SP7655_07 datasheet(5/16 Pages) SIPEX

5 / 16 page

Rev E: 3/2/07

SP7655 Wide Input Voltage Range 8A, 300kHz, Buck Regulator

tures include thermal shutdown and short-cir-

cuit detection. In the event that either a thermal,

short-circuit, or UVLO fault is detected, the

SP7655 is forced into an idle state where the

output drivers are held off for a finite period

before a restart is attempted.

Soft Start

“Soft Start” is achieved when a power converter

ramps up the output voltage while controlling

the magnitude of the input supply source cur-

rent. 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= C OUT * ( VOUT/ TSOFT-START)

The SP7655 provides the user with the option 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 10µA pullup current

present at the SS pin and the 0.8V reference

voltage. Therefore, the excess source can be

redefined as:

IVIN= C OUT * ( VOUT *10 µA / (CSS * 0.8V)

Under Voltage Lock Out (UVLO)

The SP7655 contains two separate UVLO com-

parators to monitor the bias (VCC) and conver-

sion (VIN) voltages independently. The VCC

UVLO threshold is internally set to 4.25V,

whereas the VIN UVLO threshold is program-

mable through the UVIN pin. When the UVIN

pin is greater than 2.5V, the SP7655 is permitted

to start up pending the removal of all other

faults. Both the VCC and VIN UVLO compara-

tors have been designed with hysteresis to pre-

vent noise from resetting a fault.

Thermal and Short-Circuit

Protection

Because the SP7655 is designed to drive large

output current, there is a chance that the power

converter will become too hot. Therefore, 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 SP7655 to protect against

an accidental short at the output of the power

converter. This comparator constantly 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 SP7655 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), thermal,

or short-circuit faults, the SP7655 is forced into

an idle state where the SS and COMP pins are

pulled low and both switches are held off. In the

event of UVLO fault, the SP7655 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 restart is at-

tempted immediately after the 200ms timeout

expires. Whereas, when a thermal fault is de-

tected, the 200ms delay continuously recycles

and a restart cannot be attempted until the ther-

mal fault is removed and the timer expires.

Error Amplifier and Voltage Loop

Since the heart of the SP7655 voltage error loop

is a high performance, wide bandwidth current-

limited (+/-150µA) transconductance ampli-

fier, there are many ways to compensate the

voltage loop or to control the COMP pin exter-

nally. If a simple, single pole, single zero

response is desired, then compensation can be

THEORY OF OPERATION

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