Datasheet

LM2825
www.ti.com SNVS127C –MAY 1997–REVISED APRIL 2013
LOWERING OUTPUT RIPPLE
When using the adjustable parts, one can achieve lower output 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 output voltage versions or instability will result.
See the FEED FORWARD CAPACITOR SELECTION (C
FF
) 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
FF
)
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. Table 1 shows the value for C
FF
for a given
output voltage and feedback resistor R
2
(R1 = 1 kΩ).
Table 1. C
FF
Selection Table
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
SHUTDOWN
The circuit shown in Figure 24 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 PDIP. 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.
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