MIC3808/3809 Micrel
MIC3808/3809 10 April 2005
Output Drivers
OUTA and OUTB are alternating output stages, which switch
at half the oscillator frequency. A toggle flip-flop in the
MIC3808/9 guarantee both outputs will not be on at the same
time. The RC discharge time is the dead time, where both
outputs are off. This provides an adjustable non-overlap time
to prevent shoot through currents and transformer saturation
in the power supply.
The output drivers are inhibited when V
DD
is below the
undervoltage threshold. Internal circuitry prevents the output
drivers from glitching high when V
DD
is first applied to the
MIC3808/9 controller.
Decoupling and PCB Layout
PCB layout is critical to achieve reliable, stable and efficient
operation. A ground plane is required to control EMI and
minimize the inductance in power, signal and return paths.
The following guidelines should be followed to insure proper
operation of the circuit:
• Low level signal and power grounds should be kept
separate and connected at only one location, preferably
the ground pin of the control IC. The ground signals for
the current sense, voltage feedback and oscillator
should be grouped together. The return signals for the
gate drives should be grouped together and a common
connection made at the ground pin of the controller. The
low level signals and their returns must be kept separate
from the high current and high voltage power section of
the power supply.
• Avoid running sensitive traces, such as the current
sense and voltage feedback signals next to or under
power components, such as the switching FETs and
transformer.
• If a current sense resistor is used, it’s ground end must
be located very close to the ground pin of the MIC3808/
9 controller. Careful PCB layout is necessary to keep
the high current levels in the current sense resistor from
running over the low level signals in the controller.
• A minimum 1µf bypass capacitor must be connected
directly between the V
DD
and GND pins of the MIC3808/
9. An additional 0.1uf capacitor between the V
DD
end
oscillator frequency setting resistor and the ground end
of the oscillator capacitor may be necessary if the
resistor is a distance away from the main 1µF bypass
capacitor
