Three-Phase Sensorless Fan Driver
A4941
7
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Application Information
OUTC
CP1
CP2
VCP
GND
SLEW
PWM
FG
OUTB
OUTA
SENSE
VBB
CTAP
GND
FC
TEST
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
PAD
A4941
M
R2
R1
C1
D1
D2
C3
C2
V
BB
V
BB
V
BB
CTAP
CTAP
Typical Application Circuit; speed adjusted via VBB
Name Typical Value Description
C1 10 μF / 25 V
VBB supply capacitor, minimum 10 μF,
electrolytic can be used
C2,C3 0.1 μF / 25 V Charge pump ceramic capacitors
R2 10 kΩ
FG pull-up resistor, can be pulled-up to
V
BB
if required
D1 >1.5 A rated
Optional blocking diode for supply reverse
polarity protection
D2 15 V Transient voltage suppressor (TVS)
R1 0.167 Ω
Current limiting sense resistor, required for
low resistance motors
Startup Oscillator Setting (FC)
Typically, the 50 ms setting is optimum for motors appropriate
for use with the A4941. If the motor does not produce a proper
BEMF signal at startup when power is applied, a longer setting
may be required.
SLEW Setting
Connect SLEW to ground to enable the soft switching func-
tion. For some motors, soft switching may reduce audible noise.
Enabling the soft switching function can result in motor stall for
some motors, specifically motors with large inductance that run at
higher speeds. For this situation, there are two potential solutions:
• Limit the motor speed by lowering the maximum demand, by
reducing either V
motor
(max) or the PWM duty applied.
• Disable soft switching by leaving SLEW pin open circuit.
Current Limiting
Use of the current limit circuit is not required. If motor resistance
(phase-to-phase) will limit the current below the rating in the
Absolute Maximum table, then simply connect the SENSE pin to
ground. That is:
• If (V
BB
(max) / R
motor
) < 1.25 A, eliminate R
S
.
• If (V
BB
(max) / R
motor
) > I
OUT
(max), the choice of R
S
deter-
mines the current limit setting; recommended range is
167 mΩ < R
S
< 250 mΩ.
Note: For some motor types, use of the current limit circuit may
prevent proper startup due to the effect of the chopping on the
BEMF voltage appearing on the tri-stated winding.
Layout Notes
• Connect GND pins (5,11) to exposed pad ground area under
package.
• Add thermal vias from exposed pad to bottom side ground
plane.
• Place V
BB
decoupling capacitor as close to the IC as possible.
• Place sense resistor, (if used), as close to the IC as possible.
