LX2172/73

PRODUCT DA T ABOOK 1996/1997

5

Copyright © 1998

Rev. 0.4

a 12/04

THEOR Y OF OPERA TION

IC OPERATION

Referring to the block diagram, when V

start threshold the 2.3V UVLO comparator output switches to

low state, allowing Q1 to turn on upon receiving a clock pulse

from the internal 100kHz oscillator. Once Q1 is on, current in

the inductor ramps up. Inductor current is then sensed, and

amplified by a current sense amplifier and compared to a thresh-

old set by the output of the error amplifier minus the offset

voltage (V

This allows the PWM comparator

to reset the flip flop, causing Q1 to turn off. The oscillator then

sets the flip flop, and the operation repeats itself.

CURRENT LIMIT

Current limiting is performed by sensing the peak switch cur-

rent and turning Q1 off until the next clock cycle. When the

converter goes into current limit the error amplifier output goes

to the rail and sets a maximum limit on the current.

ERROR AMPLIFIER

The function of the error amplifier is to set a threshold voltage

for inductor peak current and control the switch duty cycle so

that the power supply output voltage is closely regulated. Regu-

lation is accomplished by sensing the output voltage and com-

paring it to the internal 1.27V bandgap reference. A compensa-

tion network is placed from the output of the amplifier to ground

for closed loop stability purposes, as well as a high d.c. gain for

tight regulation. The function of V

requiring an error amplifier output to swing to ground level. It

is required the V

offset at all times. The

transfer function between the error

amplifier output (V

given by:

V

Where: I

G = Current sense gain (typically 6)

R

OSCILLATOR

The oscillator is designed to operate at 100kHz frequency. Its

function is to turn the switch Q1 on at a fixed 10µs interval and

to provide a slope compensation for the feedback loop. Slope

compensation is required for fixed frequency continuous Cur-

rent Mode regulators operating above 50% duty cycle in order

to prevent loop instability. The SYNC input is a logic input that

allows easy synchronization to an external clock. Synchroniza-

tion is done such that the external clock terminates the cycle

before the internal oscillator sets the PWM latch again.

UNDER-VOLTAGE LOCKOUT

The 2.3V internal regulator is monitored as the input supply

ramps up, ensuring glitch-free operation.

EXTRA PINS ON 8-PIN DEVICE

The 8-pin versions of the LX217x have the emitters of the power

transistors brought out separately. Connecting these pins to

ground reduces errors due to ground pin voltage drops. The E

2

pin also allows the user to half the switch current limit if the pin

is left open. Note that the switch resistance will increase when

E

increase, especially for currents over 300mA.

MICROPOWER SHUTDOWN

The LX2172/73 enter micropower shutdown mode when the V

C

pin is pulled below 0.15V. Supply current required for biassing

shutdown circuitry is less than 250µA. The LX2173 has an EN-

ABLE pin which allows ultra-low micropower shutdown opera-

tion, consuming less than 10µA.