SP7651 datasheet(5/15 Pages) SIPEX

5 / 15 page

Rev J: 3/4/07

SP765 Wide Input Voltage Range 3A, 900kHz, Buck Regulator

safely begin down conversion, and an inter-

nal VCC UVLO ensures that the controller

itselfhasenoughvoltagetoproperlyoperate.

Other protection features include thermal

shutdown and short-circuit detection. In the

event that either a thermal, short-circuit, or

UVLO fault is detected, the SP765 is forced

into an idle state where the output drivers are

held off for a finite period before a re-start

is attempted.

Soft Start

“Soft Start” is achieved when a power con-

verter ramps up the output voltage while

controlling the magnitude of the input sup-

ply source current. In a modern step down

converter, ramping up the positive terminal

of the error amplifier controls soft start. As a

result, excess source current can be defined

as the current required to charge the output

capacitor.

IVIN = COUT * (∆VOUT / ∆TSOFT-START)

The SP765 provides the user with the op-

tion to program the soft start rate by tying

a capacitor from the SS pin to GND. The

selection of this capacitor is based on the

0µA pullup current present at the SS pin

and the 0.8V reference voltage. Therefore,

the excess source can be redefined as:

IVIN = COUT * (∆VOUT *0µA / (CSS * 0.8V)

Under Voltage Lock Out (UVLO)

The SP765 contains two separate UVLO

comparatorstomonitortheinternalbias(VCC)

andconversion(VIN)voltagesindependently.

The VCC UVLO threshold is internally set

to 4.25V, whereas the VIN UVLO threshold

is programmable through the UVIN pin.

When the UVIN pin is greater than 2.5V,

the SP765 is permitted to start up pend-

ing the removal of all other faults. Both the

VCC and VIN UVLO comparators have been

designed with hysteresis to prevent noise

from resetting a fault.

Thermal and Short-Circuit

Protection

Because the SP765 is designed to drive

largeoutputcurrent,thereisachancethatthe

power converter will become too hot. There-

fore, an internal thermal shutdown (145°C)

has been included to prevent the IC from

malfunctioning at extreme temperatures.

A short-circuit detection comparator has

also been included in the SP765 to protect

against an accidental short at the output

of the power converter. This comparator

constantly monitors the positive and nega-

tive 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

SP765 is capable of detecting short-circuit

faults throughout the duration of soft start as

well as in regular operation.

Handling of Faults:

Upon the detection of power (UVLO), ther-

mal, or short-circuit faults, the SP765 is

forced into an idle state where the SS and

COMP pins are pulled low and the NFETS

are held off. In the event of UVLO fault, the

SP765 remains in this idle state until the

UVLO fault is removed. Upon the detection

of a thermal or short-circuit fault, an internal

200ms timer is activated. In the event of a

short-circuit fault, a re-start is attempted im-

mediately after the 200ms timeout expires.

Whereas, when a thermal fault is detected

the200msdelaycontinuouslyrecyclesanda

re-startcannotbeattempteduntilthethermal

fault is removed and the timer expires.

Error Amplifier and Voltage Loop

Since the heart of the SP765 voltage error

loop is a high performance, wide bandwidth

transconductance amplifier, great care

shouldbetakentoselecttheoptimalcompen-

sation network. Because of the amplifier’s

THEORY OF OPERATION

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