Datasheet

L1 =
t
ON(min)
x (V
IN(max)
- V
OUT
)
I
OR(max)
= 14.9 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
-
LM5085, LM5085-Q1
www.ti.com SNVS565G –NOVEMBER 2008–REVISED MARCH 2013
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is illustrated with the following design example. Referring
to the Block Diagram, the circuit is to be configured for the following specifications:
V
OUT
= 5V
V
IN
= 7V to 55V, 12V nominal
Maximum load current (I
OUT(max)
) = 5A
Minimum load current (I
OUT(min)
) = 600mA (for continuous conduction mode)
Switching Frequency (F
SW
) = 300kHz
Maximum allowable output ripple (V
OS
) = 5 mVp-p
Selected PFET: Vishay Si7465
R
FB1
and R
FB2
: These resistors set the output voltage. The ratio of these resistors is calculated from:
R
FB2
/R
FB1
= (V
OUT
/1.25V) - 1 (15)
For this example, R
FB2
/R
FB1
= 3. Typically, R
FB1
and R
FB2
should be chosen from standard value resistors in the
range of 1kΩ to 20kΩ which satisfy the above ratio. For this example, R
FB2
= 10kΩ, and R
FB1
= 3.4kΩ.
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:
(16)
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 (55V), is calculated to be 300ns. 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 357ns at 55V. 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 LM5085, 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 (17)
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 17 is then used in the following equation to
calculate L1:
(18)
A standard value 15µH inductor is selected. Using this inductance value, the maximum ripple current amplitude,
which occurs at maximum input voltage, calculates to 1.19 Ap-p. The peak current (I
PK
) at maximum load current
is 5.6A. However, the current rating of the selected inductor must be based on the maximum current limit value
calculated below.
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