AS3685C datasheet(9/22 Pages) AMSCO

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AS3685C

Datasheet

www.austriamicrosystems.com/AS3685

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1.1

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Detailed Functional Description

Figure 9 – Internal Circuit Diagram of AS3685C

7.1 Low Noise Charge Pump

The AS3685 charge pump uses two external flying capacitors to generate output voltages higher than the battery

voltage. The charge pump can operate in three different modes:

1:1 Bypass Mode or Off Mode

−

Battery input and output are connected by a low-impedance switch

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Battery current = output current

1:1.5 Mode

−

The output voltage is 1.5 times the battery voltage (without load)

−

Battery current = 1.5 times output current

1:2 Mode

−

The output voltage is 2 times the battery voltage (without load)

−

Battery current = 2 times output current

The flying capacitors are switched at the fixed frequency fclk.

7.1.1 Mode Switching

The AS3685 monitors the voltage at the current sink V(ILED) and if this voltage falls below VSWITCH, for a time

longer than the debounce time, the charge pump automatically switches into a higher mode. The debounce time

is set to tUP_DEB_LONG at enabling of the charge pump or immediately after a 1:1 to 1:1.5 mode change. Afterwards

the debounce time is reduced to tUP_DEB. (This allows the LED current to settle properly on startup or after a

mode change)

The charge pump enters always 1:1 mode in off mode or in case of overtemperature.

It is possible to avoid the 1:2 mode (register programmable) to limit the battery current to 1.5 times the output

(=LED) current.

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