7

Mar -07 Rev AJ

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

© 2007 Sipex Corporation

THEORY OF OPERATION

start threshold is 9.5V. A small capacitor

mayberequiredbetweenUVINandGNDto

of external resistors as shown. If external

resistors are an order of magnitude smaller

thresholdisgivenby:

startthresholdof7V,thenletR7=5KΩand

R6=9.09KΩ.

Thermal and Short-Circuit Protection

Because the SP7662 is designed to drive

largeoutputcurrent,thereisachancethatthe

power converter will become too hot. There-

fore, an internal thermal shutdown (45°C)

has been included to prevent the IC from

malfunctioning at extreme temperatures.

A short-circuit detection comparator has

also been included in the SP7662 to protect

against an accidental short at the output of

the power converter. This comparator con-

stantly monitors the positive and negative

terminals ofthe error amplifier,and ifthe

VfB pin falls more than 250mV (typical)

below the positive reference, a short-circuit

fault is set. Because the SS pin overrides the

internal 0.8V reference during soft start, the

SP7662 is capable of detecting short-circuit

faults throughout the duration of soft start as

well as in regular operation.

Over-Current Protection

The Over-current protection feature can

onlybeusedonoutputvoltages≤3.3volts.

It is limited by the common mode rating

of the op-amp used to sense the voltage

across the inductor. Over-current is de-

tected by monitoring a differential voltage

across the output inductor as shown in the

nextfigure.

ISN

SP7662

SWN

ISP

L=2.7uH,DCR=4.1mΩ

R3

5.11KΩ

R4

5.11KΩ

CS

0.uF

CSP

6.8nF

Over-current detection circuit

Inputs to an over-current detection com-

parator, set to trigger at 60 mV nominal, are

connected to the inductor as shown. Since

the average voltage sensed by the com-

parator is equal to the product of inductor

current and inductor DC resistance (DCR),

time-out is initiated, during which top and

bottom drivers are turned off. Following the

time-out, a restart is attempted. If the fault

condition persists, then the time-out is re-

peated (referred to as hiccup).

Increasing the Current Limit

this case larger than 4.6A), then a resistor

R9 should be added as shown in the next

figure.R9formsaresistordividerandre-

duces the voltage seen by the comparator.

Since:60mV

R9 = {R3 + R4 + R9}