TC1240ECH datasheet(3/11 Pages) MICROCHIP

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TC1240

Positive Doubling Charge Pump with

Shutdown in SOT Package

TC1240-1 7/7/00 DS21333A

DETAILED DESCRIPTION

The TC1240 charge pump converter doubles the volt-

age 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.

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.

APPLICATIONS INFORMATION

Output Voltage Considerations

TheTC1240 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 approximately 17

Ω nominal

at +25

°C and VIN = +2.8V. VOUT is approximately +5.6V at

light loads, and droops according to the equation below:

VDROOP = IOUT x ROUT

VOUT = 2 x 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 voltage,

temperature and oscillator frequency).

Figure 1. Ideal Swiched Capacitor Charge Pump Doubler

(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).

(4) Losses that occur during charge transfer (from

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 1(b).

(a) PLOSS (2, 3) = IOUT

2 x ROUT

(b) ≅ I

OUT

2 x

[

+8RSWITCH + 4ESRC1 + ESRC2

]

(fPUMP) C1

Equation 1.

The pump frequency in Equation 1(b) is defined as one-

half the oscillator frequency (i.e. fPUMP = fOSC/2). The

1/(fPUMP)(C1) term in Equation 1(b) is the effective output

resistance of an ideal switched capacitor circuit (Figures 2a,

2b). The value of RSWITCH can be approximated at 1.4

Ω for

the TC1240.

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

above, and are shown in Equation 2. The output voltage

ripple is given by Equation 3.

PLOSS(4) = [(0.5)(C1) (4VIN

2– VOUT2 ) + (0.5)(C2)(2VOUT

VRIPPLE – VRIPPLE

2 )] x fOSC

Equation 2.

VRIPPLE =

IOUT +2(IOUT)(ESRC2)

(fOSC)(C2)

Equation 3.

Figure 2a. Ideal Swiched Capacitor Model

V+

OUT

Figure 2b. Equivalent Output Resistance

TC1240

OSC

VIN

OUT = 2 x VIN

VIN

V+

OUT

EQUIV

EQUIV =

f x C1

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