APW7065C Ver la hoja de datos (PDF) - Anpec Electronics

Número de pieza

componentes Descripción

Fabricante

APW7065C

Synchronous Buck PWM Controller

Anpec Electronics 

APW7065C

Function Description

Power-On-Reset (POR)

The Power-On-Reset (POR) function of APW7065C con-

tinually monitors the input supply voltage (V ) and the

CC

COMP pin. The supply voltage (V ) must exceed its ris-

CC

ing POR 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

COMP

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

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

Soft-Start

Voltage(V)

FB

12.5mV

8/Fosc

Time

Figure 2. The Controlled Stepped FB Voltage

during Soft Start

The APW7065C 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, the delay time is counted from t1 to t2

and then soft-start will be enabled. 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 (0.8V) until the ramp voltage reaches

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

the oscillator frequency (300KHz). The formulation is given

by:

Tsoft−start = t3 − t2 = 512/FOSC = 1.7ms

Figure 2. shows more details 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 12.5mV in

FB, and the period of one step is about 8/F . This method

OSC

provides a controlled voltage rise and prevents the large

peak current to charge output capacitor.

Over-Current Protection

The over-current protection monitors the output current

by using the voltage drop across the lower MOSFET’s

RDS(ON) and this voltage drop will be compared with the

internal 0.29V reference voltage. If the voltage drop across

the lower MOSFET’s RDS(ON) is larger than 0.29V, an

over-current condition is detected. The IC shuts off the

converter and initiates a new soft-start process. After two

over-current events are counted, the device turns off both

high-side and low-side MOSFETs and the converter's

output is latched to be floating. It requires a POR of V to

CC

restart. The threshold of the over current limit is given by:

ILimit

=

0.29

RDS(ON)

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.

Voltage (V)

VCC

VOUT

- The MOSFET’s R is varied by temperature and

DS(ON)

gate to source voltage, the user should determine

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

- The minimum Vocset 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 and add the half of inductor ripple current.

t1 t2 t3

Figure 1. Soft Start Interval

Time

Copyright © ANPEC Electronics Corp.

10

Rev. A.2 - Mar., 2008

www.anpec.com.tw

Share Link: