MIC2570
at each cycle. At the threshold between continuous and dis-
continuous operation we can use the fact that I
1
= I
2
to get:
V
1
× t
1
= V
2
× t
2
V
1
V
2
=
t
2
t
1
L=
L=
V
I
× t
1
V
IN(min)
2 × Average I
IN(max)
duty cycle
f
OSC
× t
1
Micrel, Inc.
This relationship is useful for finding the desired oscillator
duty cycle based on input and output voltages. Since input
voltages typically vary widely over the life of the battery, care
must be taken to consider the worst case voltage for each
parameter. For example, the worst case for t
1
is when V
IN
is at its minimum value and the worst case for t
2
is when V
IN
is at its maximum value (assuming that V
OUT
, V
DIODE
and
V
SAT
do not change much).
To select an inductor for a particular application, the worst
case input and output conditions must be determined. Based
on the worst case output current we can estimate efficiency
and therefore the required input current. Remember that
this is
power conversion, so the worst case average input
current will occur at maximum output current and minimum
input voltage.
V
OUT
× I
OUT(max)
Average I
IN(max)
=
V
IN(min)
× Efficiency
where t
1
=
To illustrate the use of these equations a design example
will be given:
Assume:
MIC2570-1 (fixed oscillator)
V
OUT
= 5V
I
OUT(max)
=50mA
V
IN(min)
= 1.8V
efficiency = 75%.
Average I
IN(max)
=
L=
5V × 50mA
1.8V × 0.75
× 185.2mA
1.8V × 0.7
2 × 185.2mA × 20kHz
Referring to Figure 1, it can be seen the peak input current
will be twice the average input current. Rearranging the
inductor equation to solve for L:
L = 170µH
Use the next lowest standard value of inductor and verify
that it does not saturate at a current below about 400mA
(< 2
×
185.2mA).
MIC2570
8
August 2007
MICREL [ MICREL SEMICONDUCTOR ]
