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Data Sheet

ADP1032

Rev. 0 | Page 29 of 37

APPLICATIONS INFORMATION

COMPONENT SELECTION

Feedback Resistors

The ADP1032 provides an adjustable output voltage for the

flyback regulator. An external resistor divider sets the output

voltage, for which the divider output must equal the appropriate

feedback reference voltage, VFB1. 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. The

recommended first feedback resistor (RFB1) 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

10µF

Figure 72. Flyback Regulator Output Voltage Setting

Set the positive output for the flyback regulator by using the

following equation:

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)

Table 16. Recommended Feedback Resistor Values

Desired Output

Voltage (V)

Flyback Regulator

RFT1 (MΩ)

RFB1 (kΩ)

Calculated Output

Voltage (V)

0.715

110

6.000

1.24

121

8.998

1.54

110

12.000

2.15

121

15.015

3.48

120

24.000

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.

FLYBACK REGULATOR COMPONENTS SELECTION

Input Capacitor

An input capacitor must be placed between the VINP pin and

ground. Ceramic capacitors greater than or equal to 3.3 μF over

temperature and voltage are recommended. The input capacitor

reduces the input voltage ripple caused by the switching current.

Place the input capacitor as close as possible to the VINP pin

and PGNDP pin to reduce input voltage spikes. The voltage

rating of the input capacitor must be greater than the maximum

input voltage.

Output Capacitor

Higher output capacitor values reduce the output voltage ripple

and improve 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. A 10 μF capacitor is recommended as a

balance between performance and size.

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