Datasheet
R
S
L
V
IN(MAX)
t
ON(MIN)
+
I
LIM_PEAK
=
V
CS(TH)
¸
¹
·
¨
©
§
55V
P
RS
= 1 - x 8
2
x 0.01 = 0.58W
5V
¸
¹
·
¨
©
§
V
IN(MAX)
P
RS
= 1 - I
OUT
2
R
S
V
OUT
9.6A +
230 kHz x 15 PH
5V x 2.5
R
S
=
0.12
-
2
= 0.0096
1.32A
I
OUT(MAX)
+
V
OUT
x K
R
S
=
V
CS(TH)
-
2
I
PP
x L
f
SW
L
10 x R
S
x R
RAMP
x C
RAMP
K =
¸
¹
·
¨
©
§
55V
I
PP
=
x 1 -
5V
5V
15 PH x 230 kHz
= 1.32A
¸
¹
·
¨
©
§
V
IN(MAX)
I
PP
=
1 -
V
OUT
V
OUT
L x f
SW
x
¸
¹
·
¨
©
§
55V
L =
x 1 -
5V
5V
0.15 x 8A x 230 kHz
= 16.5 PH
LM5119/LM5119Q
SNVS676F –AUGUST 2010–REVISED FEBRUARY 2013
www.ti.com
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 15% of 8A was chosen as a compromise for the 10V
output.
(9)
The nearest standard value of 15μH was chosen for L. Using the value of 15µH for L, calculate I
PP
again. This
step is necessary if the chosen value of L differs significantly from the calculated value.
(10)
(11)
CURRENT SENSE RESISTOR
Before determining the value of current sense resistor (R
S
), it is valuable to understand the K factor, which is the
ramp slope multiple chosen for slope compensation. The K factor can be varied from 1 to 3 in practice and is
defined as:
(12)
The performance of the converter will vary depending on the selected K value (See Table 1). For this example,
2.5 was chosen as the K factor to minimize the power loss in sense resistor R
S
and the cross-talk between
channels. Crosstalk between the two regulators under certain conditions may be observed on the output as
switch jitter.
The maximum output current capability (I
OUT(MAX)
) should be 20~50% higher than the required output current, (8A
at V
OUT2
) to account for tolerances and ripple current. For this example, 120% of 8A was chosen (9.6A). The
current sense resistor value can be calculated as:
(13)
(14)
Where V
CS(TH)
is the current limit threshold voltage (120mV). A value of 10mΩ was chosen for R
S
. The sense
resistor must be rated to handle the power dissipation at maximum input voltage when current flows through the
free-wheel MOSFET for the majority of the PWM cycle. The maximum power dissipation of R
S
can be calculated:
(15)
(16)
During output short condition, the worst case peak inductor current is limited to:
(17)
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