Datasheet

'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
I
CIN(RMS)
= 3A
0.5 x 0.5 = 1.5A
D =
V
OUT
V
IN
I
CIN(RMS)
= I
OUT
D(1 - D)
www.ti.com Component Selection
6 Component Selection
This section provides a walk-through of the design process of the LM20333 evaluation board. Unless
otherwise indicated all equations assume units of amps (A) for current, farads (F) for capacitance, henries
(H) for inductance, and volts (V) for voltages.
6.1 Input Capacitors: C1, C2, C3
The required RMS current rating of the input capacitor for a buck regulator can be estimated by the
following equation:
(1)
The variable D refers to the duty cycle, and can be approximated by:
(2)
From this equation, it follows that the maximum I
CIN(RMS)
requirement will occur at a full 3A load current with
the system operating at 50% duty cycle. Under this condition, the maximum I
CIN(RMS)
is given by:
(3)
Ceramic capacitors feature a very large I
RMS
rating in a small footprint, making a ceramic capacitor ideal
for this application. A two 4.7 µF, X5R, 25V ceramic capacitor(C2, C3) from Murata are used to provide
the necessary input capacitance for the evaluation board. An additional 22uF, X5R, 25V capacitor is used
to provide additional input capacitance to counter cabling inductance to the input.
6.2 Inductor: L1
The value of the inductor was selected to allow the device to achieve a 12V to 3.3V conversion at 250kHz
to provide a peak to peak ripple current 957mA, which is about 32% 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 the equation:
(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 LM20333 evaluation board, a
Vishay IHLP4040DZER10R0M11 inductor offers a good balance between efficiency (28 mΩ DCR), size,
and saturation current rating (7.1A I
SAT
rating).
6.3 Output Capacitor: 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 the equation:
(5)
The variable R
ESR
above refers to the ESR of the output capacitor. As can be seen in the above equation,
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, a Sanyo 150 µF POSCAP output capacitor was selected to provide good
transient and DC performance in a relatively small package. From the technical specifications of this
capacitor, the ESR is roughly 35 mΩ, and RMS ripple current rating is 1.4A. With these values, the worst
case peak to peak voltage ripple on the output when operating from a 12V input can be calculated to be
37 mV.
7
SNVA325B–May 2008–Revised May 2013 AN-1791 LM20333 Evaluation Board
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