and

FBL Voltage during Soft-Start

Over-Current Protection

The over-current protection monitors the output current by

using the voltage drop across the lower MOSFET’s

R and this voltage drop will be compared with the

DS(ON)

internal 0.25V reference voltage. When the voltage drop

across the lower MOSFET’s RDS(ON) is larger than 0.25V, an

over-current condition is detected, the controller will

shutdown the IC directly, and latch the converter's output.

The threshold of the over current limit is given by:

( ) ILIMIT

=

VOCP (0.25V)

RDS(ON) Low _ Side

For the over-current is never occurred in the normal

operating load range; the variation of all parameters in

the above equation should be determined.

- The MOSFET’s RDS(ON) is varied by temperature and

gate to source voltage, the user should determine

the maximum RDS(ON) in manufacture’s datasheet.

- The minimum VOCP should be used in the above

equation.

- Note that the ILIMIT is the current flow through the lower

MOSFET; I must be greater than maximum output

LIMIT

current add the half of inductor ripple current.

Under Voltage Protection

The FB and FBL pin are monitored during converter op-

eration by their own Under Voltage (UV) comparator. If

the FB or FBL voltage drop below 50% of the refer-

ence voltage (50% of 0.8V = 0.4V), a fault signal is inter-

nally generated, and the device turns off both high-side

Copyright © ANPEC Electronics Corp.

12

Rev. A.3 - Mar., 2008

www.anpec.com.tw

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