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Date: 08/12/04

SP7652 Wide Input Voltage Range 6A, 600kHz, Buck Regulator

© Copyright 2004 Sipex Corporation

THEORY OF OPERATION

control the COMP pin externally. If a simple,

single pole, single zero response is desired, then

compensation can be as simple as an RC to

ground. If a more complex compensation is

required, then the amplifier has enough band-

width (45

° at 4 MHz) and enough gain (60dB) to

run Type III compensation schemes with ad-

equate gain and phase margins at cross over

frequencies greater than 50kHz.

The common mode output of the error amplifier

is 0.9V to 2.2V. Therefore, the PWM voltage

ramp has been set between 1.1V and 2.2V to

ensure proper 0% to 100% duty cycle capability.

The voltage loop also includes two other very

important features. One is an asynchronous start

up mode. Basically, the synchronous rectifier

can not turn on unless the high side NFET has

attempted to turn on or the SS pin has exceeded

1.7V. This feature prevents the controller from

“dragging down” the output voltage during

startup or in fault modes. The second feature is

a 100% duty cycle timeout that ensures synchro-

nized refreshing of the BST capacitor at very

high duty ratios. In the event that the high side

NFET is on for 20 continuous clock cycles, a

reset is given to the PWM flip flop half way

through the 21st cycle. This forces GL to rise for

the cycle, in turn refreshing the BST capacitor.

Power MOSFETs

The SP7652 contains a pair of integrated low

resistance N MOSFETs designed to drive up to

6A of output current. Maximum output current

could be limited by thermal limitations of a

particular application. The SP7652 incorpo-

rates a built-in over-temperature protection to

prevent internal overheating.

GH

Voltage

GL

Voltage

V(VIN)

0V

-0V

-V(Diode) V

V(VIN)+V(VCC)

BST

Voltage

V(VCC)

TIME

SWN

Voltage

VBST

VSWN

V(VCC)

The SP7652 can be set to different output

voltages. The relationship in the following

formula is based on a voltage divider from the

output to the feedback pin VFB, which is set

to an internal reference voltage of 0.80V.

Standard 1% metal film resistors of surface

mount size 0603 are recommended.

Vout = 0.80V ( R1 / R2 + 1 ) => R2 = R1 / [ (

Vout / 0.80V ) – 1 ]

Where R1 = 68.1K

Ω and for Vout = 0.80V

setting, simply remove R2 from the board.

Furthermore, one could select the value of R1

and R2 combination to meet the exact output

voltage setting by restricting R1 resistance

range such that 50K

Ω < R1 < 100KΩ for

overall system loop stability.

Setting Output Voltages