Am4961a – Diodes AM4961A User Manual
Page 11
AM4961A
Document number: DS36626 Rev. 1 - 2
11 of 17
October 2013
© Diodes Incorporated
AM4961A
A Product Line of
Diodes Incorporated
NE
W
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OD
UC
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Applications Note
(cont.)
Minimum Speed Setting
The minimum speed setting prevents the motor speed dropping below a set speed when the speed demand is too low (i.e. VPWM voltage is
closer to 3.6V)
When the VPWM pin voltage higher than the VMIN voltage, VMIN voltage is compared to the COSC pin to generate output PWM duty.
Therefore, setting VMIN to certain fixed voltage forces the VMIN to control the speed even when the PWM voltage is higher.
The output PWM duty will be clamped to the PWM duty represented by the VMIN voltage when the speed demand is lower than the set
minimum.
If VMIN is not used or application does not need to set the minimum speed, connect the VMIN to VPWM directly. When the VMIN is connected
to the VPWM the minimum output PWM duty defaults to 10%.
Rotor Lock Detect and OFF time 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 Am4961A 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 2µA. If the voltage on the C4 reaches the high threshold, V
CTH
(3.7V) before the next Hall signal change,
the output will be shutdown and the device will enter lock condition.
T
LCKDET
=
C4 (V
CTH
− V
CTL
)
I
CHG
The thresholds voltage and charge current are fixed, therefore the T
LCKDET
T
LCKDET
= C4
Ч 1 Ч 10
6
time depends only on the value of C4.
For C4 of 0.47µF, T
LCKDET
is 0.47s
If lock detection causes device to enter output shutdown, the CT pin will discharge the C4 capacitor with I
DHG
provide T
OFF
period. The T
OFF
is
the time the device waits before next auto-restart. During T
OFF
period, the C4 is discharged for the high threshold, V
CTH
to low threshold V
CTL
at the discharge current I
DHG
which is typically 0.2µA.
T
OFF
=
C4 (V
CTH
− V
CTL
)
I
DHG
The thresholds voltage and discharge current are fixed, therefore the T
OFF
T
OFF
= C4
Ч 10 Ч 10
6
time depends only on the value of C4.
For CT of 0.4µs, T
OFF
is 4.7s before the next auto restart.
Thermal Shutdown
AM4961A includes a thermal shutdown function. When the device junction temperature is higher than 176 ºC typical, the thermal shutdown
function is triggered and the low side output transistors in H- bridge driver will be turned off. When the IC junction temperature drops below
148°C typical, the device will recover.