TC7660_02 datasheet(8/18 Pages) MICROCHIP

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TC7660

DS21465B-page 8

 2002 Microchip Technology Inc.

5.4

Changing the TC7660 Oscillator

Frequency

The operating frequency of the TC7660 can be

changed in order to optimize the system performance.

The frequency can be increased by over-driving the

OSC input (Figure 5-4). Any CMOS logic gate can be

utilized in conjunction with a 1 k

Ω series resistor. The

resistor is required to prevent device latch-up. While

TTL level signals can be utilized, an additional 10 k

Ω

pull-up resistor to V+ is required. Transitions occur on

the rising edge of the clock input. The resultant output

voltage ripple frequency is one half the clock input.

Higher clock frequencies allow for the use of smaller

pump and reservoir capacitors for a given output volt-

age ripple and droop. Additionally, this allows the

TC7660 to be synchronized to an external clock, elimi-

nating undesirable beat frequencies.

At light loads, lowering the oscillator frequency can

increase the efficiency of the TC7660 (Figure 5-5). By

lowering the oscillator frequency, the switching losses

are reduced. Refer to Figure 2-3 to determine the typi-

cal operating frequency based on the value of the

external capacitor. At lower operating frequencies, it

may be necessary to increase the values of the pump

and reservoir capacitors in order to maintain the

desired output voltage ripple and output impedance.

FIGURE 5-4:

External Clocking.

FIGURE 5-5:

Lowering Oscillator

Frequency.

5.5

Positive Voltage Multiplication

Positive voltage multiplication can be obtained by

employing two external diodes (Figure 5-6). Refer to

the theory of operation of the TC7660 (Section 4.1).

During the half cycle when switch S2 is closed, capaci-

tor C1 of Figure 5-6 is charged up to a voltage of

V+ - VF1, where VF1 is the forward voltage drop of diode

D1. During the next half cycle, switch S1 is closed, shift-

ing the reference of capacitor C1 from GND to V+. The

energy in capacitor C1 is transferred to capacitor C2

through diode D2, producing an output voltage of

approximately:

EQUATION

FIGURE 5-6:

Positive Voltage Multiplier.

5.6

Combined Negative Voltage

Conversion and Positive Supply

Multiplication

Simultaneous voltage inversion and positive voltage

multiplication can be obtained (Figure 5-7). Capacitors

C1 and C3 perform the voltage inversion, while capaci-

tors C2 and C4, plus the two diodes, perform the posi-

tive voltage multiplication. Capacitors C1 and C2 are

the pump capacitors, while capacitors C3 and C4 are

the reservoir capacitors for their respective functions.

Both functions utilize the same switches of the TC7660.

As a result, if either output is loaded, both outputs will

drop towards GND.

CMOS

GATE

Ω

VOUT

“1”

V+

TC7660

10 µF

V+

VOUT

TC7660

V+

COSC

VOUT

VF1 VF2

()

–

where:

VF1 is the forward voltage drop of diode D1

and

VF2 is the forward voltage drop of diode D2.

VOUT =

TC7660

V+

(2 V+) - (2 VF)

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