LT3420 datasheet(14/20 Pages) LINER

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LT3420/LT3420-1

3420fa

risk of drawing too much current from the battery. The

input current to the LT3420/LT3420-1 circuit can be

adjusted by driving the CHARGE pin with a PWM (pulse

width modulation) signal. The microprocessor can adjust

the duty cycle of the PWM signal to achieve the desired

level of input current. Many schemes exist to achieve this

function. Once the target output voltage is reached, the

PWM signal should be halted to avoid overcharging the

photoﬂash capacitor, since the signal at the CHARGE pin

overrides the refresh timer.

A simple method to achieve adjustable input current is

shown in Figure 9. The PWM signal has a frequency of

1kHz. When ON is logic high, the circuit is enabled and the

CHARGE pin is driven by the PWM signal. When the target

output voltage is reached, DONE goes high while CHARGE

is also high. The output of A1 goes high, which forces

CHARGE high regardless of the PWM signal. The part is

now in the Refresh mode. Once the refresh period is over,

the DONE pin goes low, allowing the PWM signal to drive

the CHARGE pin once again. This function can be easily

implemented in a microcontroller. Figure 10 shows the

input current for the LT3420 and LT3420-1 as the duty

cycle of the PWM signal is varied.

APPLICATIO S I FOR ATIO

Figure 9. Simple Logic for Adjustable Input Current

Figure 10. Input Current as Duty Cycle is Varied

3420 F09

LT3420

CIRCUIT

CHARGE

DONE

1kHz PWM

SIGNAL

DUTY CYCLE (%)

3420 F10

800

LT3420

LT3420-1

400

200

600

LT3420 Datasheet(PDF) 14 Page - Linear Technology

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