TC7662ACPA datasheet(6/16 Pages) MICROCHIP

6 / 16 page

TC7662A

DS21468B-page 6

 2001-2012 Microchip Technology Inc.

4.0

TYPICAL APPLICATIONS

4.1

Simple Negative Voltage

Converter

The majority of applications will undoubtedly utilize the

TC7662A for generation of negative supply voltages.

Figure 4-1 shows typical connections to provide a

negative supply where a positive supply of +3V to +18V

is available.

FIGURE 4-1:

SIMPLE NEGATIVE

CONVERTER AND ITS

OUTPUT EQUIVALENT

The output characteristics of the circuit in Figure 4-1

are those of a nearly ideal voltage source in series with

a resistance as shown in Figure 4-1b. The voltage

source has a value of -(VDD). The output impedance

(RO) is a function of the ON resistance of the internal

MOS switches (shown in the Functional Block

Diagram), the switching frequency, the value of CP and

CR, and the ESR (equivalent series resistance) of CP

and CR. A good first order approximation for RO is:

Combining the four RSWX terms as RSW, we see that:

RSW, the total switch resistance, is a function of supply

voltage and temperature (See Section 5.0, Typical

Characteristics “Output Source Resistance” graphs),

typically 23

 at +25°C and 5V. Careful selection of C

and CR will reduce the remaining terms, minimizing the

output impedance.

High

value

capacitors

will

reduce the 1/(fPUMP x CP) component, and low ESR

capacitors will lower the ESR term. Increasing the

oscillator frequency will reduce the 1/(fPUMP x CP) term,

but may have the side effect of a net increase in output

impedance when CP > 10F and there is not enough

time to fully charge the capacitors every cycle. In a typ-

ical application when fOSC = 12kHz and C = CP = CR =

F:

Since the ESRs of the capacitors are reflected in the

output impedance multiplied by a factor of 5, a high

value could potentially swamp out a low 1/(fPUMP x CP)

term, rendering an increase in switching frequency

or filter capacitance ineffective. Typical electrolytic

capacitors may have ESRs as high as 10

.

TC7662A

μF

VDD

μF

VOUT = -V+

–

VOUT

VDD

–

RO  2(RSW1 + RSW2 + ESRCP) + 2(RSW3 + RSW4 +

ESRCP) +

+ ESRCR

fPUMP x CP

(fPUMP =

, RSWX = MOSFET switch resistance)

fOSC

RO  2 x RSW +

+ 4 x ESRCP + ESRCR

fPUMP x CP

RO  2 x 23 +

+ 4 x ESRCP + ESRCR

(5 x 123 x 10 x 10-6)

RO  (46 + 20 + 5 x ESRC)

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