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TCM828/829-4 5/22/00

TCM828

TCM829

Switched Capacitor

Voltage Converters

© 2001 Microchip Technology Inc.

DS21488A

DETAILED DESCRIPTION

The TCM828/829 charge pump converters invert the

phase operation (Figure 1). During the first phase, switches

S2 and S4 are open and S1 and S3 are closed. During this

supplied from C2. During the second phase, S2 and S4 are

closed, and S1 and S3 are open. This action connects C1

across C2, restoring charge to C2.

APPLICATIONS INFORMATION

Output Voltage Considerations

The TCM828/829 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 approximately 25

Ω nominal

at +25

loads, and droops according to the equation below:

Charge Pump Efficiency

The overall power efficiency of the charge pump is

affected by four factors:

(1) Losses from power consumed by the internal oscil-

lator, switch drive, etc. (which vary with input volt-

age, temperature and oscillator frequency).

(2) I

switches on-board the charge pump.

(3) Charge pump capacitor losses due to effective

series resistance (ESR).

Figure 1. Ideal Switched Capacitor Charge Pump

(4) Losses that occur during charge transfer (from the

commutation capacitor to the output capacitor)

when a voltage difference between the two capaci-

tors exists.

Most of the conversion losses are due to factors (2), (3)

and (4) above. These losses are given by Equation 1.

output resistance of an ideal switched capacitor circuit

(Figures 2a, 2b).

The

losses in the circuit

due to factor (4) above

are also shown in Equation 2. The output voltage ripple is

given by Equation 3.

OSC

Figure 2a. Ideal Switched Capacitor Model

Figure 2b. Equivalent Output Resistance

C1

C2

TCM828/829

IN

S1

S3

S4

S2

V

OUT

R

L

C1

C2

f

V

OUT

R

EQUIV

R

EQUIV =

R

L

C2

f x C1

1

Equation 2.

Equation 1.

Equation 3.