L7250 datasheet(26/46 Pages) STMICROELECTRONICS

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L7250

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2.8 Spindle PWM Current Limiting

Peak motor current is limited with a fixed frequency PWM scheme that works in conjunction with the Smooth-

drive PWM rate. When the current limit threshold is reached the motor is put in brake condition, and it is re-

enabled at the beginning of the next PWM cycle if the current limiting condition is false.

Spindle current is sensed via an external resistor connected from the low side driver sources to ground. This

sense voltage is compared to an internal programmable voltage reference (Reg04H Currdac[2:0]).

There is a built in digital filter, generating a SYSCLK derived delay (20 * SYSCLK period) from the over current

event. This delay appears on both edges of the current limiting comparator.

2.9 Slew Rate Control

Closed loop Voltage Slew rate control is provided on both edges for the high and low side drivers. The slew rate

value can be set with three bits in the serial port (Reg04H Spslew[2:0]). Slew rates up to 80V/us and down to

10V/us will be controllable.

2.10 Synchronous rectification

The appropriate low-side driver is enabled during the off-time phase to conduct recirculation current with a lower

voltage drop than the low side driver body diode, reducing power losses. Crossover current protection is pro-

vided to prevent shoot-through currents.

2.11 Open loop and closed loop brake

Spindle braking may be done while keeping the Smoothdrive system in sync with the motor, or not.

Closed Loop Braking means ZC's are still being detected in the same way as when normally commutating. So,

all 3 motor phases are driven low, but when the window is normally opened to look for a ZC, MOTU is tri-stated.

When the ZC occurs, MOTU is driven low as the other motor phases, until the next window comes up. A mo-

tionless motor will wait for a ZC, keeping MOTU tri-stated and the other two phases low. Open loop braking

means that all 3 motor phases are driven low, and ZC's are not detected.

Braking caused by a power fault is

always open loop braking.

CBRK provides control voltage for brake circuitry after power fails. An external cap on this pin is charged to 5V,

so that the cap stays charged after a power failure.

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