ADP1031ACPZ-2-R7 datasheet(29/38 Pages) AD

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Data Sheet

ADP1031

Rev. A | Page 29 of 38

APPLICATIONS INFORMATION

COMPONENT SELECTION

Feedback Resistors

The ADP1031 provides an adjustable output voltage for both

flyback and inverting regulators. An external resistor divider

sets the output voltage where the divider output must equal the

appropriate feedback reference voltage, VFB1 or VFB3. 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. The external feedback resistors are

not required for the fixed output versions because the feedback

resistors are already inside the chip.

VOUT1

6V TO 28V

Tx1

VINP

FB1

VOUT1

VINP

SWP

CFLYBK

SGND2

RFT1

RFB1

FLYBACK

1:1

4.7µF

Figure 70. Flyback Regulator Output Voltage Setting

Set the positive output for the flyback regulator by

VOUT1 = VFB1 × (1 + (RFT1/RFB1))

where:

VOUT1 is the 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.

Conversely, calculate the value of the top resistor for the target

VOUT1 by:

RFT1 = RFB1 × ((VOUT1/VFB1) − 1)

VOUT3

–24V TO –5V

SW3

FB3

VOUT3

CINV

RFB3

RFT3

INVERTER

100µH

4.7µF

SGND2

Figure 71. 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

disregarded).

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)

Table 15. Recommended Feedback Resistor Values

Desired

Output

Voltage (V)

Flyback/Inverting Regulator

RFT1/RFT3

(MΩ)

RFB1/RFB3

(kΩ)

Calculated Output

Voltage (V)

±6

0.715

110

±6.000

±9

1.24

121

±8.998

±12

1.54

110

±12.000

±15

2.15

121

±15.015

±24

3.48

120

±24.000

+28

3.4

100

+28.000

Capacitor Selection

Higher output capacitor values reduce the output voltage ripple

and improve the load transient response. When choosing this

value, it is also important to account for the loss of capacitance

due to the output voltage dc bias.

Ceramic capacitors are manufactured with a variety of

dielectrics, each with a different behavior over temperature and

applied voltage. Capacitors must have a dielectric adequate to

ensure the minimum capacitance over the necessary temperature

range and dc bias conditions. X5R or X7R dielectrics with

voltage ratings of 25 V to 50 V (depending on output) are

recommended for best performance. Y5V and Z5U dielectrics

are not recommended for use with any dc-to-dc converter

because of their poor temperature and dc bias characteristics.

Calculate the worst case capacitance accounting for capacitor

variation over temperature, component tolerance, and voltage

using the following equation:

CEFFECTIVE = CNOMINAL × (1 − TEMPCO) × (1 − DCBIASCO) ×

(1 − Tolerance)

where:

CEFFECTIVE is the effective capacitance at the operating voltage.

CNOMINAL is the nominal capacitance shown in this data sheet.

TEMPCO is the worst case capacitor temperature coefficient.

DCBIASCO is the dc bias derating at the output voltage.

Tolerance is the worst case component tolerance.

To guarantee the performance of the device, it is imperative to

evaluate the effects of dc bias, temperature, and tolerances on

the behavior of the capacitors for each application.

Capacitors with lower effective series resistance (ESR) and

effective series inductance (ESL) are preferred to minimize

voltage ripple.

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