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LT3468

Photoflash Capacitor Chargers in ThinSOT™

Linear Technology 

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OPERATIO

The LT3468/LT3468-1/LT3468-2 are designed to charge

photoflash capacitors quickly and efficiently. The opera-

tion of the part can be best understood by referring to

Figure 1. 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 current and when the peak current reaches 1.4A

(LT3468), 1A(LT3468-2) 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 VIN for this to 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

above VIN or lower, the output of A3 (DCM Comparator)

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 (VOUT Comparator). Resistors R1 and

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

VIN, the output of A2 goes high which resets the master

latch. This disables Q1 and halts power delivery. NPN

transistor Q3 is turned on pulling the DONE pin low,

LT3468/LT3468-1/LT3468-2

indicating that the part has finished charging. Power

delivery can only be restarted by toggling the CHARGE pin.

The CHARGE pin gives full control of the part to the user.

The charging can be halted at any time by bringing the

CHARGE pin low. Only when the final output voltage is

reached will the DONE pin go low. Figure 2 shows these

various modes in action. When CHARGE is first brought

high, charging commences. When CHARGE is brought

low during charging, the part goes into shutdown and

VOUT no longer rises. When CHARGE is brought high

again, charging resumes. When the target VOUT voltage is

reached, the DONE pin goes low and charging stops.

Finally the CHARGE pin is brought low again so the part

enters shutdown and the DONE pin goes high.

VOUT

100V/DIV

LT3468-2

VIN = 3.6V

COUT = 50µF

VDONE

5V/DIV

VCHARGE

5V/DIV

1s/DIV

3468 F02

Figure 2. Halting the Charging Cycle with the CHARGE Pin.

APPLICATIO S I FOR ATIO

Choosing The Right Device (LT3468/LT3468-1/

LT3468-2)

The only difference between the three versions of the

LT3468 is the peak current level. For the fastest possible

charge time, use the LT3468. The LT3468-1 has the lowest

peak current capability, and is designed for applications

that need a more limited drain on the batteries. Due to the

lower peak current, the LT3468-1 can use a physically

smaller transformer. The LT3468-2 has a current limit in

between that of the LT3468 and the LT3468-1.

Transformer Design

The flyback transformer is a key element for any LT3468/

LT3468-1/LT3468-2 design. It must be designed carefully

and checked that it does not cause excessive current or

voltage on any pin of the part. The main parameters that

need to be designed are shown in Table 1.

The first transformer parameter that needs to be set is the

turns ratio N. The LT3468/LT3468-1/LT3468-2 accom-

plish output voltage detection by monitoring the flyback

waveform on the SW pin. When the SW voltage reaches

31.5V higher than the VIN voltage, the part will halt power

delivery. Thus, the choice of N sets the target output

voltage as it changes the amplitude of the reflected voltage

from the output to the SW pin. Choose N according to the

following equation:

N = VOUT + 2

31. 5

346812f

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