Datasheet

L1
(min)
=
t
ON(min)
x (V
IN(max)
- V
OUT
)
I
OR(max)
= 7.85 PH
R
T
=
5 - (8V x 1MHz x 15 ns)
1MHz x 4.1 x 10
-11
- 500:= 118.5 k:
t
OFF(min)
=
V
IN(min)
- V
OUT
V
IN(min)
x F
S
= 375 ns
8V - 5V
8V x 1 MHz
=
t
ON(min)
=
V
OUT
V
IN(max)
x F
S
= 139 ns
5V
36V x 1 MHz
=
LM25011, LM25011-Q1
www.ti.com SNVS617G –APRIL 2009–REVISED FEBRUARY 2013
APPLICATIONS INFORMATION
EXTERNAL COMPONENTS
The procedure for calculating the external components is illustrated with a design example using the LM25011.
Referring to the Block Diagram, the circuit is to be configured for the following specifications:
• V
OUT
= 5V
• V
IN
= 8V to 36V
• Minimum load current for continuous conduction mode (I
OUT(min)
= 300 mA
• Maximum load current (I
OUT(max)
= 1.5 A
• Switching frequency (F
S
) = 1.0 MHz
• Soft-start time = 5 ms
R
FB2
and R
FB1
: These resistors set the output voltage, and their ratio is calculated from:
R
FB2
/R
FB1
= (V
OUT
/2.51V) - 1 (8)
For this example, R
FB2
/R
FB1
= 0.992. R
FB1
and R
FB2
should be chosen from standard value resistors in the range
of 1.0 kΩ – 10 kΩ which satisfy the above ratio. For this example, 4.99 kΩ is chosen for both resistors, providing
a 5.02V output.
R
T
: This resistor sets the on-time, and (by default) the switching frequency. First check that the desired
frequency does not require an on-time or off-time shorter than the minimum allowed values (90 ns and 150,
respectively). The minimum on-time occurs at the maximum input voltage. For this example:
(9)
The minimum off-time occurs at the minimum input voltage. For this example:
(10)
Both the on-time and off-time are acceptable since they are significantly greater than the minimum value for
each. The R
T
resistor is calculated from Equation 6 using the minimum input voltage:
(11)
A standard value 118 kΩ resistor is selected. The minimum on-time calculates to 152 ns at V
IN
= 36V, and the
maximum on-time calculates to 672 ns at Vin = 8V
L1: The parameters controlled by the inductor are the inductor current ripple amplitude (I
OR
), and the ripple
voltage amplitude across the current sense resistor R
S
. The minimum load current is used to determine the
maximum allowable ripple in order to maintain continuous conduction mode (the lower peak does not reach 0
mA). This is not a requirement of the LM25011, but serves as a guideline for selecting L1. For this example, the
maximum ripple current should be less than:
I
OR(max)
= 2 x I
OUT(min)
= 600 mA p-p (12)
For applications where the minimum load current is zero, a good starting point for allowable ripple is 20% of the
maximum load current. In this case substitute 20% of I
OUT(max)
for I
OUT(min)
in Equation 12. The ripple amplitude
calculated in Equation 12 is then used in Equation 13:
(13)
A standard value 10 µH inductor is chosen. Using this inductor value, the maximum ripple current amplitude,
which occurs at maximum V
IN
, calculates to 472 mAp-p, and the peak current is 1736 mA at maximum load
current. Ensure the selected inductor is rated for this peak current. The minimum ripple current, which occurs at
minimum V
IN
, calculates to 200 mAp-p.
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