MCP8024 datasheet(31/46 Pages) MICROCHIP

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 2013 Microchip Technology Inc.

DS20005228A-page 31

MCP8024

5.2

Device Overvoltage Protection

When a motor shaft is rotating and power is removed,

the magnetism of the motor components will cause the

motor to act like a generator. The current that was flow-

ing into the motor will now flow out of the motor. As the

motor magnetic field decays, the generator output will

also decay. The voltage across the generator terminals

will be proportional to the generator current and circuit

impedance of the generator circuit. If the power supply

is part of the return path for the current and the power

supply is disconnected, then the voltage at the genera-

tor terminals will increase until the current flows. This

voltage increase must be handled external to the driver.

A voltage suppression device must be used to clamp

the motor terminal voltage to a level that will not exceed

the maximum motor operating voltage. A voltage sup-

pressor should be connected from ground to each

motor terminal. The PCB traces must be capable of

carrying the motor current with minimum voltage and

temperature rise.

An additional method is to inactivate the high-side driv-

ers and to activate the low-side drivers. This allows cur-

rent to flow through the low-side external MOSFETs

and prevent the voltage increases at the power supply

terminals.

TABLE 5-1:

MAX INDUCTANCE FOR BUCK DISCONTINUOUS MODE OPERATION

Vin

(worst case)

Vout

Iout

Max. Inductance

250 mA

7.12 µH

3.3V

225 mA

7.05 µH

5.0V

150 mA

5.94 µH

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