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Date: 03/25/05

SP7653 Wide Input Voltage Range, 1.3MHz, Buck Regulator

© Copyright 2005 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 compen-

sation schemes with adequate 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

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 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 Boost 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 Boost capacitor.

The boost capacitor is used to generate a high

voltage drive supply for the high side switch,

Power MOSFETs

The SP7653 contains a pair of integrated low

resistance N-channel switches designed for up

to 3A. 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 current could

be limited by thermal limitations of a particular

application by taking advantage of the inte-

grated-over-temperature protective scheme em-

ployed in the SP7653. The SP7653 incorporates

a built-in overtemperature 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 SP7653 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 V

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 the

R1 and R2 combination to meet the exact

output voltage setting by restricting the

resistance range of R1 such that 50K

Ω < R1

< 100K

Ω for overall system loop stability.

Setting Output Voltages