AN1625 Datasheet, PDF (33/39 Pages) STMicroelectronics – L6235 THREE PHASE BRUSHLESS DC MOTOR DRIVER

AN1625 APPLICATION NOTE
Motor Data
Winding Resistance (2 phases): 2 Ω
Winding Inductance (2 phases): 800 µH
Supply Voltage:
24 V +/- 5%
Polar Couples:
1
Referring to approximated formulae in Figure 24, it's possible to calculate the Duty Cycle (D), the Switching Fre-
quency (f_SW), the Current Ripple (∆I). With a 8 µs off-time, we will have:
D ≅ 61%, f_SW ≅ 49 kHz, ∆I ≅ 320 mA.
The on-time is t_ON = D / f_SW ≅ 12.5 µs, which is far from the minimum allowed (1.5 µs).
The bulk capacitor needs to withstand at least 24 V + 5% + 25% ≅ 32 V. A 50 V capacitor will be used. Allowing
a voltage ripple of 200 mV, the capacitor ESR should be lower than 200 mV / 1A = 200 mΩ; the AC current
capability should be about 1.5 A (worst case).
Providing a reference voltage of 0.5 V, 0.33 Ω sensing resistor are needed. The resistors power rating is about
PR ≅ Irms² · R_SENSE · D ≅ 0.37 W. Three 1 Ω / 0.25 W - 1% resistors in parallel are used. The charge pump
uses recommended components (1N4148 diodes, ceramic capacitors and a 100 Ω resistor to reduce EMI).
R = 24 kΩ, C = 470 pF are connected to the RC pins, obtaining t_OFF ≅ 7.8 µs. On the EN pin a 5.6 nF has been
placed, and the pin is driven by the µC through a 100 kΩ resistor.
With these values, in case of short circuit between two OUT pins or an OUT pin and GND, the PowerDMOS
turns off after about 1 µs, and t_DISABLE ≅ 240 µs.
Figure 36. Application Example.
+
µC
Logic Supply
or
5V
Custom Logic
-
100 k Ω
18 kΩ 0.25 W
1%
2 kΩ 0.25 W
1%
V_ref = 0.5 V
5.6 nF
Cera mi c
3-Phase
Brushless DC Motor
1N4 148
1N4 148
100Ω
0.25W
10nF 50V
Cera mic
220nF 35V
Ce ramic
OUT1 OUT2 OUT3
VBOOT
CP
VSA VSB
H1
H2
H3
CW/CCW
BRAKE
EN
DIAG
Vref
L6235
SENSEA
SENSEB
RCPULSE TACHO
RCOFF
3X
1 Ω, 0.25 W, 1%
GND GND GND GND
47nF
Cera mi c
24 kΩ
1%
470 pF
Ceramic
100µF 50V
ESR<200mΩ
100nF 50V
Ce ramic
+
VS = 24 V
-
Referring to Figure 23, Figure 24, Figure 25, the dissipating power is about 2.37 W. If the ambient temperature
is lower than 50 °C, with 5 cm² of copper area on the PCB, a ground layer and a PowerSO36 package, the
estimated junction temperature is about 97 °C.
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