TC828AECT datasheet(3/8 Pages) TELCOM

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TC828A

Switched Capacitor

Voltage Converters

TC828A-1 5/1/00

DETAILED DESCRIPTION

The TC828A charge pump converter inverts the voltage

applied to the VIN pin. Conversion consists of a two-phase

operation (Figure 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,

and S1 and S3 are open. This action connects C1 across

C2, restoring charge to C2.

APPLICATIONS INFORMATION

Output Voltage Considerations

The TC828A performs voltage conversion 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 approximately 25

Ω nominal

at +25

°C and VIN = +5V. VOUT is approximately – 5V at light

loads, and droops according to the equation below:

VDROOP = IOUT x ROUT

VOUT = – (VIN – VDROOP)

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

2R 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).

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.

PLOSS (2, 3, 4) = IOUT

2 x ROUT

≅ IOUT2 x

[ 1 +8RSWITCH+ 4ESRC1+ ESRC2 ]

(fOSC) C1

The 1/(fOSC)(C1) term in Equation 1 is the effective

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.

PLOSS (4) =

[(0.5)(C1)(VIN2– VOUT2) + (0.5)(C2)(VRIPPLE2

– 2VOUTVRIPPLE)

] x f

OSC

VRIPPLE =

IOUT

+2(IOUT)(ESRC2)

(fOSC)(C2)

Figure 2a. Ideal Switched Capacitor Model

Figure 2b. Equivalent Output Resistance

VOUT = – (VIN)

TC828

V+

OUT

V+

OUT

EQUIV

EQUIV =

f x C1

Equation 2.

Equation 1.

Equation 3.

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