LB1696 데이터 시트보기 (PDF) - SANYO -> Panasonic

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LB1696

3-phase Brushless Motor Driver

SANYO -> Panasonic 

LB1696

7-2. Output current neglect time

While the current limiter turns on and the sink side output is off, the regeneration current flows through the external diode

used for absorbing the regeneration current above the output that was turned off. After the output off time elapses and the

sink side output is turned on again, reverse current flows momentarily through the external diode (for the diode’s reverse

recovery time), causing a current that reaches the limiter value to flow momentarily through the output. Because this current

will cause current limiter to turn on again, turning off the output, the average current decreases, causing the torque to be

decreased at motor start-up, etc. Therefore, in order to prevent this current from being detected, the current limiter is designed

so that the output current is not detected for a fixed period of time after the sink side output is turned on again. This length

of time is the output current neglect time. The output current neglect time is determined by the discharge time of the

capacitor C connected to the C pin. When current limiter turns on and C charges to 2 V, C begins discharging, and the output

current neglect time is the time it takes for C to discharge to the point where the voltage at C is 0.4 V (typ). The C discharge

current is a constant current, and is set at about 11 times the ICL1 of charge current when the motor is locked. As a result, the

output current neglect time is about 1/11 of the output off time when the motor is locked. Because the C discharge current is

the same whether the motor is locked or is rotating, the output current neglect time is also the same whether the motor is

locked or is rotating. The C discharge current ICH and the output current neglect time tsm are determined according to the

following equations:

ICH 6 1.4/R1 × 11

tsm 6 C/ICH × 1.6

6 0.10 × R1 × C

Because there is a slope to the time at which the sink side output is turned on again, the reverse current is not very large,

even if a rectifier diode (a diode in which the reverse recovery time is not short) is used as the external diode for absorbing

the regeneration current in the current limiter.

7-3. Output off time setting

It is necessary to set the output off time to a suitable level for the type of motor being used. (The output off time is set by

the external resistors connected to the R1 and R2 pins, and by the external capacitor connected to the C pin.) In the LB1696,

the output off time when the motor is rotating can be set so that it is shorter than when the motor is locked. Set the optimal

output off time for when the motor is locked, and then set the output off time for when the motor is rotating. Fig. 1 shows

the current limiter operation waveform.

(1) When the output off time is set short

The output current neglect time is set by a circuit within the IC to about 1/11 of the output off time when the motor is

locked. Therefore, if the output off time is set to a very short length of time, the output current neglect time may not be

adequate. If the output current neglect time is inadequate, the current limiter will turn on in response to reverse current

from the external diode used to absorb the regeneration current. (Refer to Section 7-2.) Furthermore, if the output off time

is short, the diode reverse current becomes large and ASO must be considered.

(2) When the output off time is set long

If the output off time when the motor is rotating (at motor start-up) is set to a very long length of time, the average

current decreases, causing the torque at motor start-up to drop. Depending on the type of motor, it may be impossible to

shift from the current limiter operation state to the normal rotation state. In current limiter operation when the motor is

locked, it is necessary to set the output time to a comparatively long length of time. Therefore, first set the output off

time toff1 for when the motor is locked, and then set the output off time toff2 for when the motor is rotating so that toff2 is

shorter than toff1.

No.5244-8/9

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