Diodes ZXBM1021 - Manual (Page 17)

ZXBM1021
Document number: DS36322 Rev. 2 - 2
17 of 25
www.diodes.com
April 2014
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ZXBM1021
Application Information
(cont.)
Off Time
If the above lock detection causes a device shutdown, C
LCK
is used again to provide the time period, t
OFF
. This is the time for which the device
will remain shut down after rotor lock is detected. The output shutdown period, t OFF , depends C LCK being discharged from the high threshold to
the low threshold at the discharge current I
LCKDL
, typically 0.7µA.
As above, the threshold voltages and discharge current are fixed, therefore the time depends only on the value of C
LCK :
For the default value of C
LCK,
0.47µF, t
OFF
is as follows:
FF
A note on probing C
LCK
Due to the very small charge and discharge currents involved with C
LCK
, putting an oscilloscope probe onto that node can have a significant
impact on the charge time. The above graph shows simulation for the impact on soft-start t
SS
. A typical 10X voltage probe will have 10MΩ
resistance, which is enough to increase soft-start time from 3 seconds to over 5 seconds. Probing this point can be useful in understanding the
operation of the system but it is important to be aware of the impact that the scope probe will have on the operation of the circuit.