LX2172CM Просмотр технического описания (PDF) - Microsemi Corporation
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LX2172CM Datasheet PDF : 6 Pages
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PRODUCT DATABOOK 1996/1997
1.25A HIGH-EFFICIENCY 100KHZ SWITCHING REGULATOR
PR E L I M I N A RY D ATA S H E E T
LX2172/73
THEORY OF OPERATION
IC OPERATION
Referring to the block diagram, when V is above the UVLO
IN
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 typically 0.9V). This allows the PWM comparator
OFS
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 is to keep Q1 off without
OFS
requiring an error amplifier output to swing to ground level. It
is required the VOL of the error amplifier is always less than the
offset at all times. The transfer function between the error
amplifier output (V ) and peak inductor current is therefore
C
given by:
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 is open, so efficiency will decrease and heat dissipation will
2
increase, especially for currents over 300mA.
MICROPOWER SHUTDOWN
The LX2172/73 enter micropower shutdown mode when the VC
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.
V
C
-
V
OFS
=
I
P
*
G
*
R
S
Where: I = Inductor peak current
P
G = Current sense gain (typically 6)
RS = Internal sense resistor
Copyright © 1998
Rev. 0.4a 12/04
5
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