PT4821 datasheet(29/32 Pages) TI

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Application Notes

Adjusting the Output Voltages of the PT4820

Triple-Output DC/DC Converters

The output voltages of the PT4820 series of triple-output

DC/DC converters, Vo1, Vo2 and Vo3 are independently

adjustable. The adjustment method uses a single external

resistor, 1 which may be used to adjust a selected output

by up to a nominal ±10% from the factory preset value.

The value of the resistor determines the magnitude of

adjustment, and the placement of the resistor determines

the direction of adjustment (up or down). Resistor val-

ues can be calculated using the appropriate formula (see

below) and the constants provided in Table 3-2. Alter-

natively the value may be selected directly from Table

3-3. The placement of each resistor is detailed as fol-

lows.

Adjust Up:

To increase a specific output, add a resistor R1

between the appropriate Vox Adj (Vo1 Adj, Vo2 Adj, or

Vo3 Adj) and the output common (COM). See Fig-

ure 3-1(a) and Table 3-1 for the resistor placement and

pin connections.

Figure 3-1b

PT4820 Series

Calculation of Adjust Values

The adjust resistor values may be calculated. Use the

applicable formula and select the appropriate constants

from Table 2 for the output and model being adjusted.

R1 [Adjust Up] 3

Ro · Vr

– Rs

Ω

Va – Vo

(R2) [Adjust Down] 3

Ro (Va – Vr )

– Rs kΩ

Vo – Va

Where: Vo = Original output voltage

Va = Adjusted output voltage

Vr = The reference voltage from Table 3-2

Ro = The resistance value in Table 3-2

Rs = The series resistance from Table 3-2

+Vo

x Adj

COM

+Vo

(Adjusted Down)

Output

Common

PT4820

9,10,14,18

# - See Table 3-1 for pin connections,

where Vo

x equals Vo 1, Vo2, or Vo 3

Figure 3-1a

+Vo

x Adj

COM

+Vo

(Adjusted Up)

Output

Common

PT4820

9,10,14,18

# - See Table 3-1 for pin connections,

where Vo

x equals Vo 1, Vo2, or Vo 3

Adjust Down:

Add a resistor (R2), between the appropriate

Vox Adj (Vo1, Vo2, or Vo3) and the output being adjusted.

See Figure 3-1(b) and Table 1 for the resistor placement

and pin connections.

Table 3-1; Adjust Resistor Pin Connections

To Adjust Up

To Adjust Down

Connect R1

Connect (R2)

from

Vox Adj

COM

Vox Adj

Vox

Vo1

Vo2

Vo3

Notes:

1. Use only a single 1% (or better) tolerance resistor in

either the R1 or (R2) location to adjust a specific output.

Place the resistor as close to the ISR as possible.

2. Never connect capacitors to any of the ‘Vox Adj’ pins. Any

capacitance added to these control pins will affect the

stability of the respective regulated output.

3. Adjustments made to any output must also comply with

the following limitations.

Vo1

≥ (Vo2 + 0.5V), and

Vo1

≥ (Vo3 + 0.5V)

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