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AS3661

Datasheet - D e t a i l e d D e s c r i p t i o n

8.6 Charge Pump Operational Description

8.6.1 Overview

The AS3661 includes a pre-regulated switched-capacitor charge pump with a programmable voltage multiplication of 1

and 1.5x. In 1.5x mode by combining the principles of a switched-capacitor charge pump and a linear regulator, it

generates a regulated 4.5V output from Li-Ion input voltage range. A two-phase non-overlapping clock generated

internally controls the operation of the charge pump. During the charge phase, both flying capacitors (CFLY1 and CFLY2)

are charged from input voltage. In the pump phase that follows, the flying capacitors are discharged to output. A

traditional switched capacitor charge pump operating in this manner will use switches with very low on-resistance,

ideally 0

Ω, to generate an output voltage that is 1.5x the input voltage. The AS3661 regulates the output voltage by

controlling the resistance of the input-connected pass-transistor switches in the charge pump.

Figure 19. Charge Pump

8.6.2 Output Resistance

At lower input voltages, the charge pump output voltage may degrade due to effective output resistance (ROUT) of the

charge pump. The expected voltage drop can be calculated by using a simple model for the charge pump illustrated in

Figure 20 below.

Figure 20. Charge Pump Output Resistance

The model shows a linear pre-regulation block (REG), a voltage multiplier (1.5x), and an output resistance (ROUT). The

output resistance models the output voltage drop that is inherent to switched capacitor converters. The output

resistance is 3.5

Ω (typ.), and it is a function of switching frequency, input voltage, flying capacitors’ capacitance value,

internal resistances of the switches and ESR of the flying capacitors. When the output voltage is in regulation, the

regulator in the model controls the voltage V to keep the output voltage equal to 4.5V (typ.). With increased output

current, the voltage drop across ROUT increases. To prevent drop in output voltage, the voltage drop across the

regulator is reduced, V increases, and VCP remains at 4.5V. When the output current increases to the point that there

is zero voltage drop across the regulator, V equals the input voltage, and the output voltage is “on the edge” of

regulation. Additional output current causes the output voltage to fall out of regulation, so that the operation is similar to

a basic open-loop 1.5x charge pump. In this mode, output current results in output voltage drop proportional to the

output resistance of the charge pump. The out-of-regulation output voltage can be approximated by:

VCP = 1.5 x VIN – IOUT x ROUT.

VBAT

Low Noise

Charge Pump

1:1, 1:1.5 Modes

CVCPOUT

CFLY2

CFLY1

CBAT

VBAT

VCP

C1+

C1-

C2+

C2-

4.5V

LED1

Mode

Switching

LED2

LED9

Voltage limit

down

5.4V

cp_max_5V4

Vin

1.5x

1.5x V

Rout

REG

Vcp

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