AAT3190
Positive/Negative Charge Pump for Voltage Bias
With R2 selected, R1 can be determined:
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:
VNEG · R2
R1 =
-VREF
VOP
VBULK(n) = (n + 1) · VIN - 2 · n · VFWD
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.
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:
VREF
R4
1.2V
IPGM
=
=
= 100µA
12kΩ
IFBP 0.1µA
IPGM 100µA
VFLY(n) = VBULK(n + 1) - VFWD
=
= 0.1%
where VBULK(0) is the input voltage (see Table 1).
Once R4 has been determined, solve for R3:
VIN = 5.0V, VFWD = 0.3V
VO
⎛
⎝
⎞
- 1
⎠
Stages (n)
VBULK(n)
9.4V
VFLY(n)
4.7V
R3 = R4 ·
V
REF
1
2
3
4
5
6
13.8V
18.2V
22.6V
27.0V
31.4V
9.1V
13.5V
17.9V
22.3V
26.7V
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.
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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