Datasheet

6.8 PH
1 x 820 pF x 8 m:x 10
R
RAMP
=
= 104 k:
L
O
K x C
RAMP
x R
S
x A
S
R
RAMP
=
[:@
8 m:
6.8 PH
36V x 100 ns
+
I
LIM_PK
=
0.12V
= 15.5A
¸
¹
·
¨
©
§
36V
P
RS
= 1 - x 9A
2
x 8 m: = 0.59W
3.3V
¸
¹
·
¨
©
§
V
IN(MAX)
P
RS
= 1 -
I
OUT
2
x R
S
V
OUT
x
[W]
9A x 1.5 +
230 kHz x 6.8 µH
3.3 x 1
R
S
=
0.12V
= 7.9 m:
0.95A
2
_
R
S
=
V
CS(TH)
>:@
I
OUT(MAX)
+
f
SW
x L
O
V
OUT
x K I
PP
2
_
LM25117
LM25117-Q1
SNVS714E –APRIL 2011–REVISED MARCH 2013 www.ti.com
CURRENT SENSE RESISTOR R
S
The performance of the converter will vary depending on the K value. For this example, K=1 was chosen to
control sub-harmonic oscillation and achieve one-cycle damping. The maximum output current capability
(I
OUT(MAX)
) should be 20~50% higher than the required output current, to account for tolerances and ripple
current. For this example, 150% of 9A was chosen. The current sense resistor value can be calculated from
Equation 9 and Equation 10 as follows:
(29)
(30)
A value of 8mΩ was chosen for R
S
. A larger value resistor can be placed in parallel with R
S
to adjust the
maximum output current capability. The sense resistor must be rated to handle the power dissipation at
maximum input voltage when current flows through the low-side NMOS for the majority of the PWM cycle. The
maximum power dissipation of R
S
can be calculated as:
(31)
(32)
The worst case peak inductor current under the output short condition can be calculated from Equation 12 as
follows:
(33)
Where t
ON(MIN)
is normally 100ns.
CURRENT SENSE FILTER R
CS
and C
CS
The LM25117 itself is not affected by the large leading edge spike because it samples valley current just prior to
the onset of the high-side switch. A current sense filter is used to minimize a noise injection from any external
noise sources. In general, a current sense filter is not necessary. In this example, a current sense filter is not
used
Adding R
CS
resistor changes the current sense amplifier gain which is defined as A
S
=10k / (1k+R
CS
). A small
value of R
CS
resistor below 100Ω is recommended to minimize the gain change which is caused by the
temperature coefficient difference between internal and external resistors.
RAMP RESISTOR R
RAMP
and RAMP CAPACITOR C
RAMP
The positive slope of the inductor current ramp signal is emulated by R
RAMP
and C
RAMP
. For this example, the
value of C
RAMP
was set at the standard capacitor value of 820pF. With the inductor, sense resistor and the K
factor selected, the value of R
RAMP
can be calculated from Equation 4 as follows:
(34)
(35)
The standard value of 105 kΩ was selected for R
RAMP
.
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