TC7660 datasheet(6/12 Pages) MICROCHIP

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TC7660

CHARGE PUMP DC-TO-DC

VOLTAGE CONVERTER

DS21465A

TC7660-7 9/30/96

The TC7660 approaches these conditions for negative

voltage multiplication if large values of C1 and C2 are used.

Energy is lost only in the transfer of charge between

capacitors if a change in voltage occurs. The energy lost

is defined by:

E = 1/2 C1 (V12 – V22)

V1 and V2 are the voltages on C1 during the pump and

transfer cycles. If the impedances of C1 and C2 are relatively

high at the pump frequency (refer to Figure 2), compared to

the value of RL, there will be a substantial difference in

voltages V1 and V2. Therefore, it is not only desirable to

make C2 as large as possible to eliminate output voltage

ripple, but also to employ a correspondingly large value for

C1 in order to achieve maximum efficiency of operation.

Dos and Don'ts

• Do not exceed maximum supply voltages.

• Do not connect LV terminal to GND for supply voltages

greater than 3.5V.

• Do not short circuit the output to V+ supply for voltages

above 5.5V for extended periods; however, transient

conditions including start-up are okay.

• When using polarized capacitors in the inverting mode,

the + terminal of C1 must be connected to pin 2 of the

TC7660 and the + terminal of C2 must be connected to

GND Pin 3.

Simple Negative Voltage Converter

Figure 3 shows typical connections to provide a nega-

tive supply where a positive supply is available. A similar

scheme may be employed for supply voltages anywhere in

the operating range of +1.5V to +10V, keeping in mind that

pin 6 (LV) is tied to the supply negative (GND) only for supply

voltages below 3.5V.

Figure 3. Simple Negative Converter

Paralleling Devices

Any number of TC7660 voltage converters may be

paralleled to reduce output resistance (Figure 4). The reser-

voir capacitor, C2, serves all devices, while each device

requires its own pump capacitor, C1. The resultant output

resistance would be approximately:

ROUT (of TC7660)

n (number of devices)

ROUT =

πf C1

The output characteristics of the circuit in Figure 3 are

those of a nearly ideal voltage source in series with 70

Ω.

Thus, for a load current of –10mA and a supply voltage of

+5V, the output voltage would be – 4.3V.

The dynamic output impedance of the TC7660 is due,

primarily, to capacitive reactance of the charge transfer

capacitor (C1). Since this capacitor is connected to the

output for only 1/2 of the cycle, the equation is:

XC =

= 3.18

Ω,

where f = 10kHz and C1 = 10µF.

TC7660

µF

VOUT*

1. VOUT = –n V

for 1.5V

10V

NOTES:

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