≈ 1.53 × R × C

• Output current ignored time

A regenerative current flows in the external diode provided to absorb regenerative currents in the upper side of the

output that was turned off while the sink side of that output is turned off by the operation of the current limiter. When

the output off time has elapsed and the sink side output has been turned on again a reverse current flows

instantaneously in this diode, due to the reverse recovery period of the diode. This results in a current that may reach

the current limit value instantaneously. If this current were to cause the current limiter circuit to operate again, the

average current produced by the IC would be reduced, and the torque produced during motor drive could be reduced.

Therefore, to assure that this current is not detected, the current limiter circuit provides a period (referred to as the

output current ignored time) during which the output current is not detected for a fixed period after the sink side output

is turned on again after being turned off. The output current ignored time is determined by the discharge time of the

capacitor C connected to the C pin. Discharge starts at the point the capacitor has been charged to 2 V after the current

limiter circuit operates, and the output current ignored time is the time until the capacitor is discharged to 0.4 V

output current ignored time tsm are related as follows:

≈ 0.41 × R × C

Since the current limiter circuit provides a slope in the on time at the point when the sink side output is turned back on,

reverse currents will not increase significantly, even if a rectifying diode (that is, a diode whose reverse recovery time

is relatively long) is used as the regenerative current absorbing diode.

• Output off time setting

Applications must set up the output off time to be optimal for the type of motor used. (This time is set by the external

resistor connected to the R pin and the external capacitor connected to the C pin.) Figure 1 shows the waveforms during

current limiter operation.

— If the output off time is too short:

Since the ratio between the output off time and the output current ignored time is fixed at about 3:1 by IC internal

circuits, it is not possible to set these parameters independently. This means that if a relatively short output off time

is used, the output current ignored time may be insufficient. If the output current ignored time is insufficient, the

current limiter circuit may operate due to the reverse current in the regenerative current absorption diode. (See

section 7 above.) Also, if the output off time is too short, the reverse current may increase leading to ASO

problems.

— If the output off time is too long:

Setting up a relatively long output off time can reduce the average current produced by the IC, which can result in

reduced motor torque. For certain motor types, this may make it impossible to switch from the current limiter

operating state to steady state operation at motor startup.

No. 5802-6/7

LB1695M