AN1625 STMicroelectronics, AN1625 Datasheet - Page 7

• Manufacturer Part Number: AN1625
• Description: L6235 THREE PHASE BRUSHLESS DC MOTOR DRIVER
• Manufacturer: STMicroelectronics

Figure 6. Two situations that must be avoided.
2.5 Sensing Resistor
Motor winding current flows through the sensing resistor, causing a voltage drop that is used, by the logic, to
control the peak value of the load current. Two issues must be taken into account when choosing the R
value:
A good compromise is calculating the sensing resistor value so that the voltage drop, corresponding to the peak
current in the load (Ipeak), is about 0.5 V: R
It should be clear that sensing resistor must absolutely be non-inductive type in order to avoid dangerous neg-
ative spikes on SENSE pins. Wire wounded resistors cannot be used here, while Metallic film resistors are rec-
ommended for their high peak current capability and low inductance. For the same reason the connections
between the SENSE pins, C6, C7, V
(see also the Layout Considerations section).
The average power dissipated by the sensing resistor is:
D is the duty-cycle of the PWM current control, I
Nevertheless, sensing resistor power rating should be chosen taking into account the peak value of the dissi-
pated power:
where I
Using multiple resistors in parallel will help obtaining the required power rating with standard resistors, and re-
The sensing resistor dissipates energy and provides dangerous negative voltages on the SENSE pins
during the current recirculation. For this reason the resistance of this component should be kept low.
The voltage drop across R
parator. The lower is the R
to the input offset of the current sense comparator: too small values of R
pk
is the peak value of the load current.
DON’T connect the Logic GND here
Voltage drop due to current in sense
L6235
path can disturb logic GND.
GND
GND
V
SA
GND
SENSE
SENSE
V
SENSE
SENSE
SB
GND
A
B
SA
is compared to the reference voltage (on Vref pin) by the internal com-
value, the higher is the peak current error due to noise on Vref pin and
, V
R5
SB
SENSE
P
and GND pins (see Figure 5) must be taken as short as possible
R
P
C6
R
rms
I
rms
= 0.5 V / Ipeak.
I
pk
is the r.m.s. value of the load current.
2
2
R
R
C7
S ENSE
S ENSE
Recirculating current cannot flow into the
bulk capacitor and causes a high voltage
V
+
S
-
D
= 8 ÷ 52 V
;
,
spike that can destroy the IC.
DON’T put a diode here!
AN1625 APPLICATION NOTE
SENSE
must be avoided.
SENSE
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