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V = -V x
OUT REF
= -1.213 V x
R
R
3
3
R
R
4
4
R3 = R4 x
= R4 x
|V
OUT
|
|V
OUT
|
V
REF
1.213
TPS65100 , TPS65101
TPS65105
SLVS496C – SEPTEMBER 2003 – REVISED APRIL 2006
Figure 22. Optimum, R
c
Is Ideal
Negative Charge Pump
The negative charge pump provides a regulated output voltage by inverting the main output voltage V
O
1. The
negative charge pump output voltage is set with external feedback resistors.
The maximum load current of the negative charge pump depends on the voltage drop across the external
Schottky diodes, the internal on resistance of the charge pump MOSFETS Q8 and Q9, and the impedance of
the flying capacitor C12. When the voltage drop across these components is larger than the voltage difference
from V
O
1 to V
O
2, the charge pump is in dropout, providing the maximum possible output current. Therefore, the
higher the voltage difference between V
O
1 and V
O
2, the higher the possible load current. See Figure 12 for the
possible output current versus boost converter voltage V
O
1.
V
O(min)
= -(V
O
- 2 V
D
- I
o
(2 x r
DS(on)Q8
+ 2 x r
DS(on)Q9
+ X
cfly
))
Setting the output voltage:
The lower feedback resistor value R4 should be in a range between 40 k Ω to 120 k Ω or the overall feedback
resistance should be within 500 k Ω to 1 M Ω . Smaller values load the reference too heavily and larger values
may cause stability problems. The negative charge pump requires two external Schottky diodes. The peak
current rating of the Schottky diode has to be twice the load current of the output. For a 20-mA output current,
the dual Schottky diode BAT54 or similar is a good choice.
Positive Charge Pump
The positive charge pump can be operated in a voltage doubler mode or a voltage tripler mode depending on
the configuration of the C2+ and C2-/MODE pins. Leaving the C2+ pin open and connecting C2-/MODE to GND
forces the positive charge pump to operate in a voltage doubler mode. If higher output voltages are required, the
positive charge pump can be operated as a voltage tripler. To operate the charge pump in the voltage tripler
mode, a flying capacitor needs to be connected to C2+ and C2-/MODE.
The maximum load current of the positive charge pump depends on the voltage drop across the internal
Schottky diodes, the internal on resistance of the charge pump MOSFETS, and the impedance of the flying
capacitor. When the voltage drop across these components is larger than the voltage difference V
O
1 x 2 to V
O
3
(doubler mode) or V
O
1 x 3 to V
O
3 (tripler mode), then the charge pump is in dropout, providing the maximum
possible output current. Therefore, the higher the voltage difference between V
O
1 x 2 (doubler) or V
O
1 x 3
(tripler) to V
O
3, the higher the possible load current. See Figure 13 and Figure 14 for the output current versus
boost converter voltage V
O
1 and the following calculations.
Voltage doubler:
V
O
3
max
= 2 x V
O
1 - (2 V
D
+ 2 x Io x (2 x r
DS(on)Q5
+ r
DS(on)Q3
+ r
DS(on)Q4
+ X
C1
))
Voltage tripler:
V
O
3
max
= 3 x V
O
1 - (3 x V
D
+ 2 x Io x (3 x r
DS(on)Q5
+ r
DS(on)Q3
+ r
DS(on)Q4
+ X
C1
+ X
C2
))
18
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