PT5821 datasheet(7/10 Pages) TI

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Application Notes

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PT5820 Series

Adjusting the Output Voltage of the

PT5820 Series of Step-Down ISRs

Using Margin Up/ Margin Down

The Margin Up (pin 4) and Margin Down (pin 3) control

inputs allow the output voltage to be easily adjusted by

approximately ±5% of the the set-point voltage. These

control inputs provide a convenient method for applying

a momentary adjustment to test a load circuit’s supply

voltage margins. To activate, simply connect the appro-

priate control input to the Sense(–) (pin 8), or the local

starpoint ground. Either a logic level MOSFET or a

p-channel JFET is recommended for this application.

Using the ‘V

Adjust’ Control

For a more permanent and precise adjustment, use the

Vo Adjust control (pin 1). The Vo Adjust control allows

adjustment in any increment by up to ±10% of the set-

point. The adjustment method requires the addition of a

single external resistor. Table 3-1 gives the allowable

adjustment range for each model of the series as Va (min)

and Va (max). The value of the external resistor can either

be calculated using the formulas given below, or simply

selected from the range of values provided in Table 3-2.

Refer to Figure 3-1 for the placement of the required

resistor. Use the resistor R1 to adjust up, and the resistor

(R2) to down.

Adjust Up: An increase in the output voltage is obtained

by adding a resistor R1, between Vo Adjust (pin 1) and

Sense(–) (pin 8). See Figure 3-1.

Adjust Down: Add a resistor (R2), between Vo Adjust (pin 1)

and Sense(+) (pin 18). See Figure 3-1.

Figure 3-1; V

Adjust Resistor Placement

Vo Adjust Resistor Calculations

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

also be calculated using the following formulas. Again, use

Figure 3-1for the placement of the required resistor;

either R1 or (R2) as appropriate.

Vr · Ro

– 24.9

Ω

Va – Vo

(R2)

Ro (Va – Vr)

– 24.9

Ω

Vo – Va

Where: Vo = Original output voltage

Va = Adjusted output voltage

Vr = The reference voltage in Table 3-1

Ro = The resistance constant in Table 3-1

Table 3-1

ISR OUTPUT VOLTAGE ADJUSTMENT RANGE AND FORMULA PARAMETERS

Series Pt. No.

PT5827

PT5821

PT5822

PT5823

PT5824

PT5825

PT5826

Vo (nom)

5.0V

3.3V

2.5V

1.8V

1.5V

1.2V

1.0V

Va (min)

4.0V

2.6V

2.0V

1.52V

1.31V

1.1V

0.94V

Va (max)

5.5V

3.63V

2.8V

2.1V

1.82V

1.52V

1.32V

1.27V

0.8V

Ro (k

Ω

Ω)

10.0

10.2

10.7

10.2

9.76

10.0

10.2

Notes:

1. Use a 1% (or better) tolerance resistor in either the R1 or

(R2) location. Place the resistor as close to the ISR as

possible.

2. Never connect capacitors from Vo Adjust to either GND or

Vout. Any capacitance added to the Vo Adjust pin will affect

the stability of the ISR.

3. If the remote sense feature is not being used, the adjust

resistor (R2) can be connected to Vout, (pins 14-17) instead

of Sense (+).

PT5820

9–13

14–17

OUT

330µF

(Optional)

GND

OUT

Sense (+) [Note 3]

Sense (–)

Adj Down

O Adj

GND

Sense(+)

Sense(–)

OUT

Adjust Up

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