Diodes ZXBM1021 - Manual (Page 16)

ZXBM1021
Document number: DS36322 Rev. 2 - 2
16 of 25
www.diodes.com
April 2014
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ZXBM1021
Application Information
(cont.)
Current limit set lower than the peak tail-end current
(Tail-end current = 145mA typ, Current limit set at 0.166mA)
C
LCK
Value
The value of C
LCK
affects three important timings for the ZXBM1021: Soft Start, Lock Detect and Off-time.
Soft Start
When the system is initially powered on, it will go through the soft-start period, during which the motor is gradually ramped to full speed. The soft
start operation relies on the current control feature being implemented and generally the current limit is set slightly above the normal running
current.
In the soft start process, at power-on the voltage at C
LCK
will rapidly increase to the low threshold, V
CLCKTHL
(0.6V). Then, C
LCK
will be charged
from 0.6V up to the high threshold, V CLCKTHH (5V) at a constant current, I LCKCL , which is typically 0.7µA. The the time taken to charge the
capacitor form V
CLCKTHL
to V
CLCKTHH
level depends on the value of C
LCK
capacitor. The soft-start time tss is given by:
Since V
CLCKTHH
, V
CLCKTHL
and I
LCKCL
are fixed, this becomes simply:
For the default value of C
LCK,
0.47µF, soft-start time is:
This is the 3s stated in the datasheet electrical parameter section.
Lock Detect
When the motor is running, the hall sensor will follow the rotor magnetic flux density to detect the commutation point. Should the rotor lock, the
signal from the hall sensor will cease. The value of C
LCK
defines the lock detect time period before the ZXBM1021 shuts down the outputs in
order to prevent damage to the coil.
The ZXBM1021 internal system returns C
LCK
voltage to the low threshold, V
CLCKTHL
(0.6V), each time the hall sensor provides a commutation
signal. C LCK then charges at the run current, I LCKCR , which is typically 5.5µA. If the voltage on C LCK reaches the high threshold, V CLCKTHH (5V),
before the hall sensor provides the next commutation signal then the system will shut down by entering the lock condition.
The thresholds voltage and charge current are fixed, therefore the time depends only on the value of C
LCK
:
For the default value of C
LCK,
0.47µF, t
LCKDET
is as follows:
This should not interfere with normal operation of a fan as the time period for lock detection is many times longer than the expected time for one
revolution. As the hall sensor will detect 4 transitions in a single revolution, 0.376s between transitions would equate to a rotational speed of
40rpm. For use in systems where extremely low rotation speeds are required, the lock detect time can be increased by making the C
LCK
capacitor value larger.