XRP7662 Datasheet, PDF(9/19 Page) Exar Corporation – 12A 300KHz Synchronous Step Down Regulator

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

XRP7662 Datasheet, PDF (9/19 Pages) Exar Corporation – 12A 300KHz Synchronous Step Down Regulator

◁

XRP7 66 2

Pow er Blox TM

1 2 A 3 0 0 KHz Sy n ch r on ou s St ep Dow n Reg u lat or

THEORY OF OPERATION

GENERAL OVERVIEW

The XRP7662 is a fixed frequency, voltage

mode, synchronous PWM regulator optimized

for high efficiency. The part has been

specifically designed for single supply

operation from a 5V to 22V input.

The heart of the XRP7662 is a wide bandwidth

transconductance amplifier

designed to

accommodate Type II and Type III

compensation 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. The output of the error amp lifier, COMP, is

compared to a 1.1V 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

accura tely sets the PWM frequency to 3 00kHz.

The XRP7662 contain s two unique control

features that are very powerful in distributed

applications. First, non - synchronous driver

control is enabled during startup, to prohibit

the low side switch from pulling down the

output until the high side switch has

attempted to turn on. Second, a 100% duty

cycle timeout ensures that the low side switch

is periodically enhanced during extended

periods at 100% duty cycle. This guarantees

the synchronized refreshing of the BST

capacitor during very large duty ratios.

The XRP7662 also co ntains a number of

valuable protection features. Programmable

VIN UVLO allows the user to set the exact

value at which the conversion voltage can

safely begin down - conversion, and an internal

VCC UVLO which ensures that the controller

itself has enough vol tage 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 XRP7662 is forced

into an idle state where the output drivers are

he ld off for a finite period before a restart is

attempted.

SOFT START

Â³6RIW 6WDUWÂ´ LV DFKLHYH- G ZKH

verter ramps up the output voltage while

controlling the magnitude of the input sup

ply

source current. In a modern step down

converter, rampin g 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.

The XRP7662 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

Ç$SXOOntXprSesenFt Xat UtheUSHS pin and

the 0.8V reference voltage. Therefore, the

excess source can be redefined as:

UNDER V OLTAGE LOCK OUT (UVLO)

The XRP7662 has two separate UV LO

comparators to monito r the bias (Vcc) and

Input (V IN ) voltages independently. The Vcc

UVLO is internally set to 4.25V. The Vin UVLO

is programmable through UVIN pin. When

UVIN pin is greater than 2.5V the

XRP7662 is

permitted to start up pending the removal of

all other faults . A pair of internal resistors is

connected to UVIN as shown in Figure 18.

VIN

UVIN

166k:

VIN UVLO

2.50V ON -

2.20V OFF

59k:

Fig. 18 : In ternal and External Bias of UVIN

Therefore without external biasing the V

start threshold is 9.5V. A small capacitor

may

be required between UVIN and GND to filter

out noise. For applications with V IN of 5V or

3.3V, connect UVIN directly to V IN . To program

9/ 19

Rev. 2. 2.0

▷