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Vout + * VREF

1 )

R3

R4 )

V

REF + *

1.213 V

1 )

R3

R4 )

1.213 V

R3

+ R4

Vout ) VREF

V

REF

* 1 + R4

Vout ) 1.213

1.213

* 1

TPS65140

TPS65145

SLVS497B – SEPTEMBER 2003 – REVISED MARCH 2004

Negative Charge Pump

The negative charge pump provides a regulated output voltage by inverting the main output voltage, Vo1. 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

Vo1 to Vo2, the charge pump is in drop out, providing the maximum possible output current. Therefore, the

higher the voltage difference between Vo1 and Vo2, the higher the possible load current. See Figure 12 for the

possible output current versus boost converter voltage Vo1 and the calculations below.

Setting the output voltage:

The lower feedback resistor value, R4, should be in a range between 40 k

Ω to 120 kΩ or the overall feedback

resistance should be within 500 k

Ω 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 Vo1

× 2 to Vo3 (doubler

mode) or Vo1

× 3 to Vo3 (tripler mode), then the charge pump is in dropout, providing the maximum possible

output current. Therefore, the higher the voltage difference between Vo1

× 2 (doubler) or Vo1 × 3 (tripler) to Vo3,

the higher the possible load current. See Figure 13 and Figure 14 for output current versus boost converter

voltage, Vo1, and the following calculations.

Voltage doubler:

17