9

Mar -07 Rev AJ

SP7662 Wide Input Voltage Range 2A, 300kHz Buck Regulator

© 2007 Sipex Corporation

Error Amplifier and Voltage Loop

The heart of the SP7662 voltage error loop

is a high performance, wide bandwidth

transconductance amplifier. Because of

theamplifier’scurrentlimited(+/-150µA)

transconductance, there are many ways to

compensatethevoltagelooportocontrolthe

COMPpinexternally. Ifasimple,single-pole,

single-zero response is desired, then com-

pensation can be as simple as an RC circuit

toGround.Ifamorecomplexcompensation

isrequired,thentheamplifierhasenough

bandwidth (45° at 4 MHz), and enough gain

(60dB)torunTypeIIIcompensationschemes

with adequate gain and phase margins at

crossover frequencies greater than 50kHz.

The common mode output of the error am-

plifieris0.9Vto2.2V.Therefore,thePWM

voltageramphasbeensetbetween.Vand

2.2V to ensure proper0% to 00% dutycycle

capability. The voltage loop also includes

two other very important features. One is a

nonsynchronousstartupmode.Basically,the

synchronousrectifiercannotturnonunless

the high side switch has attempted to turn

on or the SS pin has exceeded .7V. This

featurepreventsthecontrollerfrom“dragging

down” the output voltage during startup or

in fault modes.

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 second feature is a 00% duty cycle

timeout that ensures synchronized refresh-

ing 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

isgiventothePWMflipflophalfwaythrough

cycle, in turn refreshing the BST capacitor.

The boost capacitor is used to generate a

high voltage drive supply for the high side

Power MOSFETs

The SP7662 contains a pair of integrated

lowresistanceN-channelswitchesdesigned

to drive up to 2A of output current. Care

should be taken to de-rate the output cur-

rent based on the thermal conditions in the

systemsuchasambienttemperature,airflow

and heat sinking. Maximum output current

could be limited by thermal limitations of a

particular application by taking advantage

of the integrated-over-temperature protec-

tive scheme employed in the SP7662. The

SP7662incorporatesabuilt-inovertemperature

protection to prevent internal overheating.

Setting Output Voltages

The SP7662 can be set to different output

voltages. The relationship in the following

formula is based on a voltage divider from

set to an internal reference voltage of 0.80V.

Standard1%metalfilmresistorsofsurface

mount size 0603 are recommended.

R2 = R /

setting, simply remove R2 from the board.

Furthermore, one could select the value of

the R and R2 combination to meetthe exact

output voltage setting by restricting the R

resistance range such that 10KΩ < R1 <

100KΩforoverallsystemloopstability.

APPLICATIONS INFORMATION