AN1625 STMicroelectronics, AN1625 Datasheet - Page 16

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

AN1625 APPLICATION NOTE
2.12 Over Current Detection
To implement an Over Current (i.e. short circuit) Protection, a dedicated Over Current Detection (OCD) circuitry
(see Figure 17 for a simplified schematic) senses the current in each high side. Power DMOS are actually made
up with thousands of individual identical cells, each carrying a fraction of the total current flowing. The current
sensing element, connected in parallel to the Power DMOS, is made only with few such cells, having a 1:N ratio
compared to the power DMOS. The total drain current is split between the output and the sense element ac-
cording to the cell ratio. Sensed current is, then, a small fraction of the output current and will not contribute
significantly to power dissipation.
Figure 17. Over Current Detection simplified circuitry.
This sensed current is compared to an internally generated reference to detect an over current condition. An
internal open drain mosfet turns on when the sum of the currents in the bridges 1 and 2 or the current in the
bridge 3 reaches the threshold (5.6A typical value); the open drain is available at the DIAG pin for diagnostic
purposes or to ensure an over current protection, connecting EN and DIAG together and using an RC network
(see Figure 17).
16/39
C or LOGIC
V
DD
R
C
EN
EN
DIAG
EN
TO GATE
40 TYP.
R
LOGIC
DS(ON)
POWER SENSE
OPEN-DRAIN
INTERNAL
1 cell
COMPARATOR
OCD
HIGH SIDE DMOS
POWER DMOS
OVER TEMPERATURE
n cells
I
1
OUT
I
1
/ n
1
I
I
I
REF
REF
1
+I
VS
2
+
/ n
A
OUT
I
2
/ n
2
POWER DMOS
HIGH SIDE DMOS
I
2
n cells
D02IN1381
POWER SENSE
I
3
1 cell
/ n
OUT
3
POWER DMOS
I
3
VS
n cells
B
HIGH SIDE DMOS
POWER SENSE
1 cell