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TC1220

Switched Capacitor Voltage Converters with Shutdown in SOT Packages

Microchip Technology 

4.0 APPLICATIONS INFORMATION

4.1 Output Voltage Considerations

The TC1219/TC1220 perform voltage conversion but

do 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 approxi-

mately 25Ω nominal at +25°C and VIN = +5V. VOUT is

approximately -5V at light loads, and droops according

to the equation below:

VDROP = IOUT x ROUT

VOUT = – (VIN – VDROP)

4.2 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

the commutation capacitor to the output

capacitor) when a voltage difference between

the two capacitors exists.

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

(3) above. These losses are given by Equation 4-1(b).

EQUATION 4-1:

a) PLOSS (2, 3) = IOUT2 x ROUT

b) where ROUT = [ 1 / [fOSC(C1) ] + 8RSWITCH +

4ESRC1 + ESRC2]

The 1/(fOSC)(C1) term in Equation 4-1(b) is the

effective output resistance of an ideal switched

capacitor circuit (Figure 4-1 and Figure 4-2). The value

of RSWITCH can be approximated at 0.5Ω for the

TC1219/TC1220.

TC1219/TC1220

The remaining losses in the circuit are due to factor (4)

above, and are shown in Equation 4-2. The output

voltage ripple is given by Equation 4-3.

EQUATION 4-2:

PLOSS(4) = [(0.5)(C1) (VIN2 – VOUT2) + (0.5)

(C2) (VRIPPLE2 – 2VOUT VRIPPLE)] x fOSC

EQUATION 4-3:

VRIPPLE = [ IOUT / 2 x ( fOSC) (C2) ] + 2 ( IOUT) (ESRC2)

FIGURE 4-1:

f

V+

IDEAL SWITCHED

CAPACITOR MODEL

VOUT

C1

C2

RL

FIGURE 4-2:

EQUIVALENT OUTPUT

RESISTANCE

REQUIV

V+

REQUIV = 1

f x C1

C2

VOUT

RL

 2002 Microchip Technology Inc.

DS21366B-page 5

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