PT6641 datasheet(3/6 Pages) TI

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Application Notes

For technical support and more information, see inside back cover or visit www.ti.com/powertrends

PT6640 Series

Adjusting the Output Voltage of the PT6640 24W

Positive to Negative ISR Series

The negative output voltage of the Power Trends

PT6640 series ISRs may be adjusted higher or lower than

the factory trimmed pre-set voltage with the addition of a

single external resistor. Table 1 gives the allowable ad-

justment range for each model in the series as V

a (min)

and V

a (max).

Adjust Up:

An increase in the negative output

voltage is obtained by adding a resistor R2, between pin

14 (V

o adjust) and pins 7-10 (-Vout).

Adjust Down:

Adding a resistor (R1), between pin 14

o adjust) and pins 11-13 (GND), decreases the output

voltage magnitude.

Refer to Figure 1 and Table 2 for both the placement and

value of the required resistor, either (R1) or R2 as appro-

priate.

Notes:

1. Use only a single 1% resistor in either the (R1) or R2

location. Place the resistor as close to the ISR as possible.

2. Never connect capacitors from V

o adjust to either GND,

out, or the Remote Sense pin. Any capacitance added to

the V

o adjust pin will affect the stability of the ISR.

3. If the Remote Sense feature is being used, connecting the

resistor (R1) between pin 14 (V

o adjust) and pin 1 (Remote

Sense) can benefit load regulation.

4. The maximum allowed input voltage (V

in) will change as

out is adjusted. The difference between the input voltage

in) and the output voltage (Vout) must not exceed 30V or

× V

out, whichever is less. Use one of the following

formulas to determine the maximum allowed input voltage

for the PT6640.

V

out greater than 2.73V,

in(max)

= 30 –

V

out

Vdc

For example, if V

out = -12V,

in(max)

= 30 –

-12 = 18Vdc

V

out less than 2.73V,

in(max)

= 10

× V

out

Vdc

The values of (R1) [adjust down], and R2 [adjust up], can

also be calculated using the following formulas.

Ro (Vo – 1.25)(Va – 1.25)

(R1)

1.25 (Vo – Va)

– Rs

kΩ

Ro (Vo – 1.25)

– Rs

kΩ

Va - Vo

Where: V

= Original V

out (magnitude)

= Adjusted V

out (magnitude)

= The resistance value in Table 1

= The series resistance from Table 1

Figure 1

Table 1

PT6640 ADJUSTMENT AND FORMULA PARAMETERS

Series Pt #

PT6646

PT6641

PT6642

PT6644

PT6643

PT6645

Vo (nom)

–2.5V

–3.3V

–5.0V

–9.0V

–12.0V

–15.0V

Va (min)

–1.8V

–2.2V

–3.0V

–6.0V

–9.0V

–10.0V

Va (max)

–4.3V

–4.7V

–6.5V

–10.2V

–13.6V

–17.0V

Ro (k

Ω

Ω)

4.99

4.22

2.49

2.0

Rs (k

Ω

Ω)

2.49

4.99

12.7

560

µF

out

330

µF

(R1)

|-V

o | Down

|-V

o | Up

out

COM

PT6640

7,8,9,10

11,12,13

4,5,6

+V in

V out

GND

o(adj)

SNS(+)

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