SP4446 Datasheet, PDF (3/10 Pages) Sipex Corporation – High Output Voltage Boost Regulator LCD Bias Regulator

VI
C1
(EXTERNAL)
R1
R5
(EXTERNAL)
Q1
3
FB
R2
L1
5 ^VIN
R6
X1
+
-
Q2
R3
R4
SHDN 4
Shutdown
Logic
FUNCTIONAL DIAGRAM
D1
SW
C2
1
VOUT
DISABLE
SET
POWER
TRANSISTOR
300ns
ONE-SHOT
CLEAR
DRIVER
X2 ₊
- _52.5mV
0.35
GND
2
General Overview:
Operation can be best understood by referring to
the above block diagram. Q1 and Q2 along with
R3 and R4 form a band gap reference. The input
to this circuit completes a feedback path from
the high voltage output through a voltage di-
vider, and is used as the regulation control input.
When the voltage at the FB pin is slightly above
1.22V, comparator X1 disables most of the
internal circuitry. Current is then provided by
capacitor C2, which slowly discharges until the
voltage at the FB pin drops below the lower
hysteresis point of X1, about 6mV. X1 then
enables the internal circuitry, turns on chip
power, and the current in the inductor begins to
ramp up. When the current through the driver
transistor reaches about 150mA, comparator X2
clears the latch, which turns off the driver tran-
sistor for a preset 0.3µs. At the instant of shutoff,
inductor current is diverted to the output through
diode D1. During this 0.3µs time limit, inductor
current decreases while its energy charges C2.
THEORY OF OPERATION
At the end of the 0.3µs time period, driver
transistor is again allowed to turn on which
ramps the current back up to the 150mA level.
Comparator X2 clears the latch, it’s output turns
off the driver transistor, and this allows delivery
of L1’s stored kinetic energy to C2. This switch-
ing action continues until the output capacitor
voltage is charged to the point where FB is at
band gap (1.22V). When this condition is
reached, X1 turns off the internal circuitry and
the cycle repeats. The SP4446 contains circuitry
to provide protection during start-up and while
in short-circuit conditions. When FB pin volt-
age is less than approximately 300mV, the switch
off time is increased to about 1.2µs and the
current limit is reduced to about 70% of its
normal value. While in this mode, the average
inductor current is reduced and helps minimize
power dissipation in the SP4446, the external
inductor and diode.
Rev. 7/14/03
SP4446 High Output Voltage Boost Regualtor, LCD Bias Regulator
3
© Copyright 2003 Sipex Corporation