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LTC3441

Photoflash Capacitor Chargers with Automatic Refresh

Linear Technology 

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OPERATIO

LT3420/LT3420-1

ISW

1A/DIV

ISEC

200mA/DIV

ISW

1A/DIV

ISEC

200mA/DIV

VSW

20V/DIV

2µs/DIV

3420 F06a

Figure 6a. Switching Waveforms with

VOUT = 100V, VCC = VBAT = 3.3V

VSW

20V/DIV

2µs/DIV

3420 F06b

Figure 6b. Switching Waveforms with

VOUT = 300V, VCC = VBAT = 3.3V

of Q1 so that the peak current of Q1 is 1.4A and the mini-

mum secondary current is 40mA (typical values).

The previously described charging cycle must be halted

when the output voltage reaches the desired value. The

LT3420 monitors the output voltage via the flyback pulse

on the SW pin. When Q1 turns off, the secondary side

conducts current turning on diode D1. Since the diode is

conducting and the SEC pin is at nearly ground, the voltage

across the secondary is nearly equal to VOUT. The voltage

across the primary is therefore close to VOUT/N. A current

proportional to VOUT/N flows through R1 and into the RFB

pin. The current flows out of the RREF pin through a resis-

tor creating a ground referred voltage. When this voltage

exceeds an internal 1V reference voltage, the output of

comparator A3 goes high which resets the master latch. The

Q output of the master latch goes low, disabling the entire

power delivery block and enabling the refresh timer.

Leakage Spike Blanking

Another function of the LT3420 is leakage spike blanking

when the power switch, Q1, turns off. Right after Q1 turns

off, a one-shot turns on Q2 for 200ns (typ). With Q2 on,

comparator A3 is disabled. This function may prevent A3

from false tripping on the leakage inductance spike on the

SW pin. In practice, the PNP transistor Q3 filters out the

leakage spike.

Refresh Timer

When the refresh timer is enabled, a 2.5µA current source

is switched on, charging up the external timing capacitor,

C3, from its initial voltage towards 1V. When the voltage

on C3 reaches 1V, the polarity of the current source changes

and 2.5µA discharges C3. When the voltage on C3 reaches

0.5V, the refresh timer sends a set pulse to the master latch,

which puts the LT3420 into the charging mode.

Interface/Control

The CHARGE pin serves two functions. The first is to

enable or shutdown the part depending on the level of the

pin (high = enable, low =shutdown). The second is to

force the part into the charging mode (low→high transi-

tion). The LT3420 also has a DONE pin, which signals

whether or not the part is done charging the photoflash

capacitor. The DONE pin is an open collector NPN switch

(Q5) so an external pull-up resistor is needed. Whenever

the part is in charging mode, DONE will be low. DONE will

go high when the charging mode is complete. Both the

CHARGE and DONE pins can be easily interfaced to a

microprocessor in a digital or film camera.

LT3420-1 Differences

The LT3420-1 has different primary and secondary cur-

rent limit levels. The primary current limit level of the

LT3420-1 is 1A (typ) and the secondary current limit is

15mA (typ). The LT3420-1 has no leakage spike blanking

which causes no problems since the PNP transistor, Q3,

provides adequate filtering. Finally, the breakdown voltage

of the SW pin of the LT3420-1 is higher at 50V.

3420fa

9

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