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TC1235

Inverting Dual (–VIN, –2VIN) Charge Pump Voltage Converters with Shutdown

Microchip Technology 

Inverting Dual (–VIN, –2VIN)

Charge Pump Voltage Converters

with Shutdown

TC1235

TC1236

TC1237

DETAILED DESCRIPTION

The TC1235/1236/1237 dual charge pump converters

perform both a –1x and –2x multiply of the voltage applied

to the VIN pin. Output ‘– VIN’ provides a negative voltage

inversion of the VIN supply, while output ‘-2 VIN’ provides a

negative doubling inversion of VIN. Conversion is performed

using two synchronous switching matrices and four exter-

nal capacitors. When the shutdown input is held at a logic

low both stages go into a very low power mode of operation

consuming less than 1uA of supply current.

Figure 1 (below) is a block diagram representation of the

TC1235/1236/1237 architecture. The first switching stage

inverts the voltage present at VIN and the second stage uses

the ‘–VIN’ output generated from the first stage to produce

the ‘–2VIN’ output function from the second stage switching

matrix.

Each device contains an on-board oscillator that syn-

chronously controls the operation of the charge pump switch-

ing matrices. The TC1235 synchronously switches at 12KHz,

the TC1236 synchronously switches at 35KHz, and the

TC1237 synchronously switches at 125KHz. The different

oscillator frequencies for this device family allow the user to

trade-off capacitor size versus supply current. Faster oscil-

lators can use smaller external capacitors but will consume

more supply current (see Electrical Characteristics Table).

When the shutdown input is in a low state, the oscillator

and both switch matrices are powered off placing the TC1235/

1236/1237 in the shutdown mode. When the VIN supply

input is powered from an external battery, the shutdown

mode minimizes power consumption, which in turn will

extend the life of the battery.

VIN

+

C1

OSCILLATOR

ENABLE

SWITCH MATRIX

(1st STAGE)

ENABLE

—VIN

COUT1

+

SHDN

+

C2

SWITCH MATRIX

(2nd STAGE)

ENABLE

—2VIN

COUT2

+

APPLICATIONS INFORMATION

Output Voltage Considerations

The TC1235/1236/1237 performs voltage conversions

but does not provide any type of regulation. The two output

voltage stages will droop in a linear manner with respect to

their respective load currents. The value of the equivalent

output resistance of the ‘-VIN’ output is approximately 50Ω

nominal at +25°C and VIN = +5V. The value of the ‘-2VIN’

output and is approximately 140Ω nominal at +25°C and VIN

= +5V. In this particular case, ‘-VIN’ is approximately – 5V

and ‘–2VIN’ is approximately –10V at very light loads, and

each stage will droop according to the equation below:

VDROOP = IOUT x ROUT

[-VIN OUTPUT] = VOUT1 = – (VIN – VDROOP1)

[-2VIN OUTPUT] = VOUT2 = VOUT1 – (VIN – VDROOP2)

where VDROOP1 is the output voltage droop contributed

from stage 1 loading , and VDROOP2 is the output voltage

droop from stage 2 loading.

Charge Pump Efficiency

The overall power efficiency of the two charge pump

stages 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).

(2) I2R losses due to the on-resistance of the MOSFET

switches on-board each 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 factor (2), (3)

and (4) above. The losses for the first stage are given by

Equation 1a and the losses for the second stage are given

by Equation 1b.

P1LOSS (2, 3, 4) = IOUT1 2 x ROUT1

where ROUT1 = [ 1 / [ fOSC(C1) ] + 8RSWITCH1 +

4ESRC1 + ESRCOUT1 ]

Figure 1. Functional Block Diagram

© 2001 Microchip Technology Inc. DS21371A

3

TC1235/6/7-1 3/24/00

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