Datasheet

10V/V x 10 m:
5 PA/V x 6 PH
C
RAMP
=
= 300 pF
A x R
S
g
m
x L
C
RAMP
,
5V
7A +
0.11V
R
S
2 x 6 PH x 250 kHz
5V
7V
1 +
x
0.011:
d d
V
OUT
I
O
+
V
CS(TH)
R
S
2 x L x f
SW
V
OUT
V
IN(MIN)
1 +
x
d
I
LIM
=
V
CS(TH)
R
S
0.4 x 7A x 250kHz
5V
L =
x
5V
42V
1 -
= 6.3 PH
I
PP
x f
SW
V
OUT
L =
x
V
OUT
V
IN(MAX)
1 -
LM25116
www.ti.com SNVS509D –APRIL 2007–REVISED FEBRUARY 2013
Knowing the switching frequency (f
SW
), maximum ripple current (I
PP
), maximum input voltage (V
IN(MAX)
) and the
nominal output voltage (V
OUT
), the inductor value can be calculated:
(8)
The maximum ripple current occurs at the maximum input voltage. Typically, I
PP
is 20% to 40% of the full load
current. When running diode emulation mode, 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 low output ripple voltage. For this example, 40% ripple current was chosen for a smaller sized
inductor.
(9)
The nearest standard value of 6 µH will be used. The inductor must be rated for the peak current to prevent
saturation. During normal operation, the peak current occurs at maximum load current plus maximum ripple.
During overload conditions with properly scaled component values, the peak current is limited to V
CS(TH)
/ R
S
(See CURRENT SENSE RESISTOR). 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. The peak current in steady state
will increase to V
IN(MAX)
x t
ON(min)
/ L above this level. The chosen inductor must be evaluated for this condition,
especially at elevated temperature where the saturation current rating may drop significantly.
CURRENT SENSE RESISTOR
The current limit is set by the current sense resistor value (R
S
).
(10)
For a 5V output, the maximum current sense signal occurs at the minimum input voltage, so R
S
is calculated
from:
(11)
For this example VCCX = 0V, so V
CS(TH)
= 0.11V. The current sense resistor is calculated as:
(12)
The next lowest standard value of 10 mΩ was chosen for R
S
.
RAMP CAPACITOR
With the inductor and sense resistor value selected, the value of the ramp capacitor (C
RAMP
) necessary for the
emulation ramp circuit is:
where
• L is the value of the output inductor in Henrys, g
m
is the ramp generator transconductance (5 µA/V), and A is
the current sense amplifier gain (10 V/V). For the 5V output design example, the ramp capacitor is calculated
as: (13)
(14)
The next lowest standard value of 270 pF was selected for C
RAMP
. A COG type capacitor with 5% or better
tolerance is recommended.
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