7-1. Output off time

The current limiter is so designed that current limit function turns on to turn off the sink side output transistor and then turn

on the transistor again after off period of a fixed time (output off time) has elapsed. Since the LB1694N uses this output

switching method for the current limiter, it is hardly necessary to consider ASO in current limiter operated mode as compared

with the output unsaturated current limited one. The output off time depends on charge time of capacitor C connected to the

C pin. When the current limiter turns on, C begins charging and the output is kept off until C is charged up to 2 V (typ).

When C has been charged up to 2 V, the sink side output turns on again. The C charging current is a constant-regulated

as follows:

6 1.53 × R × C

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 the 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

according to the following equations:

6 0.41 × R × 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 R pin, and by the external capacitor connected to the C pin.) 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 1/3 of the output off time. 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.

LB1694N

No.5243-7/8