Datasheet

R3=R4x
=R4x
|V
OUT
|
|V
OUT
|
V
REF
1.213
V =-V x
OUT REF
=-1.213Vx
R
R
3
3
R
R
4
4
TPS65140, TPS65141
TPS65145
SLVS497E –SEPTEMBER 2003–REVISED NOVEMBER 2012 www.ti.com
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 drop out, 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 and the calculations below.
Vout
min
= -(V
O
1 - 2 V
D
- I
O
(2 x r
DS(on)Q8
+ 2 x r
DS(on)Q9
+ X
cfly
))
Setting the output voltage:
(1)
(2)
The lower feedback resistor value, R4, should be in a range between 40 kΩ to 120 kΩ or the overall feedback
resistance should be within 500 kΩ to 1 MΩ. Smaller values load the reference too heavy 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 output current versus boost converter
voltage, V
O
1, and the following calculations.
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