MIC2571 Datasheet, PDF (8/11 Pages) Micrel Semiconductor – Single-Cell Switching Regulator Preliminary Information

MIC2571
Micrel
When the output switch turns off, the voltage across the
inductor changes sign and flies high in an attempt to maintain
a constant current. The inductor voltage will eventually be
clamped to a diode drop above V_OUT. Therefore, when the
output switch is off, the voltage across the inductor is:
V₂ = V_OUT + V_DIODE – V_IN
For normal operation the inductor current is a triangular
waveform which returns to zero current (discontinuous mode)
at each cycle. At the threshold between continuous and
discontinuous operation we can use the fact that I₁ = I₂ to get:
V₁ × t₁ = V₂ × t₂
V_{1 =} t₂
V₂
t₁
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=
V
I
× t₁
L=
V_IN(min)
2 × Average I_IN(max)
× t₁
where t₁ =
duty cycle
f_OSC
To illustrate the use of these equations a design example will
be given:
Assume:
This relationship is useful for finding the desired oscillator
MIC2571-1 (fixed oscillator)
duty cycle based on input and output voltages. Since input
V_OUT = 5V
voltages typically vary widely over the life of the battery, care
must be taken to consider the worst case voltage for each
I_OUT(max) =5mA
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
V_IN(min) = 1.0V
efficiency = 75%.
at its maximum value (assuming that V_OUT, V_DIODE and V_SAT
do not change much).
Average I_IN(max) =
5V × 5mA
1.0V × 0.75
= 33.3mA
To select an inductor for a particular application, the worst
case input and output conditions must be determined. Based
L=
1.0V × 0.7
4
on the worst case output current we can estimate efficiency
2 × 33.3mA × 20kHz
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.
L = 525µH
Use the next lowest standard value of inductor and verify that
it does not saturate at a current below about 75mA
(< 2 × 33.3mA).
Average I_IN(max) =
V_OUT × I_OUT(max)
V_IN(min) × Efficiency
1997
4-83