Datasheet
I
CL(nom)
=
0.01:
(2.1 k: x 40 PA)
= 8.4A
R
ADJ
=
32 PA
6.44A
x 0.01:
= 2.01 k:
SW Node
I
PK
Inductor Current
I
OUT
I
OR
1/F
S
L1 =
t
ON(min)
x (V
IN(max)
-
V
OUT
)
I
OR(max)
= 13.5 PH
R
T
=
5 x (12 - 1.56V)
1.45 x 10
-7
x 12 x 300 kHz
- 1.4= 90.9
(50 ns + 57 ns) x (12 - 1.56V)
1.45 x 10
-7
-
LM25085, LM25085-Q1
www.ti.com
SNVS593H –OCTOBER 2008–REVISED MARCH 2013
• R
T
, PFET: Before selecting the R
T
resistor, the PFET must be selected as its turn-on and turn-off delays
affect the calculated value of R
T
. For the Vishay Si7465 PFET, the difference of its typical turn-off and turn-on
delays is 57ns. Using Equation 5 at nominal input voltage, R
T
calculates to be:
A standard value 90.9kΩ resistor is selected. Using Equation 3 the minimum on-time at the PGATE pin, which
occurs at maximum input voltage (42V), is calculated to be 381ns. This minimum one-shot period is
sufficiently longer than the minimum recommended value of 150ns. The minimum on-time at the SW node is
longer due to the delay added by the PFET (57ns). Therefore the minimum SW node on-time is 438ns at
42V. At the SW node the maximum on-time is calculated to be 2.55µs at 7V.
• L1: The main parameter controlled by the inductor value is the current ripple amplitude (I
OR
). See Figure 27.
The minimum load current for continuous conduction mode is used to determine the maximum allowable
ripple such that the inductor current’s lower peak does not fall below 0mA. Continuous conduction mode
operation at minimum load current is not a requirement of the LM25085, but serves as a guideline for
selecting L1. For this example, the maximum ripple current is:
I
OR(max)
= 2 x I
OUT(min)
= 1.2 Amp (13)
If an application’s minimum load current is zero, a good initial estimate for the maximum ripple current
(I
OR(max)
) is 20% of the maximum load current. The ripple calculated in Equation 13 is then used in the
following equation to calculate L1:
(14)
A standard value 15µH inductor is selected. Using this inductance value, the maximum ripple current
amplitude, which occurs at maximum input voltage, calculates to 1.08Ap-p. The peak current (I
PK
) at
maximum load current is 5.54A. However, the current rating of the selected inductor must be based on the
maximum current limit value calculated below.
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 LM25085 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 + 1.08A/2 = 5.54A
At this current level the voltage across R
SEN
is 55.4mV. Adding the current limit comparator offset of 9mV
(max) increases the required current limit threshold to 6.44A. Using Equation 7 with the minimum value for
the ADJ pin current (32µA), the required R
ADJ
resistor calculates to:
A standard value 2.1kΩ resistor is selected. The nominal current limit threshold calculates to:
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