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PT4842 Datasheet(PDF) 11 Page - Texas Instruments |
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PT4842 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 14 page Application Notes For technical support and more information, see inside back cover or visit www.ti.com Adjusting the Output Voltage of the PT4840 Dual Output Voltage DC/DC Converters The Vo2 and Vo3 output voltages from the PT4840 se- ries of DC/DC converters can be independantly adjusted by up to ±10% from their factory pre-set voltage. The method of adjustment uses a single external resistor. 1 The value of the resistor determines the magnitude of adustment, and the placement of the resistor determines the direction of adjustment (up or down). The resistor value can be calculated directly from a simple formula. The constants are given in Table 1. Alternatively, Table 2 provides the resistor values for a select number of output voltages. The placement of each resistor is as follows. Vo2 Adjust Up: To increase the output, add a resistor R1 between pin 17 (Vo2 Adjust) and pin 18 (COM). Vo2 Adjust Down: Add a resistor (R2), between pin 17 (Vo2 Adjust) and pin 16 (+Vo1). Vo3 Adjust Up: Add a resistor R3 between pin 21 (Vo3 Adjust) and pins 20 (COM). Vo3 Adjust Down: Add a resistor (R4) between pin 21 (Vo3 Adjust) and pin 22 (+Vo3). Refer to Figure 1 for further information on resistor placement. Figure 1 Notes: 1. Use only a single 1% resistor in either the R1 or (R2) location to adjust Vo2, and in the R3 or (R4) location to adjust Vo3. Place the resistor as close to the ISR as possible. 2. Never connect capacitors to either the Vo2 Adjust or Vo3 Adjust pins. Any capacitance added to these control pins will affect the stability of the respective regulated output. The adjust resistor values may be calculated. Use the applicable formula and select the appropriate constants from Table 1 for the output and model being adjusted. R1 or R3 = Ro · Vr – Rs k Ω Va – Vo (R2) or (R4) = Ro (Va – Vr ) – Rs k Ω Vo – Va Where: Vo = Original output voltage, (Vo2 or Vo3) Va = Adjusted output voltage Vr = The reference voltage from Table 1 Ro = The resistance value in Table 1 Rs = The series resistance from Table 1 PT4840 Series +V IN – V IN +Vo 1 +Vo 2 +Vo 3 PT4840 1 2 11 15, 16 22, 23 3 Vo 1 COM COM Vo 2 Vo 3 +V IN -V IN EN 1 Vo 2 ad j Vo 3 ad j 17 21 C OUT 1 C OUT 3 C OUT 2 C IN 4 EN 2 ( R 4 ) Ad j Down R 3 Ad j ust Up ( R 2 ) Ad j Down R 1 Ad j ust Up 12, 13 18 – 20 COM |
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