7.

Current limiter

The current limiter turns off the sink side output transistor when the output current-set current value (limiter value) is reached. The

output current is limited by the limit value. The RF pin is used to detect the output current. The output current is detected as

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 LB1696 uses this output

switching method for the current limiter, the ASO problems when current limitation goes into operated mode as compared

with the output unsaturated current limited one. In addition, by separating current limiter operation into two modes, one when

the motor is locked and one when the motor is rotating (during start-up), it was possible to implement a current limiter circuit

with excellent motor start-up characteristics. The explanation of current limiter operation below is divided into two parts: one

for the mode used when the motor is locked and one for the mode used when the motor is rotating. The output off time

depends on the 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 current, which depends on resistor R1 connected to the R1 pin and

resistor R2 connected to the R2 pin. In the LB1696, the charge current can be switched for when the motor is locked and for

when the motor is rotating in order to support motors for a large number of applications. As a result, it is possible to set the

output off time so that it is different for when the motor is locked and for when the motor is rotating. By setting the output

off time so that it is shorter when the motor is rotating (at start-up) as opposed to when the motor is locked, it is possible to

reduce the decrease in torque at start-up caused by the output off time. The charge currents and output off times for when the

motor is locked and for when the motor is rotating are as follows:

6 1.42 × R1 × C

(R1 must be set between 14 k

Ω and 100 kΩ.)

6 1.42 × R × C{R=R1 × R2/(R1 + R2)}

(R2 must be set between 7 k

Ω and 100 kΩ.)

LB1696

No.5244-7/9