LM2833 Datasheet, PDF (16/30 Pages) National Semiconductor (TI) – 1.5MHz/3MHz 3.0A Step-Down DC-DC Switching Regulator

LM2833
SNVS505E – MAY 2008 – REVISED APRIL 2013
www.ti.com
ESR and low ESL. A 22µF multilayer ceramic capacitor (MLCC) is a good choice for most applications. In cases
where large capacitance is required, use surface mount capacitors such as Tantalum capacitors and place at
least a 4.7µF ceramic capacitor close to the V_IN pin. For MLCCs it is recommended to use X7R or X5R
dielectrics. Consult capacitor manufacturer datasheet to see how rated capacitance varies over operating
conditions.
OUTPUT CAPACITOR
The output capacitor is selected based upon the desired output ripple and transient response. The initial current
of a load transient is provided mainly by the output capacitor. The output ripple of the converter is:
1
'V_OUT = 'I_L R_ESR + _{8 x fSW x COUT}
(12)
When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the
output ripple will be approximately sinusoidal and 90° phase shifted from the switching action. Given the
availability and quality of MLCCs and the expected output voltage of designs using the LM2833, there is really no
need to review any other capacitor technologies. Another benefit of ceramic capacitors is their ability to bypass
high frequency noise. A certain amount of switching edge noise will couple through parasitic capacitances in the
inductor to the output. A ceramic capacitor will bypass this noise while a tantalum will not. Since the output
capacitor is one of the two external components that control the stability of the regulator control loop, most
applications will require a minimum of 22µF output capacitance. In the case of low output voltage, a larger output
capacitance is required to ensure sufficient phase margin. Capacitance can often, but not always, be increased
significantly with little detriment to the regulator stability. Like the input capacitor, recommended multilayer
ceramic capacitors are X7R or X5R types. Again, verify actual capacitance at the desired operating voltage and
temperature. Check the RMS current rating of the capacitor. The maximum RMS current rating of the capacitor
is:
(13)
One may select a 1206 size MLCC for output capacitor, since its current rating is typically above 1A, more than
enough for the requirement.
CATCH DIODE
The catch diode conducts during the switch off-time. A Schottky diode is recommended for its fast switching time
and low forward voltage drop. The catch diode should be chosen such that its current rating is greater than:
I_D = I_OUT x (1-D)
(14)
The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin.
To improve efficiency, choose a Schottky diode with a low forward voltage drop.
OUTPUT VOLTAGE
The output voltage is set using the following equation where R2 is connected between the FB pin and GND, and
R1 is connected between V_OUT and the FB pin. A good value for R2 is 2kΩ.
R1 =
V_OUT
-1
x R2
V_REF
(15)
V_REF = 0.60V
(16)
16
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