AN1625 STMicroelectronics, AN1625 Datasheet - Page 17

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

Figure 18. Over Current Operation after a short circuit between an OUT pin and GND. EN and DIAG
Figure 18 shows the device operating in overcurrent condition (short to ground). When an over current is de-
tected the internal open drain mosfet pulls the EN pin to GND switching off all 6 power DMOS of the device and
allowing the current to decay. Under a persistent over current condition, like a short to ground or a short between
two output pins, the external RC network on the EN pin (see Figure 17) reduces the r.m.s. value of the output
current by imposing a fixed disable-time after each over current occurrence. The values of R
selected to ensure proper operation of the device under a short circuit condition. When the current flowing
through the high side DMOS reaches the OCD threshold (5.6 A typ.), after an internal propagation delay
(t
(V
decay as it circulates through the freewheeling diodes. Since the DMOS are off, there is no current flowing
through them and no current to sense so the OCD circuit, after a short delay (t
open drain device off, and R
(V
rent can be very high, the external RC network provides a disable time (t
(see Figure 18).
The maximum value reached by the current depends on its slew-rate, so on the short circuit nature and supply
voltage, and on the total intervention delay (t
imum value reached by the output current becomes lower, because the capacitor on EN and DIAG pins is dis-
charged starting from a lower voltage, resulting in a shorter t
The following approximate relations estimate the disable time and the first OCD intervention delay after the short
circuit (worst case).
The time the device remains disabled is:
V
OCD(ON)
EN(LOW)
TH(OFF)
TH(ON)
I
S OVER
t
OCD(ON)
), after the t
) the open drain starts discharging C
) all the Power DMOS turn off after the internal propagation delay (t
is the minimum voltage reached by the EN pin, and can be estimated with the relation:
V
pins are connected together.
TH(OFF)
t
OCD(OFF)
t
DELAY
D(ON)EN
t
EN(FALL)
delay, the DMOS turn on and the current restarts. Even if the maximum output cur-
EN
t
D(OFF)EN
can charge C
Output Current
t
t
DIS ABLE
E N RISE
t
DISABLE
EN = DIAG
=
=
DELAY
t
R
OCD OF F
EN
EN
EN
. When the voltage at EN pin reaches the turn-on threshold
. When the EN pin voltage falls below the turn-off threshold
). It can be noticed that after the first current peak, the max-
C
EN
+
t
ln
EN RISE


V
------------------------------------------- -
DELAY
V
DD
DD
V
–
V
–
t
DISABLE
TH(ON)
V
.
+
EN(LOW)
V
EN LOW
t
T H O N
D O N E N
DISABLE
AN1625 APPLICATION NOTE


t
D(ON)EN
t
D(OFF)EN
EN(RISE)
OCD(OFF)
Output Current
) to ensure a safe r.m.s. value
). The current begins to
), switches the internal
EN = DIAG
EN
and C
EN
17/39
are