Datasheet

¸
¹
·
¨
©
§
36V
I
PP(MAX)
=
x 1 -
3.3V
3.3V
6.8 PH x 230 kHz
= 1.9A
¸
¹
·
¨
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§
36V
L
O
=
x 1 -
3.3V
3.3V
9A x 0.2 x 230 kHz
= 7.2 PH
( )
OUT OUT
O
PP(MAX) SW
IN MAX
V V
L 1 [H]
I f V
æ ö
ç ÷
= ´ -
ç ÷
´
è ø
5.2 x 10
9
R
T
=
230 x 10
3
- 948 = 21.7 k:
LM25117
LM25117-Q1
www.ti.com SNVS714E –APRIL 2011–REVISED MARCH 2013
DESIGN EXAMPLE
OPERATING CONDITIONS
• Output Voltage V
OUT
= 3.3V
• Full Load Current I
OUT
= 9A
• Minimum Input Voltage V
IN(MIN)
= 6V
• Maximum Input Voltage V
IN(MAX)
= 36V
• Switching Frequency f
SW
= 230kHz
• Diode Emulation Yes
• External VCC Supply No
TIMING RESISTOR R
T
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 from Equation 3.
(25)
A standard value of 22.1kΩ was chosen for R
T
.
OUTPUT INDUCTOR L
O
The maximum inductor ripple current occurs at the maximum input voltage. Typically, 20% to 40% of the full load
current is a good compromise between core loss and copper loss of the inductor. Higher ripple current allows for
a smaller inductor size, but places more of a burden on the output capacitor to smooth the ripple voltage on the
output. For this example, a ripple current of 20% of 9A was chosen. Knowing the switching frequency, maximum
ripple current, maximum input voltage and the nominal output voltage, the inductor value can be calculated as
follows:
(26)
(27)
The closest standard value of 6.8μH was chosen for L
O
. Using the value of 6.8μH for L
O
, calculate I
PP
again. This
step is necessary if the chosen value of L
O
differs significantly from the calculated value.
From Equation 11,
(28)
At the minimum input voltage, this value is 0.95A.
DIODE EMULATION FUNCTION
The DEMB pin is left floating since this example uses diode emulation to reduce the power loss under no load or
light load conditions.
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