MIC2196 Datasheet, PDF (10/11 Pages) Micrel Semiconductor – 400kHz SO-8 Boost Control IC

MIC2196
A minimum 1µF ceramic capacitor is required to decouple the
V_IN. The capacitor should be placed near the IC and con-
nected directly between pins 8 (VCC) and 6 (GND). For V_IN
greater than 8V, use a 4.7µF or a 10µF ceramic capacitor to
decouple the VDD pin.
Efficiency Calculation and Considerations
Efficiency is the ratio of output power to input power. The
difference is dissipated as heat in the boost converter. The
significant contributors at light output loads are:
• The VIN pin supply current which includes the
current required to switch the external
MOSFETs.
• Core losses in the inductor.
To maximize efficiency at light loads:
• Use a low gate charge MOSFET or use the
smallest MOSFET, which is still adequate for the
maximum output current.
• Use a ferrite material for the inductor core, which
has less core loss than an MPP or iron power
core.
The significant contributors to power loss at higher output
loads are (in approximate order of magnitude):
• Resistive on-time losses in the MOSFET
• Switching transition losses in the MOSFET
• Inductor resistive losses
Micrel
• Current sense resistor losses
• Output capacitor resistive losses (due to the
capacitor’s ESR)
To minimize power loss under heavy loads:
• Use logic level, low on resistance MOSFETs.
Multiplying the gate charge by the on-resistance
gives a figure of merit, providing a good balance
between switching and resistive power dissipa-
tion.
• Slow transition times and oscillations on the
voltage and current waveforms dissipate more
power during the turn-on and turn-off of the low
side MOSFET. A clean layout will minimize
parasitic inductance and capacitance in the gate
drive and high current paths. This will allow the
fastest transition times and waveforms without
oscillations. Low gate charge MOSFETs will
switch faster than those with higher gate charge
specifications.
• For the same size inductor, a lower value will
have fewer turns and therefore, lower winding
resistance. However, using too small of a value
will increase the inductor current and therefore
require more output capacitors to filter the output
ripple.
• Lowering the current sense resistor value will
decrease the power dissipated in the resistor.
However, it will also increase the overcurrent
limit and may require larger MOSFETs and
inductor components to handle the higher
currents.
• Use low ESR output capacitors to minimize the
power dissipated in the capacitor’s ESR.
MIC2196
10
August 2004