MIC2570_07 Datasheet, PDF (8/15 Pages) Micrel Semiconductor – Two-Cell Switching Regulator

MIC2570
at each cycle. At the threshold between continuous and dis-
continuous operation we can use the fact that I₁ = I₂ to get:
V₁ × t₁ = V₂ × t₂
V₁
V₂
=
t₂
t₁
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 t₁ is when V_IN
is at its minimum value and the worst case for t₂ 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) = _{VIN(min) × Efficiency}
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:
V
L=
I
× t₁
V_IN(min)
L = _{2 × Average IIN(max)} × t₁
Micrel, Inc.
duty cycle
where t₁ =
f_OSC
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%.
5V × 50mA
Average I_IN(max) = _{1.8V × 0.75} × 185.2mA
1.8V × 0.7
L=
2 × 185.2mA × 20kHz
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