MIC3263 Datasheet, PDF (26/32 Pages) Micrel Semiconductor – Six-Channel WLED Driver for Backlighting Applications with Flicker-Free Dimming

Micrel, Inc.
This is the minimum value that should be used. To
protect the IC from inductive spikes or any overshoot, a
larger value of input capacitance may be required. Use
2.2µF or higher as a good safe min.
Rectifier Diode Selection
A Schottky diode is best used here because of the lower
forward voltage and the low reverse recovery time. The
voltage stress on the diode is the maximum V_OUT and
therefore, a diode with a higher rating than maximum
V_OUT should be used. An 80% de-rating is recommended
here as well.
Equation 14:
I_{DIODE_(MAX)} = I_OUT(MAX) = 0.18A
Equation 15:
P_DIODE(MAX) ≈ V_DIODE × I_{DIODE_(MAX)}
A SK34A is used in this example, it’s V_DIODE is 0.5V.
P_DIODE(MAX) ≈ 0.5V × 0.18A≈ 0.09W
MIC3263 Power Losses
To find the power losses in the MIC3263: There is about
25mA to 35mA input from V_IN into the V_DD pin. The
internal bipolar power switch has an V_{CE(ON MAX)} of about
0.5V.
V_{CE(ON MAX)} ≈ 0.5V
Equation 16:
P_MIC3263(MAX) = V_IN(MAX) × 35mA + PWR_SW(MAX)
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)}
Equation 17:
MIC3263
PWR_{SW_ON( MAX)} = I_{SW_RMS(MAX)} × V_{CE_ON_RMS (MAX)}
I_{SW_RMS(MAX)} =
( ) ^⎛
D_(MAX)×^⎜⎜I_{IN_AVE(MAX)}² +
⎜⎝
I_{IN_PP}
12
2⎞
⎟
⎟
⎟⎠
≈ 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
tsw ≈ 20ns is the internal power switch on an off
transition time
PWR_{SW_SWITCHING (MAX)} = 34V × 0.9 × 20ns ×1MHz = 0.61W
Therefore:
P_MIC3263(MAX) = 14V × 35mA + 0.97 = 1.46W
Snubber
If a high-frequency ringing is present 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.
Equation 18:
R_SNUBBER =
L_PARISITIC
C_DS
January 2010
26
M9999-012110