Datasheet
t
SS
=
0.6V x Css
I
SS
'V
OUT
= 'I
P-P
x
1
8 x f
SW
x C
OUT
R
ESR
+
'I
P-P
=
(V
IN
- V
OUT
) x D
L x f
SW
Component Selection
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The input capacitors also keep the input stable during load transient conditions. If the input capacitance is
too low, the input can drop below the UVLO threshold and cause the device to disable the output. This
may result in repetitive dropout and re-enable oscillation, or "motorboating". To give you the ability to
operate with a low V
IN
voltage, three 100 µF ceramic capacitors were used on the input.
7.2 Inductor: L1
The value of the inductor was selected to allow the device to achieve a 5V to 1.2V conversion at 500kHz
to provide a peak to peak ripple current of 3.2A, which is about 27% of the maximum output current. To
have an optimized design, generally the peak to peak inductor ripple current should be kept to within 20%
to 40% of the rated output current for a given input voltage, output voltage and operating frequency. The
peak to peak inductor ripple current can be calculated by Equation 4:
(4)
Once an inductance value is calculated, an actual inductor needs to be selected based on a trade-off
between physical size, efficiency, and current carrying capability. For the LM21212-2 evaluation board, a
Vishay IHLP4040DZERR56M01 inductor offers a good balance between efficiency (1.8 mΩ DCR) and
size.
7.3 Output Capacitor: C3, C4, C5, C9
The value of the output capacitor in a buck regulator influences the voltage ripple that will be present on
the output voltage as well as the large signal output voltage response to a load transient. Given the peak-
to-peak inductor current ripple (ΔI
P-P
) the output voltage ripple can be approximated by Equation 5:
(5)
The variable R
ESR
above refers to the ESR of the output capacitor. As can be seen in Equation 5, the
ripple voltage on the output can be divided into two parts, one of which is attributed to the AC ripple
current flowing through the ESR of the output capacitor and another due to the AC ripple current actually
charging and discharging the output capacitor. The output capacitor also has an effect on the amount of
droop that is seen on the output voltage in response to a load transient event.
For the evaluation board, three 100µF ceramic capacitors were selected to provide good transient and DC
performance. Ceramic capacitors give the lowest R
ESR
of any standard capacitor chemistries, resulting in
the lowest output ripple for the given ripple current. Ceramic capacitors (especially high capacitance, small
package multi-layer types, or MLCC) lose their capacitance as the DC voltage is increased. For this
configuration, the actual capacitance value was approximated to be 50 µF per capacitor, or 150 µF total.
This is lower than measured capacitance values for 1.2V, but will allow you to change the output voltage
up to 3.3V and maintain stability.
7.4 Soft-Start Capacitor: C
SS
A soft-start capacitor can be used to control the startup time of the LM21212-2 voltage regulator. The
startup time of the regulator when using a soft-start capacitor can be estimated by Equation 6:
(6)
For the LM21212-2, I
SS
is nominally 5 µA. For the evaluation board, the soft-start time has been designed
to be roughly 10 ms, resulting in a C
SS
capacitor value of 33 nF.
7.5 Compensation Components: C
C1
, C
C2
, C
C3
, R
C1
, R
C2
These components are used in conjunction with the error amplifier to create a type 3 voltage-mode
compensation network. The analysis of type 3 compensation is outside the scope of this document, but an
example of the step-by-step procedure to generate the compensation component values is given. The
parameters needed for the compensation values are given in Table 3.
8
AN-2140 LM21212-2 Evaluation Board SNVA480A–May 2011–Revised May 2013
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