TC1240_13 Datasheet, PDF(7/18 Page) Microchip Technology – Positive Doubling Charge Pumps with Shutdown in a SOT-23 Package

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TC1240_13 Datasheet, PDF (7/18 Pages) Microchip Technology – Positive Doubling Charge Pumps with Shutdown in a SOT-23 Package

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4.0 DETAILED DESCRIPTION

The TC1240/TC1240A charge pump converter dou-

bles the voltage applied to the VIN pin. Conversion con-

sists of a two-phase operation (Figure 4-1). During the

first phase, switches S2 and S4 are open and S1 and S3

are closed. During this time, C1 charges to the voltage

on VIN and load current is supplied from C2. During the

second phase, S2 and S4 are closed, while S1 and S3

are open.

During this second phase, C1 is level-shifted upward by

VIN volts. This connects C1 to the reservoir capacitor

C2, allowing energy to be delivered to the output as

needed. The actual voltage is slightly lower than 2 x VIN

since the four switches (S1-S4) have an on-resistance

and the load drains charge from reservoir capacitor C2.

VIN

S2 TC1240/TC1240A

VOUT = 2 x VIN

VIN

OSC

FIGURE 4-1:

Ideal Switched Capacitor

Charge Pump Doubler.

TC1240/TC1240A

5.0 TYPICAL APPLICATIONS

5.1 Output Voltage Considerations

The TC1240/TC1240A performs voltage doubling but

does not provide regulation. The output voltage will

droop in a linear manner with respect to load current.

The value of this equivalent output resistance is approx-

imately 12ï nominal at +25Â°C and VIN = +5.0V for the

TC1240A and 17ï nominal at +25Â°C and VIN = +2.8V

for the TC1240. VOUT is approximately +10.0V at light

loads for the TC1240A and +5.6V for the TC1240, and

droops according to the equation below:

EQUATION

VDROOP = IOUT ï´ ROUT

VOUT = 2 ï´ VIN â VDROOP

5.2 Charge Pump Efficiency

The overall power efficiency of the charge pump is

affected by four factors:

1. Losses from power consumed by the internal

oscillator, switch drive, etc. (which vary with

input voltage, temperature and oscillator

frequency).

2. I2R losses due to the on-resistance of the

MOSFET switches on-board the charge pump.

3. Charge pump capacitor losses due to effective

series resistance (ESR).

4. Losses that occur during charge transfer (from

commutation capacitor to the output capacitor)

when a voltage difference between the two

capacitors exist.

Most of the conversion losses are due to factors (2) and

(3) above. These losses are given by Equation 5-1.

EQUATION 5-1:

a) PLOSS(2,3) = IO2UT ï´ ROUT

ï£ 2001-2012 Microchip Technology Inc.

DS21516D-page 7

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