LT3468/LT3468-1

6

34681i

BLOCK DIAGRA

OPERATIO

SW

+

DONE

CHARGE

C1

T1

R2

60k

Q1

ENABLE

PHOTOFLASH

CAPACITOR

36mV

20mV

TO BATTERY

D2

Q2

D1

3486 BD

1

GND

5

3

4

2

+

–

–

+

–

+

–

+

+–

ONE-

SHOT

ONE-

SHOT

DRIVER

PRIMARY

SECONDARY

MASTER

LATCH

DCM COMPARATOR

R1

2.5k

S

RQ

1.25V

REFERENCE

Q3

Q1

QQ

SR

A3

A1

A2

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

happen.

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

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,

R2 are sized so that when the SW voltage is 31.5V above

latch. This disables Q1 and halts power delivery. Power

delivery can only be restarted by toggling the CHARGE Pin.

Figure 3