Datasheet
¸
¹
·
¨
©
§
36V
I
PP
=
x 1 -
3.3V
3.3V
6.8 PH x 230 kHz
= 1.92A
¸
¹
·
¨
©
§
V
IN(MAX)
I
PP
=
1 -
V
OUT
V
OUT
L x f
SW
x
¸
¹
·
¨
©
§
36V
L =
x 1 -
3.3V
3.3V
0.25 x 8A x 230 kHz
= 6.5 PH
¸
¹
·
¨
©
§
V
IN(MAX)
L =
x 1 -
V
OUT
V
OUT
I
PP
x f
SW
I
PP
I
O
0
T =
1
f
SW
5.2 x 10
9
R
T
=
f
SW
- 948 = 21.66 k:
LM25119/25119Q
www.ti.com
SNVS680G –AUGUST 2010–REVISED JANUARY 2014
TIMING RESISTOR
R
T
sets the switching frequency of each regulator channel. Generally, higher frequency applications are smaller
but have higher losses. Operation at 230 kHz was selected for this example as a reasonable compromise
between small size and high efficiency. The value of R
T
for 230 kHz switching frequency can be calculated as
follows:
(7)
A standard value or 22.1 kΩ was chosen for R
T
. The internal oscillator frequency is twice the switching frequency
and is about 460 kHz.
OUTPUT INDUCTOR
The inductor value is determined based on the operating frequency, load current, ripple current and the input and
output voltages.
Figure 7. Inductor Current
Knowing the switching frequency, maximum ripple current (I
PP
), maximum input voltage and the nominal output
voltage (V
OUT
), the inductor value can be calculated:
(8)
The maximum ripple current occurs at the maximum input voltage. Typically, I
PP
is 20% to 40% of the full load
current. When operating in the diode emulation mode configuration, the maximum ripple current should be less
than twice the minimum load current. For full synchronous operation, higher ripple current is acceptable. Higher
ripple current allows for a smaller inductor size, but places more of a burden on the output capacitor to smooth
the ripple current. For this example, a ripple current of 25% of 8 A was chosen as a compromise for the 1.8 V
output.
(9)
The nearest standard value of 6.8 μH was chosen for L. Using the value of 6.8 µH for L, calculate I
PP
again. This
step is necessary if the chosen value of L differs significantly from the calculated value.
(10)
(11)
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