Datasheet

t
SS
=
0.8V x C6
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
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6.2 Inductor: L1
The value of the inductor was selected to allow the device to achieve a 12V to 3.3V conversion at 750kHz
to provide a peak-to-peak ripple current 679mA, which is about 23% 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 LM20343 evaluation board, a
Vishay IHLP4040DZER4R7M11 inductor offers a good balance between efficiency (13 mΩ DCR), size,
and saturation current rating (7.6A 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 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, 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
25 mV.
6.4 Soft-Start Capacitor: C6
A soft-start capacitor can be used to control the startup time of the LM20343 voltage regulator. The startup
time of the regulator when using a soft-start capacitor can be estimated by Equation 6:
(6)
For the LM20343, I
SS
is nominally 5 µA. For the evaluation board, the soft-start time has been designed to
be roughly 15 ms, resulting in a C
SS
capacitor value of 100 nF.
6.5 VCC Bypass: C5
The capacitor C5 is used to bypass the internal 4.5V sub-regulator. A value of 1 µF is sufficient for most
applications.
6.6 Boot Capacitor: C4
C4 is the boot capacitor which is used to provide the charge needed to drive the high-side FET. An
optimal value for this capacitor is 0.1 µF.
6.7 Compensation Capacitor: C8
The capacitor C8 is used to set the crossover frequency of the LM20343 control loop. Since this board
was optimized to be stable over the full input and output voltage range, the value of C8 was selected to be
820 pF. Once the operating conditions for the device are known, the transient response can be optimized
by reducing the value of C8 and calculating the value for R7 as outlined in the next section.
7
SNVA326B–May 2008–Revised May 2013 AN-1792 LM20343 Evaluation Board
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