AS3685A/AS3685B

Datasheet

www.austriamicrosystems.com/

AS3685

(ptr)

Revision 2.2

8 - 24

7

Detailed Functional Description

Figure 12 – Internal Circuit Diagram of AS3685A/AS3685B

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

−

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

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

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 (factory programmable) to limit the battery current to 1.5 times the output

(=LED) current.

C1+

C1-

C2+

C2-

VCP

ILED

ISET

VBAT

EN1

EN2

EN1

EN2

TXMask

Low Noise

Charge Pump

1:1, 1:1.5, 1:2 Modes

Digital

Control

1.3V

Vswitch

5.3V

soft_limit

AS3685

Different Control Logic

for AS3685A and AS3685B

1MHz

Overtemp

Detect

VSS