ADP1034ACPZ-1-R7 datasheet(32/41 Pages) AD

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Data Sheet

ADP1034

APPLICATIONS INFORMATION

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Rev. 0 | 32 of 41

COMPONENT SELECTION

Feedback Resistors

The ADP1034 provides an adjustable output voltage for both fly-

back and inverting regulators. An external resistor divider sets the

output voltages for VOUT1 and VOUT3. To limit the output voltage

accuracy degradation due to the feedback bias current, ensure that

the current through the divider is at least 10 times IFB1 or IFB3. The

recommended RFB1 and RFB3 values are in the range of 50 kΩ to

250 kΩ to minimize the output voltage error due to the bias current

and to lessen the power dissipation across the feedback resistors.

Figure 82. Flyback Regulator Output Voltage Setting

Set the maximum output for the flyback regulator by

VOUT1_MAX = 252/256 × VFB1 × (1 + (RFT1/RFB1))

where:

VOUT1_MAX is the maximum flyback output voltage.

VFB1 is the flyback feedback voltage.

RFT1 is the feedback resistor from VOUT1 to FB1.

RFB1 is the feedback resistor from FB1 to SGND2.

Table 17. Recommended Feedback Resistor Values for Flyback Regulator

Maximum VOUT1 (V)

Flyback Regulator

RFT1 (MΩ)

RFB1 (kΩ)

Calculated VOUT1_MAX (V)

1.5

105

12.037

2.05

113

15.074

3.48

118

24.012

3.3

95.3

28.056

Calculate the desired VOUT1_PPC for PPC code by

VOUT1_PPC = ((PPCCODE + 1)/252) × VOUT1_MAX

where:

PPCCODE is the PPC data that corresponds to the desired flyback

output voltage, VOUT1_PPC.

VOUT1_PPC is the target flyback output voltage through PPC pro-

gramming.

The LSB size for the VOUT1_PPC is calculated by

VOUT1_PPC_LSB = VOUT1_MAX/252

Code 251 is the maximum PPC code that corresponds to the

maximum VOUT1. PPC Code 252 to Code 255 are reserved for

factory trimming purposes and is clipped to Code 251 when written

through PPC programming. Figure 83 shows the VOUT1 output vs.

PPCCODE.

Figure 83. VOUT1 Output vs. PPCCODE

The secondary side circuitry is supplied from VOUT1 with a minimum

allowable VOUT1 level of 6 V. Avoid programming the VOUT1 lower

than 6 V because doing so may cause the regulators to reset.

Figure 84 shows the output response after programming PPC to 4

Figure 84. Output Response After Programming PPC to 4 V

Figure 85. Inverting Regulator Output Voltage Setting

Set the negative output for the inverting regulator by

VOUT3 = VFB3 × (1 + (RFT3/RFB3))

where:

VOUT3 is the inverting regulator output voltage (negative sign disre-

garded).

VFB3 is the inverting regulator feedback voltage in reference to

VOUT3.

RFT3 is the feedback resistor from FB3 to SGND2.

RFB3 is the feedback resistor from VOUT3 to FB3.

As with the flyback regulator, calculate the value of the top resistor

for the target VOUT3 by the following equation:

RFT3 = RFB3 × ((VOUT3/VFB3) − 1)

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