L6238 datasheet(20/35 Pages) STMICROELECTRONICS

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torque. The spin sense waveform at the bottom of

the figure indicates that this output signal toggles

with each zero crossing.

5.2 Brake Delay

When Run/Brake is brought low, a brake is initi-

ated. Referring to figure 16, SWI is opened and

the brake delay capacitor, Cbrake, is allowed to

discharge towards ground via Rbrake. At the same

time, switches SW2 through SW7 bring the gates

of the output FETs to ground halting conduction,

causing the motor to coast. While the motor is

coasting, the Bemf is used to park the heads.

When Cbrake reaches a voltage that is below the

turn ON threshold of Q I, Switches SW8, 9 and 10

bring the gates of the lower drivers to Vbrake po-

tential. This enables the lower FETs causing a

braking action. This braking action also occurs if

the logic supply is lost. The analog supply is not

monitored in the L6238 since the L6243 already

monitors this voltage and initiates a Park function

when this supply drops to a predetermined level.

If multiple logic supplies are used in the applica-

tion, all logic signals to the L6238 including the

reference and clock signals should be buffered

with gates powered by the same supply as the

L6238 in order to prevent erroneous operation.

This would occur, for example, if the 5V supply to

the controller were lost while 5V were still present

at one of the logic pins. This would partially power

the chip, causing unpredictable operation.

5.3 Charge Pump

The charge pump circuitry is used as a means of

doubling the analog supply voltage in order to al-

low the upper N-channel DMOS transistors to be

driven like P-channel devices. The energy stored

in the reservoir capacitor is also used to drive the

Figure 16: Brake Delay.

L6238

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