TC7660CPA datasheet(8/12 Pages) MICROCHIP

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TC7660

CHARGE PUMP DC-TO-DC

VOLTAGE CONVERTER

DS21465A

TC7660-7 9/30/96

Figure 6. External Clocking

Figure 7. Lowering Oscillator Frequency

Positive Voltage Multiplication

The TC7660 may be employed to achieve positive

voltage multiplication using the circuit shown in Figure 8. In

this application, the pump inverter switches of the TC7660

are used to charge C1 to a voltage level of V+– VF (where V+

is the supply voltage and VF is the forward voltage drop of

diode D1). On the transfer cycle, the voltage on C1 plus the

supply voltage (V+) is applied through diode D2 to capacitor

C2. The voltage thus created on C2 becomes (2 V+) – (2 VF),

or twice the supply voltage minus the combined forward

voltage drops of diodes D1 and D2.

The source impedance of the output (VOUT) will depend

on the output current, but for V+ = 5V and an output current

of 10 mA, it will be approximately 60

Ω.

Figure 8. Positive Voltage Multiplier

Combined Negative Voltage Conversion

and Positive Supply Multiplication

Figure 9 combines the functions shown in Figures 3 and

8 to provide negative voltage conversion and positive volt-

age multiplication simultaneously. This approach would be,

for example, suitable for generating +9V and – 5V from an

existing +5V supply. In this instance, capacitors C1 and C3

perform the pump and reservoir functions, respectively, for

the generation of the negative voltage, while capacitors C2

and C4 are pump and reservoir, respectively, for the multi-

plied positive voltage. There is a penalty in this configuration

which combines both functions, however, in that the source

impedances of the generated supplies will be somewhat

higher due to the finite impedance of the common charge

pump driver at pin 2 of the device.

Figure 9. Combined Negative Converter and Positive Multiplier

Efficient Positive Voltage

Multiplication/Conversion

Since the switches that allow the charge pumping op-

eration are bidirectional, the charge transfer can be per-

formed backwards as easily as forwards. Figure 10 shows

a TC7660 transforming – 5V to +5V (or +5V to +10V, etc.).

The only problem here is that the internal clock and switch-

drive section will not operate until some positive voltage has

been generated. An initial inefficient pump, as shown in

Figure 9, could be used to start this circuit up, after which it

will bypass the other (D1 and D2 in Figure 9 would never turn

on), or else the diode and resistor shown dotted in Figure 10

can be used to "force" the internal regulator on.

TC7660

V +

CMOS

GATE

µF

VOUT

µF

1 k

Ω

V +

VOUT

COSC

TC7660

V +

VOUT =

(2 V +) – (2 VF)

VOUT =

– (V+– VF)

TC7660

V+

VOUT =

(2 V+) – (2 VF)

TC7660

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