Datasheet

LM2825
SNVS127C –MAY 1997–REVISED APRIL 2013 www.ti.com
APPLICATION INFORMATION
PROGRAMMING OUTPUT VOLTAGE
(Selecting R1 and R2 as shown in Figure 18)
The LM2825 is available in two adjustable output versions. The LM2825-ADJ has been optimized for output
voltages between 1.23V and 8V, while the LM2825H-ADJ covers the output voltage range of 7V to 15V. Both
adjustable versions are set in the following way.
(1)
Select a value for R1 between 240Ω and 1.5 kΩ. The lower resistor values minimize noise pickup at the sensitive
adjust pin. (For lowest temperature coefficient and the best stability with time, use 1% metal film resistors.)
Select R2 with the following equation.
(2)
When programming V
OUT
, keep in mind that V
IN
must be greater than V
OUT
+ 2V for proper operation.
OPTIONAL EXTERNAL COMPONENTS
SOFT-START CAPACITOR
C
SS
: A capacitor on this pin provides the regulator with a Soft-start feature (slow start-up). The current drawn
from the source starts out at a low average level with narrow pulses, and ramps up in a controlled manner as the
pulses expand to their steady-state width. This reduces the startup current considerably, and delays and slows
down the output voltage rise time.
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 19 indicates the
input voltage and ambient temperature conditions for which a Soft-start capacitor may be required.
This curve is typical for full ensured output current and can be used as a guideline. As the output current
decreases, the operating area requiring a Soft-start capacitor decreases. Capacitor values between 0.1 μF and 1
μF are recommended. Tantalum or ceramic capacitors are appropriate for this application.
INPUT CAPACITOR
C
IN
: An optional input capacitor is required if the package is more than 6″ away from the main filter or bypass
capacitor. A low ESR aluminum or tantalum bypass capacitor is recommended 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 capacitor should be selected to be at least ½ the DC load current. 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 × V
IN
) would be needed.
Solid tantalum input capacitors should only be used where the input source is impedance current limited. High
dV/dt applied at the input can cause excessive charge current through low ESR tantalum capacitors. This high
charge current can result in shorting within the capacitor. It is recommended that they be surge current tested by
the manufacturer.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 bypassing, because it may cause ringing at the V
IN
pin.
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