MIC2570 Datasheet, PDF (8/14 Pages) Micrel Semiconductor – Two-Cell Switching Regulator Preliminary Information

MIC2570
Micrel
at each cycle. At the threshold between continuous and
discontinuous operation we can use the fact that I₁ = I₂ to get:
L=
V
I
× t₁
V₁ × t₁ = V₂ × t₂
V_{1 =} t₂
V₂
t₁
This relationship is useful for finding the desired oscillator
duty cycle based on input and output voltages. Since input
L=
V_IN(min)
2 × Average I_IN(max)
× t₁
where t₁ =
duty cycle
f_OSC
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 illustrate the use of these equations a design example will
be given:
Assume:
MIC2570-1 (fixed oscillator)
V_OUT = 5V
To select an inductor for a particular application, the worst
I_OUT(max) =50mA
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
V_IN(min) = 1.8V
efficiency = 75%.
Average I_IN(max) =
5V × 50mA
1.8V × 0.75
= 185.2mA
voltage.
L=
1.8V × 0.7
Average I_IN(max) =
V_OUT × I_OUT(max)
V_IN(min) × Efficiency
2 × 185.2mA × 20kHz
L = 170µH
Use the next lowest standard value of inductor and verify that
4
Referring to Figure 1, it can be seen the peak input current will _{it does not saturate at a current below about 400mA}
be twice the average input current. Rearranging the inductor _{(< 2 × 185.2mA).}
equation to solve for L:
1997
4-69