lb1694n Sanyo Semiconductor Corporation, lb1694n Datasheet - Page 7

• Manufacturer Part Number: lb1694n
• Description: 3-phase Brushless Motor Driver
• Manufacturer: Sanyo Semiconductor Corporation

7-1. Output off time
7-2. Output current neglect time
7-3. Output off time setting
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
current which depends on resistor R connected to the R pin. The C charging current I
as follows:
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
current is a constant current, and is set at about 3 times charge current I
about 1/3 of the output off time. The C discharge current I
according to the following equations:
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.
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.
I
t
I
t
off
sm
CL
CL
6 C/I
6 1.53
6 C/I
6 0.41
6 1.3/R (R must be set to be 13 k
6 1.3/R
CL
CL
R
R
2.0
1.6
3
C
C
LB1694N
CH
and the output current neglect time t
and 100 k .)
CL
, As a result, the output current neglect time is
CL
and output off time t
sm
are determined
off
are calculated
No.5243-7/8