Datasheet

L1 =
t
ON(min)
x (V
IN(max)
- V
OUT
)
I
OR(max)
= 5.79 PH
R
T
=
1 x (12 - 1.56V)
1.45 x 10
-7
x 12 x 200 kHz
- 1.4= 20.9
(50 ns + 57 ns) x (12 - 1.56V)
1.45 x 10
-7
-
LM25085A
www.ti.com SNVS601B –JANUARY 2009–REVISED MARCH 2013
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is illustrated with the following design example. Referring
to Block Diagram, the circuit is to be configured for the following specifications:
• V
OUT
= 1.0V
• V
IN
= 4.5V to 24V, 12V Nominal
• Maximum load current (I
OUT(max)
) = 5A
• Minimum load current (I
OUT(min)
) = 500 mA (for continuous conduction mode)
• Switching Frequency (F
SW
) = 200 kHz
• Maximum allowable output ripple (V
OS
) = 10 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
/0.9V) - 1
For this example, R
FB2
/ R
FB1
= 0.111. Typically, R
FB1
and R
FB2
should be chosen from standard value
resistors in the range of 1 kΩ to 20 kΩ which satisfy the above ratio. For this example, R
FB2
= 1.1 kΩ, and
R
FB1
= 10 kΩ.
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 57 ns. Using Equation 5 at nominal input voltage, R
T
calculates to be:
A standard value 21 kΩ resistor is selected. Using Equation 3 the minimum on-time at the PGATE pin, which
occurs at maximum input voltage (24V), is calculated to be 195 ns. This minimum one-shot period is
sufficiently longer than the minimum recommended value of 150 ns. The minimum on-time at the SW node is
longer due to the delay added by the PFET (57 ns). Therefore the minimum SW node on-time is 252 ns at
24V. At the SW node the maximum on-time is calculated to be 1.21 µs at 4.5V.
• 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 0 mA. Continuous conduction mode
operation at minimum load current is not a requirement of the LM25085A, but serves as a guideline for
selecting L1. For this example, the maximum ripple current is:
I
OR(max)
= 2 x I
OUT(min)
= 1.0 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 6.8 µH inductor is selected. Using this inductance value, the maximum ripple current
amplitude, which occurs at maximum input voltage, calculates to 0.85 Ap-p. The peak current (I
PK
) at
maximum load current is 5.43A. However, the current rating of the selected inductor must be based on the
maximum current limit value calculated below.
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