Datasheet

10 PH
1 x 820 pF x 7.41 m:x 10
R
RAMP
=
= 165 k:
L
O
K x C
RAMP
x R
S
x A
S
R
RAMP
=
[:@
7.41 m:
10 PH
55V x 100 ns
+
I
LIM_PK
=
0.12V
= 16.7A
¹
·
©
§
55V
P
RS
= 1 - x 9A
2
x 7.41 m: = 0.47W
12V
¸
¹
·
¨
©
§
V
IN(MAX)
P
RS
= 1 -
I
OUT
2
x R
S
V
OUT
x
[W]
9A x 1.3 +
230 kHz x 10 µH
12 x 1
R
S
=
0.12V
= 7.3 m:
1.04A
_
2
R
S
=
V
CS(TH)
>:@
I
OUT(MAX)
+
f
SW
x L
O
V
OUT
x K I
PP
2
_
LM5117, LM5117-Q1
SNVS698E –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, 130% of 9A was chosen. The current sense resistor value can be calculated from
Equation 9 and Equation 10 as follows:
(24)
(25)
A value of 7.41mΩ was realized for R
S
by placing an additional 0.1Ω sense resistor in parallel with 8mΩ. 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:
(26)
(27)
The worst case peak inductor current under the output short condition can be calculated from Equation 12 as
follows:
where
• t
ON(MIN)
is normally 100ns (28)
CURRENT SENSE FILTER R
CS
and C
CS
The LM5117 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:
(29)
(30)
The standard value of 165 kΩ was selected for R
RAMP
.
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