
LB1660N, 1661, 1664N
No.2551-6/8
Setting Method of Lock Detect Time
(1) Generation of discharge pulse
Electric charges on C charged through R3 are discharged via route of C
→ r → TRr with constant width pulses at each
phase change. When a fan has a high rotational speed, a lot of discharge pulses are generated and the CR pin voltage
( = VCR) drops. By contrast, when a fan has a low rotational speed, VCR rises and the protector is operated.
The discharge pulses are generated by two methods as shown below.
1) Generated internally at the time of phase change. The power transistor base storage time is used for discharge
pulses.
t = 15 to 30
μsec
2) When the kickback voltage induced by L of a motor coil is absorbed by the zener diode across collector and base of
the output transistor, this interval of time during which the transistor is turned on is used for discharge pulses.
Therefore, the LB1661 or LB1660N, 1664N use external zener diodes and capacitors, discharge occurs with pulses
of 1) only and VCR voltage gets higher than the circuit generating discharge pulses of 2) even if the CR time
constant is the same.
Generation of discharge pulse
(2) CR time constant setting method
1) Bias current of CR pin and effect of leakage of C. The value of the charge current at the threshold point is VIN ×
0.2/R3. Therefore, this value and the leakage of C must be large enough for the bias current.
2) The CR pin voltage must not exceed 0.6
× VIN (LB1660N, 1664N) or 0.8 × VIN-0.3V (LB1661), until the normal
rotational speed of a fan is reached and while a fan keeps the normal rotational speed ; a malfunction may be caused
by instantaneous drop of power supply.
3) It should be noted that the CR pin voltage is varied with the combination C and R, even if the CR time constant is
the same.
(3) Restart after lock detection
The LB1660 series are so designed that when motor lock is detected and the output stage transistor is turned off the
drive-off state is kept even after motor lock is released. The method of restart is shown below.
1) Turn off the power supply and turn on again. Turn on after the VCR has dropped enough.
2) Set the CR pin voltage to be less than 1V, and then turn off the power supply in order to cause the lock detect
function to return to normal. (The CR pin can be also used to provide on-off control.)
VIN VIN
VIN
0.8VIN
+
-
CR
RD
output control
C
R3
TRr
TR1 base
TR2 base
r
Output transistor lock
Pulse of 1
Pulse of 2
→t (s)