ICL7660S Просмотр технического описания (PDF) - Intersil

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ICL7660S

Super Voltage Converters

Intersil 

ICL7660S, ICL7660A

Typical Applications

Simple Negative Voltage Converter

The majority of applications will undoubtedly utilize the

ICL7660S and ICL7660A for generation of negative supply

voltages. Figure 15 shows typical connections to provide a

negative supply where a positive supply of +1.5V to +12V is

available. Keep in mind that pin 6 (LV) is tied to the supply

negative (GND) for supply voltage below 3.5V.

V+

10µF

+

-

1

8

2 ICL7660S 7

3 ICL7660A 6

4

5

- VOUT = -V+

10µF +

RO VOUT

-

V+

+

15A.

15B.

FIGURE 15. SIMPLE NEGATIVE CONVERTER AND ITS

OUTPUT EQUIVALENT

The output characteristics of the circuit in Figure 15 can be

approximated by an ideal voltage source in series with a

resistance as shown in Figure 15B. The voltage source has

a value of -(V+). The output impedance (RO) is a function of

the ON resistance of the internal MOS switches (shown in

Figure 14), the switching frequency, the value of C1 and C2,

and the ESR (equivalent series resistance) of C1 and C2. A

good first order approximation for RO is shown in

Equation 2:

R0 ≅ 2((RSW1 + RSW3 + ESRC1) + 2(RSW2 + RSW4 + ESRC1))

---------------1----------------

fPUMP × C1

+

ESRC2

(EQ. 2)

fPUMP

=

f--O-----S----C--

2

(RSWX = MOSFET Switch Resistance)

Combining the four RSWX terms as RSW, we see in

Equation 3 that:

R0

≅

2x

RSW

+

---------------1----------------

fPUMP × C1

+

4

xESRC

1

+

E

S

RC2

(EQ. 3)

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

voltage and temperature (see the output source resistance

graphs, Figures 2, 3, and 11), typically 23Ω at +25°C and 5V.

Careful selection of C1 and C2 will reduce the remaining

terms, minimizing the output impedance. High value

capacitors will reduce the 1/(fPUMP x C1) component, and low

ESR capacitors will lower the ESR term. Increasing the

oscillator frequency will reduce the 1/(fPUMP x C1) term, but

may have the side effect of a net increase in output

impedance when C1 > 10µF and is not long enough to fully

charge the capacitors every cycle. Equation 4 shows a typical

application where fOSC = 10kHz and C = C1 = C2 = 10µF:

R0

≅

2

x

23

+

-------------------------1-------------------------

5 × 103 × 10 × 10–6

+

4x

E

S

RC1

+

E

S

RC

2

R0 ≅ 46 + 20 + 5 × ESRC

(EQ. 4)

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 C1 term, rendering an

increase in switching frequency or filter capacitance

ineffective. Typical electrolytic capacitors may have ESRs as

high as 10Ω.

Output Ripple

ESR also affects the ripple voltage seen at the output. The

peak-to-peak output ripple voltage is given by Equation 5:

VRI

P

P

LE

≅

⎛

⎝

--------------------1---------------------

2 × fPUMP × C2

+

2

ESRC

2

×

IOU

⎞

T⎠

(EQ. 5)

A low ESR capacitor will result in a higher performance

output.

Paralleling Devices

Any number of ICL7660S and ICL7660A voltage converters

may be paralleled to reduce output resistance. The reservoir

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

its own pump capacitor, C1. The resultant output resistance

is approximated in Equation 6:

ROUT

=

----R-----O----U----T----(--o---f---I-C----L---7---6---6---0---S----)----

n(number of devices)

(EQ. 6)

Cascading Devices

The ICL7660S and ICL7660A may be cascaded as shown to

produce larger negative multiplication of the initial supply

voltage. However, due to the finite efficiency of each device,

the practical limit is 10 devices for light loads. The output

voltage is defined as shown in Equation 7:

VOUT = –n(VIN)

(EQ. 7)

where n is an integer representing the number of devices

cascaded. The resulting output resistance would be

approximately the weighted sum of the individual ICL7660S

and ICL7660A ROUT values.

Changing the ICL7660S and ICL7660A Oscillator

Frequency

It may be desirable in some applications, due to noise or other

considerations, to alter the oscillator frequency. This can be

achieved simply by one of several methods.

By connecting the Boost Pin (Pin 1) to V+, the oscillator

charge and discharge current is increased and, hence, the

oscillator frequency is increased by approximately 3.5 times.

The result is a decrease in the output impedance and ripple.

9

FN3179.7

January 23, 2013

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