TC3682 datasheet(5/10 Pages) MICROCHIP

5 / 10 page

 2002 Microchip Technology Inc.

DS21556B-page 5

TC3682/TC3683/TC3684

4.0

APPLICATIONS INFORMATION

4.1

Output Voltage Considerations

The

TC3682/TC3683/TC3684

perform

inverting

voltage conversions but do not provide any type of

regulation. The output voltage will droop in a linear

manner with respect to the output load current. The

value of the equivalent output resistance of the ‘-VIN’

output is approximately 50

Ω nominal at +25°C and

VIN = +5V. The value of the ‘-2VIN’ output is approxi-

mately 140

Ω nominal at +25°C and VIN = +5V. In this

particular case, ‘-VIN’ is approximately -5V and ‘-2VIN’

is approximately -10V at very light loads and each

stage will droop according to the equation below:

VDROOP = IOUT x ROUT

4.2

Capacitor Selection

In order to maintain the lowest output resistance and

output ripple voltage, it is recommended that low ESR

capacitors be used. Additionally, larger values of C1

and C2 will lower the output resistance and larger

values of COUT will reduce output ripple.

Note:

For proper charge pump operation, C1

must have a voltage rating greater than or

equal to VIN, while C2 and COUT must have

a voltage rating greater than or equal to

2VIN.

Table 4-1 shows various values of C1/C2 and the

corresponding output resistance values for VIN = 5V

@+25°C.

Table 4-2 shows the output voltage ripple for various

values of COUT (again assuming VIN = 5V @ +25°C).

The VRIPPLE values assume a 1mA output load current

and a 0.1

Ω ESRCOUT.

TABLE 4-1:

OUTPUT RESISTANCE

VS. C1/C2 (ESR = 0.1

Ω)

TABLE 4-2:

OUTPUT VOLTAGE RIPPLE

VS. COUT (ESR = 0.1Ω)

IOUT = 1mA

4.3

Input Supply Bypassing

The VIN input should be capacitively bypassed to

reduce AC impedance and minimize noise effects due

the

switching

internal

the

device.

recommended that a large value capacitor (at least

equal to C1) be connected from VIN to GND for optimal

circuit performance.

4.4

Shutdown Input

The TC3682/TC3683/TC3684 is enabled when SHDN

is high, and disabled when SHDN is low. This input

cannot be allowed to float. (If SHDN is not required, see

the TC2682/TC2683/TC2684 data sheet.) The SHDN

input should be limited to 0.3V above VIN.

4.5

Inverting Voltage Doubler

The most common application for the TC3682/TC3683/

TC3684 devices is the inverting voltage doubler

(Figure 4-1). This application uses three external

capacitors: C1, C2 and COUT.

Note:

power

supply

bypass

capacitor

recommended.

The output is equal to -2VIN plus any voltage drops due

to loading. Refer to Table 4-1 and Table 4-2 for

capacitor selection guidelines.

FIGURE 4-1:

INVERTING VOLTAGE

DOUBLER TEST CIRCUIT

4.6

Layout Considerations

As with any switching power supply circuit, good layout

practice is recommended. Mount components as close

together as possible to minimize stray inductance and

capacitance. Also use a large ground plane to minimize

noise leakage into other circuitry.

C1, C2

(

µF)

TC3682

ROUT(Ω)

TC3683

ROUT(Ω)

TC3684

ROUT(Ω)

0.33

633

184

120

262

120

102

3.3

120

COUT

(

µF)

TC3682

VRIPPLE

(mV)

TC3683

VRIPPLE

(mV)

TC3684

VRIPPLE

(mV)

0.33

192

632116

3.3

VIN

CIN

VOUT

VIN

COUT

TC3682

TC3683

TC3684

C1–

C2–

C1+

C2+

GND

Device

CIN

COUT

TC3682

3.3

µF

3.3

µF

3.3

µF

3.3

µF

TC3683

µF

TC3684

0.33

µF

0.33

µF

0.33

µF

0.33

µF

SHDN

-2VIN

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