6

Date: /20/06

SP7653 Wide Input Voltage Range, .3MHz, Buck Regulator

© Copyright 2006 Sipex Corporation

6

Date: 2/17/06

SP7653 Wide Input Voltage Range, 1.3MHz, Buck Regulator

© Copyright 2006 Sipex Corporation

THEORY OF OPERATION

pin externally. If a simple, single-pole single-

zero response is desired, then compensation can

be as simple as an RC circuit to Ground. If a

more complex compensation is required, then

the amplifier has enough bandwidth (45° at 4

MHz) and enough gain (60dB) to run Type III

compensation schemes with adequate gain and

phase margins at crossover 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

startup mode. Basically, the synchronous recti-

fier cannot turn on unless the high side switch

has attempted to turn on or the SS pin has

exceeded 1.7V. This feature prevents the con-

troller from “dragging down” the output voltage

during startup or in fault modes. The second

feature is a 100% duty cycle timeout that en-

sures synchronized refreshing of the Boost ca-

pacitor 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-

GL to rise for the cycle, in turn refreshing the

Boost capacitor. The boost capacitor is used to

generate a high voltage drive supply for the high

Power MOSFETs

The SP7653 contains a pair of integrated low

resistance N-channel switches designed for up

to 3Amps. Care should be taken to de-rate the

output current based on the thermal conditions

in the system such as ambient temperature,

airflow and heat sinking. Maximum output cur-

rent could be limited by thermal limitations of a

particular application by taking advantage of

the integrated-over-temperature protective

scheme employed in the SP7653. The SP7653

incorporates a built-in overtemperature protec-

tion 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 SP7653 can be set to different output volt-

ages. The relationship in the following formula

is based on a voltage divider from the output to

the feedback pin V

reference voltage of 0.80V. Standard 1% metal

film resistors of surface mount size 0603 are

recommended.

R2 =

R1

.

Where R1 = 68.1K

setting, simply remove R2 from the board. Fur-

thermore, one could select the value of the R1

and R2 combination to meet the exact output

voltage setting by restricting the resistance range

of R1 such that 50K < R1 < 100K for overall

system loop stability.

Setting Output Voltages