RT6203E
The output stage of a synchronous buck converter is
composed of an inductor and capacitor, which stores and
delivers energy to the load, and forms a second-order low-
pass filter to smooth out the switch node voltage to
maintain a regulated output voltage.
in input voltage. The waveform of CIN ripple voltage and
ripple current are shown in Figure 1. The peak-to-peak
voltage ripple on input capacitor can be estimated as
equation below :
1D
IN SW
V
= DI
+ I
ESR
CIN
OUT
OUT
C
f
where
D =
Inductor Selection
V
OUT
The inductor selection trade-offs among size, cost,
efficiency, and transient response requirements.Generally,
three key inductor parameters are specified for operation
with the device: inductance value (L), inductor saturation
current (ISAT), andDC resistance (DCR).
V
IN
For ceramic capacitors, the equivalent series resistance
(ESR) is very low, the ripple which is caused by ESR can
be ignored, and the minimum input capacitance can be
estimated as equation below :
A good compromise between size and loss is to choose
the peak-to-peak ripple current equals to 10% to 50% of
the IC rated current. The switching frequency, input
voltage, output voltage, and selected inductor ripple current
determines the inductor value as follows :
D 1D
C
IN_MIN
= I
OUT_MAX
V
f
CIN_MAX SW
Where VCIN_MAX 200mV
V
(V V
)
OUT
IN
OUT
L =
V
CIN
V f
I
IN SW
L
C
Ripple Voltage
IN
Once an inductor value is chosen, the ripple current (ΔIL)
is calculated to determine the required peak inductor
current.
V
= I
x ESR
OUT
ESR
(1-D) x I
OUT
V
(V V
)
I
OUT
IN
OUT
L
C
IN
Ripple Current
I =
L
and I
= I
+
L(PEAK)
OUT_MAX
V f
IN SW
L
2
D x I
OUT
IL(PEAK) should not exceed the minimum value of IC's upper
current limit level. Besides, the current flowing through
the inductor is the inductor ripple current plus the output
current. During power up, faults or transient load
conditions, the inductor current can increase above the
calculated peak inductor current level calculated above.
In transient conditions, the inductor current can increase
up to the switch current limit of the device. For this reason,
the most conservative approach is to specify an inductor
with a saturation current rating equal to or greater than
the switch current limit rather than the peak inductor
current.
D x t
SW (1-D) x tSW
Figure 1. CIN Ripple Voltage and Ripple Current
In addition, the input capacitor needs to have a very low
ESR and must be rated to handle the worst-case RMS
input current of :
V
V
V
IN
V
OUT
OUT
I
I
1
RMS
OUT_MAX
IN
It is commonly to use the worse IRMS ≅ IOUT/2 at VIN
=
2VOUT for design. Note that ripple current ratings from
capacitor manufacturers are often based on only 2000
hours of life which makes it advisable to further de-rate
the capacitor, or choose a capacitor rated at a higher
temperature than required.
For more conservative, the rating for inductor saturation
current must be equal to or greater than switch current
limit of the device rather than the inductor peak current.
Several capacitors may also be paralleled to meet size,
height and thermal requirements in the design. For low
input voltage applications, sufficient bulk input capacitance
is needed to minimize transient effects during output load
changes.
Input Capacitor Selection
Input capacitance, CIN, is needed to filter the pulsating
current at the drain of the high-side power MOSFET. CIN
should be sized to do this without causing a large variation
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14
DS6203E-00 January 2019
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
