ML4880
Inductor ripple currents in the range of 30% to 70% of the
maximum output current are typical. As a good starting
point set the inductor ripple current to 50% of the
maximum output current:
DESIGN CONSIDERATIONS
A typical design can be implemented by using the
following design procedure. Note that this procedure is
not intended to give final values, but to give a good
starting point, and provide the relationships necessary to
make trade-off decisions. Some experimentation will be
necessary to optimize values and to verify that the design
operates over worst case conditions.
T
× (V − V
)
ON
IN
OUT
∆I =
= F ×I
IRC OUT(MAX)
(4)
L
L
where F
= ratio of inductor ripple current to the
IRC
maximum output current, or:
DESIGN SPECIFICATIONS
T
ON × (VIN − VOUT
0.5 ×IOUT(MAX)
)
It is important to start with a clear definition of the design
specifications. Make sure the specifications reflect worst
case conditions. Key specifications include the minimum
and maximum input voltage and the output voltage and
load current for each output.
L =
(5)
Calculate the inductance using the volt-seconds value
given in Figure 5 at the maximum input voltage. Choose
the nearest standard value, realizing the trade-offs
mentioned before. Then, using the inductance value
chosen, determine the actual inductor ripple current at the
maximum and minimum input voltage using Equation 4
and Figure 5.
BUCK REGULATORS - INDUCTOR AND SENSE
RESISTOR SELECTION
Figure 6 shows the inductor current of the step down
regulators. The inductor current is made up of two
components: the DC current level set by the
The sense resistor value can be determined using the
inductor ripple current value calculated above and
Equation 3 rearranged as follows:
transconductance amplifier, I
, and the inductor ripple
SENSE
current, ∆I . The figure also shows that I
is the
L
OUT
summation of I
and 1/
∆I :
L
2
SENSE
V
0.14
SENSE(MIN)
R
=
=
SENSE
VSENSE
RSENSE
TON × (VIN − VOUT)
1
2
1
2
1
(6)
IOUT = ISENSE
+
∆IL =
+
I
−
∆I
I
−
∆I
L(MIN)
(3)
OUT(MAX)
L(MIN)
OUT(MAX)
2 ×L
2
Therefore, the selection of the inductance value
determines how much of the output current is made up of
the ripple current. Higher inductor ripple current allows
smaller inductor values, but results in higher peak
currents, lower efficiency, and higher output voltage
ripple.
18
45
40
35
30
25
20
15
10
5
5V BUCK
16
14
12
10
12V FLYBACK
3.3V BUCK
5V BUCK
3.3V BUCK
8
12V FLYBACK
6
4
2
0
5
10
(V)
15
20
0
5
10
(V)
15
20
V
V
IN
IN
Figure 4. T
vs. V
IN
ON
Figure 5. Volt-seconds vs V
IN
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
