Am4964 – Diodes AM4964 User Manual
Page 16
AM4964
Document number: DS37241 Rev. 2 - 2
16 of 21
June 2014
© Diodes Incorporated
AM4964
A Product Line of
Diodes Incorporated
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P
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Applications Note
(cont.)
Adjustable Voltage
f
PWM
=25kHz
Low Speed
PWM Variable
High Speed
V
MIN
V
H
=3.6V
V
L
=2.0V
All Parameters Are Tested under V
CC
=12V
V
CF
(T
≤T
L
)
V
CF
(T
≥T
H
)
FG Output(T
≤T
L
)
FG Output(T
≥T
H
)
Speed Control and Minimum Speed Setting
Minimum Speed Setting
The minimum speed setting prevents the motor speed dropping below a setting speed when the speed demand is too low (i.e. PWM voltage is
closer to 3.6V)
When the CF pin voltage is higher than the VMIN voltage, the VMIN voltage is compared with the internal triangular wave to generate the output
PWM duty. Therefore, setting the VMIN to certain fixed voltage forces the VMIN to control the speed even when the CF voltage is higher.
If VMIN setting is not used or application does not need to set the minimum speed, connect the VMIN to CF directly.
Rotor Lock Detect and OFF Tme Setting
The capacitor C4 from CT pin to the ground provides the timing for the lock detect and auto-restart. The capacitor C4 is charges and discharged by
the CT pin at a fixed rate depending on the mode of operation (fan operation status) and therefore the value of the C4 to gives lock-detect time
(T
LCKDET
) and lock time (T
OFF
) before next auto-start retry.
The AM4964 returns the C4 voltage to the low threshold, V
CTL
(1.7
7
V), each time the Hall sensor provides the commutation signal. C4 is charged
with I
CHG
which is typically 2
μA. If the voltage on the C4 reaches the high threshold, V
CTH
(3.7V) before the next Hall signal change, the output will
be shut down and the device will enter lock condition.
C DET
CTH
CT
CHG
The thresholds voltage and charge current are fixed, therefore the t
LCKDET
time depends only on the value of C4.
C DET
For C4 of 0.47
μF, t
LCKDET
is 0.47s.