MIC3808 データシートの表示（PDF） - Micrel

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MIC3808

Push-Pull PWM Controller

Micrel 

MIC3808/3809

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 VDD is below the

undervoltage threshold. Internal circuitry prevents the output

drivers from glitching high when VDD 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

Micrel

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 VDD and GND pins of the MIC3808/

9. An additional 0.1uf capacitor between the VDD 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

MIC3808/3809

10

April 2005

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