SP6691_07 Datasheet, PDF (3/15 Pages) Sipex Corporation – SOT23Micro Power Boost Regulator Series White LED Driver

PIN NUMBER
1
2
3
4
5
PIN DESCRIPTION
PIN NAME
SW
GND
FB
SHDN
V_IN
DESCRIPTION
Switch input to the internal power switch.
Ground
Feedback
Shutdown. Pull high (on) to enable. Pull low (off) for shutdown.
Input Voltage. Bypass this pin with a capacitor as close to the device
as possible.
FUNCTIONAL DIAGRAM
5 ^VIN
SW
1
R1
Q1
FB 3
R2
Q2
X1
+
-
R3
R4
SHDN 4
Shutdown
Logic
DISABLE
SET
POWER
TRANSISTOR
250ns
ONE-SHOT
CLEAR
DRIVER
X2 ₊
-
GND
2
THEORY OF OPERATION
Operation can be best understood by referring to
the functional diagram above and the typical
application circuit in the front page. Q1 and Q2
along with R3 and R4 form a band gap refer-
ence. The input to this circuit completes a feed-
back path from the high voltage output through
a voltage divider, 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 dis-
charges 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
450mA, comparator X2 clears the latch, which
turns off the driver transistor for a preset 250nS.
At the instant of shutoff, inductor current is
diverted to the output through diode D1. During
this 250nS time limit, inductor current decreases
while its energy charges C2.
At the end of the 250ns time period, driver
transistor is again allowed to turn on which
ramps the current back up to the 450mA 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.
Jun26-07 Rev D
Micro Power Boost Regulator Series White LED Driver

© 2007 Sipex Corporation