Datasheet

C
OUT
=
1.19A
8 x 300 kHz x 0.005V
= 99.2 PF
C
OUT
=
I
OR(max)
8 x F
S
x V
RIPPLE
I
CL(min)
=
0.01:
(2.1 k: x 32 PA) - 9 mV
= 5.82A
I
CL(max)
=
0.01:
(2.1 k: x 48 PA) + 9 mV
= 11A
I
CL(nom)
=
0.01:
(2.1 k: x 40 PA)
= 8.4A
R
ADJ
=
32 PA
6.5A x 0.01:
= 2.03 k:
SW Node
I
PK
Inductor Current
I
OUT
I
OR
1/F
S
LM5085, LM5085-Q1
SNVS565G –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com
Figure 27. Inductor Current Waveform
R
SEN
, R
ADJ
: To achieve good current limit accuracy and avoid over designing the power stage components, the
sense resistor method is used for current limiting in this example. A standard value 10mΩ resistor is selected for
R
SEN
, resulting in a 50mV drop at maximum load current, and a maximum 0.25W power dissipation in the
resistor. Since the LM5085 uses peak current detection, the minimum value for the current limit threshold must
be equal to the maximum load current (5A) plus half the maximum ripple amplitude calculated above:
I
CL(min)
= 5A + 1.19A/2 = 5.6A (19)
At this current level the voltage across R
SEN
is 56mV. Adding the current limit comparator offset of 9mV (max)
increases the required current limit threshold to 6.5A. Using Equation 9 with the minimum value for the ADJ pin
current (32µA), the required R
ADJ
resistor calculates to:
(20)
A standard value 2.1kΩ resistor is selected. The nominal current limit threshold calculates to:
(21)
Using the tolerances for the ADJ pin current and the current limit comparator offset, the maximum current limit
threshold calculates to:
(22)
The minimum current limit thresholds calculate to:
(23)
The load current in each case is equal to the current limit threshold minus half the current ripple amplitude. The
recommended value of 1000pF for C
ADJ
is used in this example.
C
OUT
: Since the maximum allowed output ripple voltage is very low in this example (5 mVp-p), the minimum
ripple configuration (R3, C1, and C2 in the Block Diagram) must be used. The resulting ripple at V
OUT
is then due
to the inductor’s ripple current passing through C
OUT
. This capacitor’s value can be selected based on the
maximum allowable ripple voltage at V
OUT
, or based on transient response requirements. The following
calculation, based on ripple voltage, provides a first order result for the value of C
OUT
:
(24)
where I
OR(max)
is the maximum ripple current calculated above, and V
RIPPLE
is the allowable ripple at V
OUT
.
(25)
A 100µF capacitor is selected. Typically the ripple amplitude will be higher than the calculations indicate due to
the capacitor’s ESR.
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