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AFL28XXD Series

General Application Information

The AFL28XXD series of converters are capable of

providing large transient currents to user loads on demand.

Because the nominal input voltage range in this series is

relatively low, the resulting input current demands will be

correspondingly large. It is important therefore, that the line

impedance be kept very low to prevent steady state and

transient input currents from degrading the supply voltage

between the voltage source and the converter input. In

applications requiring high static currents and large

transients, it is recommended that the input leads be made

of adequate size to minimize resistive losses, and that a

good quality capacitor of approximately 100

µfd be

connected directly across the input terminals to assure an

adequately low impedance at the input terminals. Table I

relates nominal resistance values and selected wire sizes.

Figure V. Connection for V

Input Filter

Undervoltage Lockout

The AFL28XXD series converters incorporate a two stage

LC input filter whose elements dominate the input load

impedance characteristic during the turn-on sequence. The

input circuit is as shown in Figure IV.

Figure IV. Input Filter Circuit

A minimum voltage is required at the input of the converter

to initiate operation. This voltage is set to 14V

± 0.5V. To

preclude the possibility of noise or other variations at the

input falsely initiating and halting converter operation, a

hysteresis of approximately 1.0V is incorporated in this

circuit. Thus if the input voltage droops to 13V

± 0.5V, the

converter will shut down and remain inoperative until the

input voltage returns to

≈14V.

Output Voltage Adjust

Connect R

Pin 1

Pin 2

900nH

130nH

6 µfd

11.2 µfd

By use of the trim pin (10), the magnitude of output voltages

can be adjusted over a limited range in either a positive or

negative direction. Connecting a resistor between the trim

pin and either the output return or the positive output will

raise or lower the magnitude of output voltages. The span

of output voltage adjustment is restricted to the limits shown

in Table I.

Enable 2

Share

Trim

- Vout

Return

+ Vout

To

Loads

R

ADJ

AFL28xxD

7

12

+

-

Table 1. Output Voltage Trim Values and Limits

AFL2805D

AFL2812D

AFL2815D

V

out

R

adj

V

out

R

adj

V

out

R

adj

5.5

0

12.5

0

15.5

0

5.4

12.5K

12.4

47.5K

15.4

62.5K

5.3

33.3K

12.3

127K

15.3

167K

5.2

75K

12.2

285K

15.2

375K

5.1

200K

12.1

760K

15.1

1.0M

5.0

∞

12.0

∞

15.0

∞

4.9

190K

11.7

975K

14.6

1.2M

4.8

65K

11.3

288K

14.0

325K

4.7

23K

10.8

72.9K

13.5

117K

4.6

2.5K

10.6

29.9K

13.0

12.5K

4.583

0

10.417

0

12.917

0

Note that the nominal magnitude of output voltage resides in

the middle of the table and the corresponding resistor value

is set to

adjust resistor is connected to output return. To set the

magnitude less than nominal, the adjust resistor is connected

to the positive output. (Refer to Figure V.)

For output voltage settings that are within the limits, but

between those listed in Table I, it is suggested that the

resistor values be determined empirically by selection or by

use of a variable resistor. The value thus determined can

then be replaced with a good quality fixed resistor for

permanent installation.

When use of this adjust feature is elected, the user should

be aware that the temperature performance of the converter

output voltage will be affected by the temperature

performance of the resistor selected as the adjustment

element and therefore, is advised to employ resistors with a

tight temperature coefficient of resistance.