APW7098 datasheet(18/30 Pages) ANPEC

18 / 30 page

Rev. A.7 - Oct., 2011

APW7098

www.anpec.com.tw

Function Description (Cont.)

Reference Voltage Selection and Shutdown Control

(Cont.)

REFIN/EN pin determines which reference voltage is

used. If this REFIN/EN pin is driven by an external

voltage ranged from 0.4V to 2V, the IC uses the V

REFIN/EN

voltage as reference voltage of the converter with soft-

start control. If external reference is not available, con-

nect this pin to 5VCC for internal 0.6V reference used.

Once the internal or external reference is selected, the

reference source is latched. Cycling the POR signal re-

sets the latch.

The other function of REFIN/EN pin is used to enable or

shut off the IC. Pulling the V

REFIN/EN voltage below 0.4V

(typical) shuts down the two-phase PWM controller. In

the shutdown mode, the two-phase UGATE and LGATE

signals are pulled to PHASE and PGND respectively, the

output is floating.

Operation Phase Selection

The MODE pin programs single- or two- phase operation.

It has a typical value for rising threshold of 0.8V, V

MODE_THR,

with 0.16V of hysteresis (0.64V), V

MODE_THF. When the MODE

pin voltage is higher than V

MODE_THR, the device operates

in single-phase; when the MODE pin voltage is lower

than V

MODE_THF and VIN2 supply voltage is above approxi-

mate 4V, the device operates in two-phase operation.

This function makes the APW7098 ideally suitable for

dual power input applications like PCIE interfaced graphic

cards.

The figure 2 shows the power sources of the two

channels. The input power of PWM1 converter is sup-

plied by PCIE bus power and the input power of PWM2

converter is supplied by an external power. If the input

power connector of PWM2 converter is not plugged into

the socket before start-up, the internal V

IN2 sensing circuit

can sense the absence of V

IN2 and set the IC to operate in

single-phase mode with PWM2 disabled. When the IC

operates in two-phase mode, it can switch the operating

mode from two-phase to single-phase operation. Once

operating in single-phase mode, the operation mode is

latched. It is required to toggle SS, REFIN/EN, or 5VCC

pin to reset the IC.

Figure 2. V

IN2 Sensing Circuit

Over-Voltage Protection (OVP)

The over-voltage protection function monitors the output

voltage through the FB pin. When the FB voltage in-

creases over 125% of the reference voltage (V

R) due to

the high-side MOSFET failure or other reasons, the over-

voltage protection comparator designed with a 2

µs

noise filter will force the low-side MOSFET gate drivers

high. This action actively pulls down the output voltage

and eventually attempts to trigger the over-current shut-

down of an ATX power supply. As soon as the output

voltage is within regulation, the OVP comparator is

disengaged. The chip will restore its normal operation.

When the OVP occurs, the REFOUT/POK will drop to low

as well.

Under-Voltage Protection (UVP)

In the process of operation, when a short-circuit occurs,

the output voltage will drop quickly. Before the over-cur-

rent protection responds, the output voltage will fall

out of the required regulation range. The under-voltage

continually monitors the V

FB voltage after soft-start is

completed. If a load step is strong enough to pull the

output voltage lower than the under-voltage threshold,

the IC shuts down converter’s output. Cycling the 5VCC

POR or REFIN/EN signal resets the fault latch and starts

a start-up process. The under-voltage threshold is 50% of

the nominal output voltage. The under-voltage compara-

tor has a built-in 2

µs noise filter to prevent the chips

from wrong UVP shutdown being caused by noise.

PWM 1

converter

PCIE

+12V

PWM 2

converter

External

Power

IN2

PHASE2

VCC

MODE

IN2 sensing

circuit

Operation

Phase

Selection

This OVP scheme only clamps the voltage overshoot,

and does not invert the output voltage when otherwise

activated with a continuously high output from low-side

MOSFETs driver, which is a common problem for OVP

schemes with a latch.

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