Datasheet

6.8 PH
10 x 0.008: x 3 x 820 pF
R
RAMP
=
= 34.5 k:
L
10 x R
S
x K x C
RAMP
R
RAMP
=
0.008:
6.8 PH
36V x 100 ns
+
I
LIM_PEAK
=
0.12
= 15.53A
R
S
L
V
IN(MAX)
t
ON(MIN)
+
I
LIM_PEAK
=
V
CS(TH)
¸
¹
·
¨
©
§
36V
P
RS
= 1 - x 8
2
x 0.008 = 0.46W
3.3V
¸
¹
·
¨
©
§
V
IN(MAX)
P
RS
= 1 - I
OUT
2
R
S
V
OUT
10.4A +
230 kHz x 6.8 PH
3.3V x 3
R
S
=
0.12
-
2
= 0.0076
1.92A
I
OUT(MAX)
+
f
SW
x L
V
OUT
x K
R
S
=
V
CS(TH)
-
2
I
PP
L
10 x R
S
x R
RAMP
x C
RAMP
K =
LM25119/25119Q
SNVS680G –AUGUST 2010–REVISED JANUARY 2014 www.ti.com
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, 3
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, (8
A at V
OUT1
) to account for tolerances and ripple current. For this example, 130% of 8 A was chosen (10.4 A). The
current sense resistor value can be calculated as:
(13)
(14)
Where V
CS(TH)
is the current limit threshold voltage (120 mV). A value of 8 mΩ 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)
(18)
Where t
ON(MIN)
is the minimum HO on-time which is nominally 100 ns. The chosen inductor must be evaluated for
this condition, especially at elevated temperature where the saturation current rating of the inductor may drop
significantly. At the maximum input voltage with a shorted output, the valley current must fall below V
CS(TH)
/ R
S
before the high-side MOSFET is allowed to turn on.
RAMP RESISTOR AND RAMP CAPACITOR
The value of ramp capacitor (C
RAMP
) should be less than 2nF to allow full discharge between cycles by the
discharge switch internal to the LM25119. A good quality, thermally stable ceramic capacitor with 5% or less
tolerance is recommended. For this design the value of C
RAMP
was set at the standard capacitor value of 820 pF.
With the inductor, sense resistor and the K factor selected, the value of the ramp resistor (R
RAMP
) can be
calculated as:
(19)
(20)
The standard value of 34 kΩ was selected.
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Product Folder Links: LM25119/25119Q