Datasheet

L1 =
0.40A x 152 kHz x 40V
5V x (40V - 5V)
= 72 PH
L1 =
I
OR
x F
S(min)
x V
IN(max)
V
OUT
x (V
IN(max)
- V
OUT
)
L1 Current
0 mA
I
OR
1/Fs
I
PK+
I
O
I
PK-
R
ON
=
5V x (8V - 1.4V)
8V x 175 kHz x 1.18 x 10
-10
- 1.4 k: = 198 k:
LM25010
SNVS419D –DECEMBER 2005–REVISED FEBRUARY 2013 www.ti.com
APPLICATIONS INFORMATION
External Components
The procedure for calculating the external components is illustrated with a design example. Referring to the
Block Diagram, the circuit is to be configured for the following specifications:
• V
OUT
= 5V
• V
IN
= 6V to 40V
• F
S
= 175 kHz
• Minimum load current = 200 mA
• Maximum load current = 1.0A
• Softstart time = 5 ms.
R1 and R2: These resistors set the output voltage, and their ratio is calculated from:
R1/R2 = (V
OUT
/2.5V) – 1 (10)
R1/R2 calculates to 1.0. The resistors should be chosen from standard value resistors in the range of 1.0 kΩ to
10 kΩ. A value of 1.0 kΩ will be used for R1 and for R2.
R
ON
, F
S
: R
ON
can be chosen using Equation 7 to set the nominal frequency, or from Equation 6 if the on-time at
a particular V
IN
is important. A higher frequency generally means a smaller inductor and capacitors (value, size
and cost), but higher switching losses. A lower frequency means a higher efficiency, but with larger components.
Generally, if PC board space is tight, a higher frequency is better. The resulting on-time and frequency have a
±25% tolerance. Using Equation 7 at a nominal V
IN
of 8V,
(11)
A value of 200 kΩ will be used for R
ON
, yielding a nominal frequency of 161 kHz at V
IN
= 6V, and 203 kHz at V
IN
= 40V.
L1: The guideline for choosing the inductor value in this example is that it must keep the circuit’s operation in
continuous conduction mode at minimum load current. This is not a strict requirement since the LM25010
regulates correctly when in discontinuous conduction mode, although at a lower frequency. However, to provide
an initial value for L1 the above guideline will be used.
Figure 11. Inductor Current
To keep the circuit in continuous conduction mode, the maximum allowed ripple current is twice the minimum
load current, or 400 mA
P-P
. Using this value of ripple current, the inductor (L1) is calculated using the following:
(12)
where F
S(min)
is the minimum frequency of 152 kHz (203 kHz – 25%) at V
IN(max)
.
(13)
12 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated
Product Folder Links: LM25010