6

Mar -07 Rev AJ

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

© 2007 Sipex Corporation

THEORY OF OPERATION

General Overview

The SP7662 is a fixed frequency, volt-

age mode, synchronous PWM regulator

optimizedforhighefficiency.Theparthas

beenspecificallydesignedforsinglesupply

operation from a 5V to 22V input.

The heart of the SP7662 is a wide bandwidth

transconductanceamplifierdesignedtoac-

commodate Type II and Type III compensa-

tion schemes. A precision 0.8V reference,

present on the positive terminal of the error

amplifier, permits the programming of the

output voltage down to 0.8V via the VFB

pin.Theoutputoftheerroramplifier,COMP,

is compared to a .V peak-to-peak ramp,

which is responsible for trailing edge PWM

control. This voltage ramp and PWM control

logic are governed by the internal oscillator

that accurately sets the PWM frequency to

300kHz.

The SP7662 contains two unique control

features that are very powerful in distributed

applications. First, nonsynchronous driver

control is enabled during startup, to prohibit

the low side switch from pulling down the

output until the high side switch has at-

tempted to turn on. Second, a 00% duty

cycletimeoutensuresthatthe lowsideswitch

is periodically enhanced during extended

periods at 00% duty cycle. This guaran-

tees the synchronized refreshing of the

BST capacitor during very large duty ratios.

The SP7662 also contains a number of valu-

UVLO allows the user to set the exact value

at which the conversion voltage can safely

UVLO which ensures that the controller itself

has enough voltage to properly operate.

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 SP7662 is forced

into an idle state where the output drivers

areheldoffforafiniteperiodbeforearestart

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

oftheerroramplifiercontrolssoftstart.Asa

result,excesssourcecurrentcanbedefined

as the current required to charge the output

capacitor.

The SP7662 provides the user with the op-

tion to program the soft start rate by tying

acapacitorfromtheSSpintoGND.The

selection of this capacitor is based on the

0µA pull up current present at the SS pin

and the 0.8V reference voltage. Therefore,

theexcesssourcecanberedefinedas:

Under Voltage Lock Out (UVLO)

The SP7662 has two separate UVLO com-

parators to monitor the bias (Vcc) and Input

programmable through UViN pin. When

UVIN pin is greater than 2.5V the SP7662

is permitted to start up pending the removal

of all other faults. A pair of internal resistors

isconnectedtoUVINasshowninthefigure

below.

SP7662

R6

R7

+

-

140KΩ

50KΩ

2.5V ON

2.2V OFF

GND

UVIN

VIN

Internal and external bias of UVIN