F72820

Jul 2007

V0.24P

-7-

F72820

6.6 Output Voltage Selection

The output voltage of the PWM converter can be programmed to any level between VIN (i.e. 5+V) and the internal

reference, 0.8V. An external resistor divider is used to scale the output voltage relative to the reference voltage and

feed it back to the inverting input of the error amplifier, see Figure 8. Depending on the value chosen for R1 (the

resistor connected between VOUT1 and FB), R4 (the resistor connected from FB to GND) can be calculated based

on the following equation:

V

VOUT

V

R

R

8

.

0

8

.

0

1

4

−

×

=

(EQ1)

If the desired output voltage is 0.8V, simply route VOUT back to FB through R1, but do not populate R4.

5V

VOUT

Q1

Q2

UGATE

LGATE

PHASE

BOOT

VCC

FB

R4

R1

Figure 8: Output voltage selection

6.7 Converter Shut Down

Pulling low the OCSET pin by a small signal transistor can shutdown the F72820 PWM controller as show is typical

application circuit.

6.8 PWM Controller Feedback Compensation

The F72820 is a voltage mode step down PWM controller. The gain of error amp is fixed at 34dB for simplified

design and saving external components. The amplitude of internal Tri-wave OSC is 1.75V. The loop gain and loop

Zero/Pole are calculated as follow equations.