www.SiliconStandard.com

6 of 8

SS7529

Rev.1.01 4/26/2004

FUNCTIONAL DESCRIPTION

Operation Overview

As graphic-card power design is getting more and

more complicated, engineers need a smart solution to

reduce not only the design effort, but to further reduce the

development time. The SS7529 is targeted at providing

application to provide an easy and cost effective solution.

The SS7529 control circuit is supplied by 5v and the high-

side MOSFET driver is supplied by 12V to eliminate the

bootstrap circuit.

The SS7529 integrates a synchronous-rectified buck

controller and a linear power controller to control the

supply of both the high-current requirement of the GPU

and the low current requirement of memory. To address

fault conditions, UVP (Under-Voltage Protection) is

implemented on both outputs and OVP (Over-Voltage

Protection) on the linear controller.

The high switching frequency (600kHz) helps to reduce

component sizes and the output ripple.

Initialization

There is a smart power-on-reset (POR) circuit to monitor

both 5Vcc and 12Vcc to identify if controller has started

operation or not. This is to prevent fault conditions from

undesirable power sequencing. With this, users can

easily implement circuits without worrying about the

power sequencing.

Soft-Start

The POR function initiates the soft-start function after the

5Vcc and 12Vcc reach their threshold voltage. The built-in

soft-start function is to prevent inrush current and output

voltage overshoot during power on. An internal digital

counter controls the soft-start voltage. It clamps the

ramping of reference voltage at the input of the error

amplifier and increases the pulse width of the output driver

slowly. The typical soft-start duration is 3.4mS.

Under-Voltage Protection

The under-voltage protection (UVP) of the SS7529 is

implemented by monitoring the feedback signal at FB and

FB2 pins. Whenever one of these two signals drops below

51% of the internal reference, the UVP will be triggered

and then both outputs will be quickly shut down. Unless

the fault condition is removed, both outputs will keep in

hiccup mode operation.

Output Voltage Setting

Both SS7529 outputs achieve regulation by feedback

from the voltage dividers. Therefore, the output voltage

can be easily programmed by the resistor values in the

following equation.

For switching output (figure 7),

=0.8V

V

R

R

R

V

OUT

Ref

4

1

4

1

=

+

×

SS7529

12VCC

5VCC

+5V

+12V

+3.3V

Vout1

UGATE

LGATE

FB

COMP

R4

R2

C1

C2

R1

C3

R3

Figure 7. OUTPUT VOLTAGE SELECTION OF THE PWM

OUTPUT

For linear output (figure 8),

R

R

R

V

OUT

6

5

6

2

+

×

= 0.8V

Vout2

+3.3Vin

SS7529

R5

R6

DRIVER2

FB2

Cout2

Figure 8. OUTPUT VOLTAGE SELECTION OF THE LINEAR

OUTPUT

Converter Shutdown

Forcing the FB2 pin to be higher than a threshold of 1.28V

will shutdown both regulators. When the applied voltage

is removed, the regulators will return to the re-start cycle

and begin the soft-start process.