ISL78268 Datasheet, PDF (28/33 Pages) Intersil Corporation – Integrated 2A sourcing

ISL78268
Output Inductor Selection
While the Buck Converter is operating in stable continuous
conduction mode (CCM), the output voltage and on-time of the
high-side transistor is determined by Equation 17:
V_OUT
=
VIN  -^t--^O---^N-- =
T
VIN  D
(EQ. 17)
Where T is the switching cycle (1/f_SW) and D = t_ON/T is the
on-duty of the high-side transistor.
Under this CCM condition, the inductor ripple current can be
defined as Equation 18:
I_L(P-P)
=
t_ON

V_----I_--N_{------–----}V_{----O---U----T- =}
L
t_O
F
F

-^V---^O----^U---^T-
L
(EQ. 18)
From the previous equations, the inductor value will be
determined as Equation 19:
L = ^V----^I--^N------–----^V----^O---^U----^T-  -^V---^O----^U---^T-
f_SW
VIN
(EQ. 19)
In general, once the inductor value is determined, the ripple
current varies by the input voltage. At the maximum input
voltage, the on-duty becomes minimum and the ripple current
becomes maximum. So, the minimum inductor value can be
estimated from Equation 20.
L_min
=
-^V---^I--^N----^m-----^a----^x---–-----^V---^o----^u---^t
f_SW  IL_max

----^V----^O---^U----^T----
VIN_max
(EQ. 20)
In DC/DC converter design, this ripple current will be set around
20% to 50% of maximum DC output current. A reasonable
starting point to adjust the inductor value will be around 30% of
the maximum DC output current.
Increasing the value of inductor reduces the ripple current and
thus ripple voltage. However, the large inductance value may
reduce the converter’s response time to a load transient. Also,
this reduces the ramp signal and may cause a noise sensitivity
issue.
Under stable operation, the peak current flow in the inductor will
be the sum of output current and 1/2 of ripple current.
I_L
=
I_{--L---(--P-----P---)}
2

I_O
UT
(EQ. 21)
This peak current at maximum load condition must be lower
than the saturation current rating of the inductor with enough
margin. In the actual design, the largest peak current may be
observed at the start-up or heavy load transient. Therefore, the
inductor’s size needs to be determined with the consideration of
these conditions. In addition, to avoid exceeding the inductor’s
saturation rating, it is recommended to set the OCP trip point
between the maximum peak current and the inductor’s
saturation current rating.
Output Capacitor
To filter the inductor current ripples and to have sufficient
transient response, an output capacitor is required.
The current mode control loop allows the usage of lower ESR
ceramic capacitors and thus enables smaller board layout.
Electrolytic and polymer capacitors may also be used.
However, additional consideration may be needed to use the
ceramic capacitors. While the ceramic capacitor offers excellent
overall performance and reliability, the actual capacitance may
be considerably lower than the advertised value if used DC
biased condition. The effective capacitance can be easily 50%
lower than that of the rated value.
The following are equations for the required capacitance value to
meet the desired ripple voltage level. Additional capacitance may
be used to lower the ripple voltage and to improve transient
response.
For the ceramic capacitor (low ESR):
V_OUTripple
=
---------------^---^I--^L---------------
8  f_SW  C_OUT
(EQ. 22)
Where I_L is the inductor’s peak-to-peak ripple current, f_SW is the
switching frequency and C_OUT is the output capacitor.
Required minimum output capacitance based on ripple current
will be:
C_OUTmin
=
-------------------^----^I--^L--------------------
8  f_SW  V_OUTmin
(EQ. 23)
If using electrolytic capacitors, the ESR will be the dominant
portion of the ripple voltage.
V_OUTripple= I_L  ESR
(EQ. 24)
So, to reduce the ripple voltage, reduce the ripple current with
increasing the inductor value or use multiple capacitors in
parallel to reduce the ESR.
The other factor which may affect the selection of the output
capacitor will be the transient response. To estimate the
capacitance value related to transient response, a good starting
point is to determine the allowable overshoot in V_OUT if the load
is suddenly reduced. In this case, energy stored in the inductor
will be transferred to C_OUT and causing its voltage rise.
Equation 25 determines the required output capacitor value in
order to achieve a desired overshoot level relative to the
regulated voltage.
C_OUTtran
=
------------------------^I--^O----^U---^T---²----^---^L-------------------------
V_O
2
UT



V_{----O--V--U--o-T--u-m---t--a---x-}
2
–
1_
(EQ. 25)
Where V_OUTmax/V_OUT is the relative maximum overshoot
allowed during the removal of the load.
After calculating the required capacitance for both ripple and
transient needs, choose the larger of the calculated values as the
output capacitance. To keep enough capacitance over the biased
voltage and temperature range, a good quality capacitor such as
X7R or X5R is recommended.
Input Capacitor
Depending upon the system input power rail conditions, the
aluminum electrolytic type capacitor is normally used to provide
the stable input voltage and restrict the switching frequency
pulse current in small areas over the input trace for better EMC
performance. The input capacitor should be able to handle the
RMS current from the switching power devices.
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FN8657.3
December 12, 2014