It is especially useful in situations where the input power

source is limited in the amount of current it can deliver, since

you avoid loading down this type of power supply.

Under some operating conditions, a Soft-start capacitor is

required for proper operation.

Figure 5 indicates the input

voltage and ambient temperature conditions for which a

Soft-start capacitor may be required.

This curve is typical for full guaranteed output current and

can be used as a guideline. As the output current decreases,

the operating area requiring a Soft-start capacitor de-

creases. Capacitor values between 0.1 µF and 1 µF are rec-

ommended. Tantalum or ceramic capacitors are appropriate

for this application.

INPUT CAPACITOR

C

more than 6" away from the main filter or bypass capacitor. A

low ESR aluminum or tantalum bypass capacitor is recom-

mended between the input pin and ground to prevent large

voltage transients from appearing at the input. In addition, to

be conservative, the RMS current rating of the input capaci-

With a 1A load, a capacitor with a RMS current rating of at

least 500 mA is recommended.

The voltage rating should be approximately 1.25 times the

maximum input voltage. With a nominal input voltage of 12V,

an aluminum electrolytic capacitor (Panasonic HFQ series or

Nichicon PL series or equivalent) with a voltage rating

greater than 15V (1.25 x V

Solid tantalum input capacitors should only be used where

the input source is impedance current limited. High dV/dt ap-

plied at the input can cause excessive charge current

through low ESR tantalum capacitors. This high charge cur-

rent can result in shorting within the capacitor. It is recom-

mended that they be surge current tested by the manufactur-

er.The TPS series available from AVX, and the 593D series

from Sprague are both surge current tested.

Use caution when using ceramic capacitors for input bypass-

ing, because it may cause ringing at the V

LOWERING OUTPUT RIPPLE

When using the adjustable parts, one can achieve lower out-

put ripple voltage by shorting a resistor internal to the

LM2825. However, if this resistor is shorted, a feed forward

capacitor must be used to keep the regulator stable. For this

reason, this resistor must be left open on all of the fixed out-

put voltage versions or instability will result. See the feed for-

ward capacitor selection below. Shorting the internal resistor

is accomplished by shorting pins 8 and 9 on the LM2825,

and will typically reduce output ripple by 25 to 33%.

FEED FORWARD CAPACITOR SELECTION (C

When using an adjustable part and pins 8 and 9 are shorted

to reduce output ripple, a feed forward capacitor is required.

This capacitor is typically between 680 pF and 2700 pF. The

table of

Figure 4 shows the value for C

voltage and feedback resistor R

SHUTDOWN

The circuit shown in

Figure 10 shows 2 circuits for the

Shutdown/Soft-start feature using different logic signals for

shutdown and using a 0.1 µF Soft-start capacitor.

THERMAL CONSIDERATIONS

The LM2825 is available in a 24-pin through hole DIP. The

package is molded plastic with a copper lead frame. When

the package is soldered to the PC board, the copper and the

board are the heat sink for the LM2825.

V

OUT

R2

C

FF

LM2825-ADJ

2

630

N/A

3

1.43k

N/A

4

2.26k

2700 pF

5

3.09k

2700 pF

6

3.92k

2200 pF

7

4.75k

1800 pF

8

5.49k

1500 pF

LM2825H-ADJ

7

4.75k

2700 pF

8

5.49k

2200 pF

9

6.34k

1800 pF

10

7.15k

1500 pF

11

8.06k

1000 pF

12

8.87k

820 pF

13

9.53k

680 pF

14

10.5k

680 pF

15

11.3k

680 pF

FIGURE 4. C

DS012661-22

FIGURE 5. Usage of the Soft-start Capacitor

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