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Micrel, Inc.
This is the minimum value that should be us ed. To
protect the IC from inductive spikes or any oversh oot, a
larger value of input capa citance may be required. Use
2.2μF or higher as a good safe min.
MIC3263
Equation 17:
PWR SW_ON( MAX) =I SW_RMS(MAX) ×V CE_ON_RMS (MAX)
?
12
?
Rectifier Diode Selection
A Schottky diode is best used here because of the lower
forward voltage and the low reverse recove ry time. The
?
I SW_RMS(MAX) = D (MAX) × ? ? I IN_AVE(MAX) 2 +
?
( I IN_PP ) 2 ? ?
?
?
voltage s tress on the diode is the maximum V OUT and
therefore, a diode with a higher rating than maximum
V OUT shou ld be used. An 80% de-rating is recommended
here as well.
Equation 14:
I DIODE _(MAX ) = I OUT (MAX) = 0.18A
≈ D (MAX) ×I IN_AVE(MAX)
V CE_ON_RMS (MAX) = D (MAX) ×V CE_ON( MAX)
PWR SW_ON( MAX) = D (MAX) ×I AVE(MAX) × V CE_ON (MAX)
PWR SW_ON( MAX) = 0.8 × 0.9A × 0.5V = 0.36W
Equation 18:
PWR SW_SWITCHING (MAX ) = V OUT(MAX) × I IN_AVE(MAX) × tsw × F sw
Equation 15:
tsw ≈ 20ns is the intern al power switch on an off
P DIODE(MAX) ≈ V DIODE × I DIODE_(MAX)
A SK34A is used in this example, it’s V DIODE i s 0.5 V.
P DIODE(MAX) ≈ 0.5V × 0.18A ≈ 0.09W
MIC3263 Pow er Losses
To find the power losses in the MIC3263: There is about
25mA to 35m A i npu t from V IN in to t he V DD pin. The
internal bipolar power switch has an V CE(ON MAX) o f about
0.5V.
V CE(ON MAX) ≈ 0.5V
Equation 16:
transition time
PWR SW_SWITCHING (MAX) = 34V ×0.9 × 20ns ×1MHz = 0.61W
Therefore:
P MIC3263(MAX) = 14V × 35m A + 0.97 = 1.46W
Snubber
If a high-frequency ringing is p resent at V SW , a snubber
may be needed. A snubber is a damping resistor in
series with a DC blocking capacitor in parallel with the
power switch. When the power switch turns off, the drain
to source capacitance and parasitic inductance will
cause a high frequency ringing at the switch node. A
snubber circuit as shown in the application schematics
may be required if ringing is present at the switch node.
A critically damped circuit at the switch node is where R
equals the characteristic impedance of the switch node.
P MIC3263(MAX) = V IN(MAX) × 35mA + PWR SW(MAX)
Equation 18:
Where PWR SW(MAX) is the power loss of the internal
bipolar power switch. The power switch power losses
are the sum of the on-time losses; PWR SW(MAX) and the
switching losses: PWR SW(SWITCHING MAX) .
PWR SW(MAX) = PWR SW(MAX) + PWR SW(SWITCHING MAX)
January 2010
26
R SNUBBER =
L PARISITIC
C DS
M9999-012110
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