7

Feb2-07 Rev O

SP7663 Wide Input Voltage Range 6A, 600kHz 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

R5=9.09KΩ.

Thermal and Short-Circuit Protection

Because the SP7663 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 SP7663 to protect

against an accidental short at the output of

the power converter. This comparator con-

stantly monitors the positive and negative

terminals of the error amplifier, and if the

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

SP7663 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

theoperationalamplifierusedtosensethe

voltage across the inductor. Over-current is

detected by monitoring a differential voltage

across the output inductor as shown in the

nextfigure.

ISN

SP7663

SWN

ISP

L = 1.5uH, DCR = 9.2mΩ

R3

5.11KΩ

R4

5.11KΩ

CS

0.1uF

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

comparator is equal to the product of in-

ductor current and inductor DC resistance

thisequationforthespecificinductorincir-

a 20 ms time-out is initiated, during which

top and bottom drivers are turned off. Fol-

lowing the time-out, a restart is attempted.

If the fault condition persists, then the time-

out is repeated (referred to as hiccup).

Increasing the Current Limit

thiscaselargerthan9.6A),thenaresistor R9

shouldbeaddedasshowninthenextfigure.

R9 forms a resistor divider and reduces the

voltage seen by the comparator.

Since:60mV

R9 = {R3 + R4 + R9}

SolvingforR9weget:

R9 = [60mV • (R3 + R4)]