LT3468 Linear Technology, LT3468 Datasheet - Page 6

• Manufacturer Part Number: LT3468
• Description: Photoflash Capacitor Charger in ThinSOT
• Manufacturer: Linear Technology

BLOCK DIAGRA
LT3468/LT3468-1
OPERATIO
The LT3468/LT3468-1 are designed to charge photoflash
capacitors quickly and efficiently. The operation of the part
can be best understood by referring to Figure 3. When the
CHARGE pin is first driven high, a one shot sets both SR
latches in the correct state. The power NPN device, Q1,
turns on and current begins ramping up in the primary of
transformer T1. Comparator A1 monitors the switch cur-
rent and when the peak current reaches 1.4A (LT3468) or
0.7A (LT3468-1), Q1 is turned off. Since T1 is utilized as
a flyback transformer, the flyback pulse on the SW pin will
cause the output of A3 to be high. The voltage on the SW
pin needs to be at least 36mV higher than V
happen.
6
CHARGE
4
LT3468: R
LT3468-1: R
MASTER
LATCH
Q3
3
DONE
SHOT
ONE-
SENSE
Q
S
SENSE
= 0.015
U
Q
R
ENABLE
= 0.03
TO BATTERY
Q1
W
V
OUT
A2
COMPARATOR
+
–
C1
5
REFERENCE
V
IN
1.25V
IN
Q2
for this to
R1
2.5k
60k
R2
Figure 3
SHOT
ONE-
During this phase, current is delivered to the photoflash
capacitor via the secondary and diode D1. As the second-
ary current decreases to zero, the SW pin voltage will begin
to collapse. When the SW pin voltage drops to 36mV
above V
will go low. This fires a one shot which turns Q1 back on.
This cycle will continue to deliver power to the output.
Output voltage detection is accomplished via R2, R1, Q2,
and comparator A2 (V
R2 are sized so that when the SW voltage is 31.5V above
V
latch. This disables Q1 and halts power delivery. Power
delivery can only be restarted by toggling the CHARGE Pin.
IN
, the output of A2 goes high which resets the master
R
S
DCM COMPARATOR
IN
Q
A1
A3
or lower, the output of A3 (DCM Comparator)
+
–
+
–
DRIVER
+
–
20mV
+–
36mV
OUT
PRIMARY
1
Comparator). Resistors R1 and
Q1
R
SW
SENSE
3486 BD
T1
2
D2
GND
SECONDARY
D1
+
C
PHOTOFLASH
CAPACITOR
V
OUT
OUT
34681i