Datasheet

LMZ14203H
www.ti.com SNVS692D –JANUARY 2011–REVISED OCTOBER 2013
INPUT CAPACITOR, C
IN
, SELECTION
The LMZ14203H module contains an internal 0.47 µF input ceramic capacitor. Additional input capacitance is
required external to the module to handle the input ripple current of the application. This input capacitance should
be located as close as possible to the module. Input capacitor selection is generally directed to satisfy the input
ripple current requirements rather than by capacitance value. Worst case input ripple current rating is dictated by
Equation 12.
I(C
IN(RMS)
) ≊ 1 / 2 x I
O
x √ (D / 1-D) (12)
where D ≊ V
O
/ V
IN
(As a point of reference, the worst case ripple current will occur when the module is presented with full load
current and when V
IN
= 2 x V
O
).
Recommended minimum input capacitance is 10uF X7R ceramic with a voltage rating at least 25% higher than
the maximum applied input voltage for the application. It is also recommended that attention be paid to the
voltage and temperature deratings of the capacitor selected. It should be noted that ripple current rating of
ceramic capacitors may be missing from the capacitor data sheet and you may have to contact the capacitor
manufacturer for this rating.
If the system design requires a certain maximum value of input ripple voltage ΔV
IN
to be maintained then
Equation 13 may be used.
C
IN
≥ I
O
x D x (1–D) / f
SW-CCM
x ΔV
IN
(13)
If ΔV
IN
is 1% of V
IN
for a 24V input to 12V output application this equals 240 mV and f
SW
= 400 kHz.
C
IN
≥ 3A x 12V/24V x (1– 12V/24V) / (400000 x 0.240 V) (14)
C
IN
≥ 7.8μF (15)
Additional bulk capacitance with higher ESR may be required to damp any resonant effects of the input
capacitance and parasitic inductance of the incoming supply lines.
ON TIME, R
ON
, RESISTOR SELECTION
Many designs will begin with a desired switching frequency in mind. As seen in the TYPICAL PERFORMANCE
CHARACTERISTICS section, the best efficiency is achieved in the 300kHz-400kHz switching frequency range.
Equation 16 can be used to calculate the R
ON
value.
f
SW(CCM)
≊ V
O
/ (1.3 x 10
-10
x R
ON
) (16)
This can be rearranged as
R
ON
≊ V
O
/ (1.3 x 10
-10
x f
SW(CCM)
(17)
The selection of R
ON
and f
SW(CCM)
must be confined by limitations in the on-time and off-time for the COT Control
Circuit Overview section.
The on-time of the LMZ14203H timer is determined by the resistor R
ON
and the input voltage V
IN
. It is calculated
as follows:
t
ON
= (1.3 x 10
-10
x R
ON
) / V
IN
(18)
The inverse relationship of t
ON
and V
IN
gives a nearly constant switching frequency as V
IN
is varied. R
ON
should
be selected such that the on-time at maximum V
IN
is greater than 150 ns. The on-timer has a limiter to ensure a
minimum of 150 ns for t
ON
. This limits the maximum operating frequency, which is governed by Equation 19.
f
SW(MAX)
= V
O
/ (V
IN(MAX)
x 150 nsec) (19)
This equation can be used to select R
ON
if a certain operating frequency is desired so long as the minimum on-
time of 150 ns is observed. The limit for R
ON
can be calculated as follows:
R
ON
≥ V
IN(MAX)
x 150 nsec / (1.3 x 10
-10
) (20)
If R
ON
calculated in Equation 17 is less than the minimum value determined in Equation 20, a lower frequency
should be selected. Alternatively, V
IN(MAX)
can also be limited in order to keep the frequency unchanged.
Additionally, the minimum off-time of 260 ns (typ) limits the maximum duty ratio. Larger R
ON
(lower F
SW
) should
be selected in any application requiring large duty ratio.
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