LT3484-0/LT3484-1/LT3484-2

7

3484012f

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

reached, the DONE pin goes low and charging stops.

Finally the CHARGE pin is brought low again so the part

these pins goes below its UVLO voltage, the DONE pin

terminate the charge. The application diagrams suggest

values adequate for most applications.

OPERATIO

The LT3484-0/LT3484-1/LT3484-2 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

(LT3484-0), 1A(LT3484-2) or 0.7A (LT3484-1), 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

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 40mV

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. NPN

transistor Q3 is turned on pulling the DONE pin low,

indicating that the part has finished charging. Power

delivery can only be restarted by toggling the CHARGE pin.

LT3484-2

100V/DIV

5V/DIV

5V/DIV

1s/DIV

3484 F02

Figure 2. Halting the Charging Cycle with the CHARGE Pin

APPLICATIO S I FOR ATIO

Choosing the Right Device

(LT3484-0/LT3484-1/LT3484-2)

The only difference between the three versions of the

LT3484 is the peak current level. For the fastest possible

charge time, use the LT3484-0. The LT3484-1 has the

lowest peak current capability, and is designed for appli-

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

to the lower peak current, the LT3484-1 can use a physi-

cally smaller transformer. The LT3484-2 has a current

limit in between that of the LT3484-0 and the LT3484-1.

Transformer Design

The flyback transformer is a key element for any

LT3484-0/LT3484-1/LT3484-2 design. It must be de-

signed carefully and checked that it does not cause exces-

sive 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 LT3484-0/LT3484-1/LT3484-2 accom-

plish output voltage detection by monitoring the flyback

waveform on the SW pin. When the SW voltage reaches

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