Datasheet

REF
V
¨
¨
©
§
OUT
V
=
2
R
1
¸
¸
¹
·
- x
1
R
LOAD
R
O
V
3
C
2
R
1
R
FB
V
¨
¨
©
§
+
x
=
ESRLOUT
RÂIÂV
xxx
OUTLoadSW
CRF2
x
OUT
DV
¹
¸
¸
·
LMR62421
www.ti.com SNVS734B –OCTOBER 2011–REVISED APRIL 2013
OUTPUT CAPACITOR
The LMR62421 operates at frequencies allowing the use of ceramic output capacitors without compromising
transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple.
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 impedance will therefore determine the
maximum voltage perturbation. The output ripple of the converter is a function of the capacitor’s reactance and
its equivalent series resistance (ESR):
(8)
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 LMR62421,
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 at 4.7 µF of output capacitance. Like the input capacitor,
recommended multilayer ceramic capacitors are X7R or X5R. Again, verify actual capacitance at the desired
operating voltage and temperature.
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the following equation where R1 is connected between the FB pin and GND, and
R2 is connected between V
OUT
and the FB pin.
Figure 16. Setting Vout
A good value for R1 is 10kΩ.
(9)
Copyright © 2011–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LMR62421