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AAT3190

Positive/Negative Charge Pump for Voltage Bias

Advanced Analogic Technologies 

AAT3190

Positive/Negative Charge Pump for Voltage Bias

With R2 selected, R1 can be determined:

R1 =

VNEG · R2

-VREF

VOP

The positive output voltage is set by way of a resis-

tive divider from the output (VOP) to the FBP and

ground pin. Limiting the size of R4 reduces the

effect of the FBP bias current. For less than 0.1%

error, limit R4 to less than 12kΩ.

IPGM

=

VREF

R4

=

1.2V

12kΩ

=

100µA

IFBP

IPGM

=

0.1µA

100µA =

0.1%

Once R4 has been determined, solve for R3:

R3

=

R4

·

⎛

⎝

VO

VREF

-

1⎞⎠

Flying and Output Capacitor

The flying capacitor minimum value is limited by

the output power requirement, while the maximum

value is set by the bandwidth of the power supply.

If CFLY is too small, the output may not be able to

deliver the power demanded, while too large of a

capacitor may limit the bandwidth and time

required to recover from load and line transients. A

0.1µF X7R or X5R ceramic capacitor is typically

used. The voltage rating of the flying and reservoir

output capacitors varies with the number of charge

pump stages. The reservoir output capacitor

should be roughly 10 times the flying capacitor.

Use larger capacitors for reduced output ripple.

Positive Output Capacitor Voltage

Ratings

The absolute steady-state maximum output voltage

(neglecting the internal RDS(ON) drop of the internal

MOSFETs) for the nth stage is:

VBULK(n) = (n + 1) · VIN - 2 · n · VFWD

where VFWD is the estimated forward drop of the

Schottky diode. This is also the voltage rating

required for the nth bulk capacitor in the positive

output charge pump.

The voltage rating for the nth flying capacitor in the

positive stage is:

VFLY(n) = VBULK(n + 1) - VFWD

where VBULK(0) is the input voltage (see Table 1).

VIN = 5.0V, VFWD = 0.3V

Stages (n)

VBULK(n)

1

9.4V

2

13.8V

3

18.2V

4

22.6V

5

27.0V

6

31.4V

VFLY(n)

4.7V

9.1V

13.5V

17.9V

22.3V

26.7V

Table 1: Positive Output Capacitor Voltages.

Negative Output Capacitor Voltage

Ratings

The absolute steady-state maximum output voltage

(neglecting the internal RDS(ON) drop of the internal

MOSFETs) for the nth stage is:

VBULK(n) = -n · VIN + 2 · n · VFWD

This is also the voltage rating required for the nth

bulk capacitor in the negative output charge pump.

3190.2006.01.1.2

11

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