LT1932 Linear Technology, LT1932 Datasheet - Page 4

• Manufacturer Part Number: LT1932
• Description: Constant-Current/ Constant-Voltage 1.4MHz Step-Up DC/DC Converter
• Manufacturer: Linear Technology

BLOCK DIAGRA
PI FU CTIO S
SW (Pin 1): Switch Pin. This is the collector of the internal
NPN power switch. Minimize the metal trace area con-
nected to this pin to minimize EMI.
GND (Pin 2): Ground Pin. Tie this pin directly to local
ground plane.
LED (Pin 3): LED Pin. This is the collector of the internal
NPN LED switch. Connect the cathode of the bottom LED
to this pin.
OPERATIO
LT1932
4
The LT1932 uses a constant frequency, current mode
control scheme to regulate the output current, I
Operation can be best understood by referring to the
block diagram in Figure 1. At the start of each oscillator
cycle, the SR latch is set, turning on power switch Q1. The
signal at the noninverting input of the PWM comparator
A2 is proportional to the switch current, summed to-
gether with a portion of the oscillator ramp. When this
signal reaches the level set by the output of error amplifier
A1, comparator A2 resets the latch and turns off the
U
U
V
IN
U
U
C1
W
5
SHDN
Q
DRIVER
R
S
6
V
IN
L1
Figure 1. LT1932 Block Diagram
OSCILLATOR
0.04
1.2MHz
2
1
GND
Q1
SW
LED
+
–
.
5
A2
+
–
R
programs the LED current (that flows into the LED pin).
This pin is also used to provide LED dimming.
SHDN (Pin 5): Shutdown Pin. Tie this pin higher than
0.85V to turn on the LT1932; tie below 0.25V to turn it off.
V
capacitor to ground as close to the device as possible.
power switch. In this manner, A1 sets the correct peak
current level to keep the LED current in regulation. If A1’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered. A1 senses the
LED current in switch Q2 and compares it to the current
reference, which is programmed using resistor R
R
I
than 100mV will pull down the output of A1, turning off
power switch Q1 and LED switch Q2.
+
LED
IN
SET
SET
D1
+
, is regulated to 225 • I
(Pin 6): Input Supply Pin. Bypass this pin with a
pin is regulated to 100mV and the output current,
(Pin 4): A resistor between this pin and ground
A1
DRIVER
–
+
I
LED CURRENT
SET
REFERENCE
4
R
R
SET
SET
Q2
SET
3
. Pulling the R
1932 F01
V
LED
OUT
I
LED
C2
SET
pin higher
SET
. The