Rainbow Electronics ATA6834 - Manual (Page 11)

11
9122B–AUTO–10/08
ATA6833/ATA6834 [Preliminary]
3.8 Short Circuit Detection
Short circuits in the motor bridge circuitry are sensed by S1 to S3 inputs. Internal comparators
monitor the voltage differences between the drain and the source terminals of the external
power-NMOS transistors. If one transistor switches on and the voltage drop from its drain to
source stays higher than the threshold V
SC
(typical 4V) for a longer time than t
SC
(typically 6 µs),
a short circuit in this branch is detected. In this case, all output drivers are switched off immedi-
ately and DG1 pin will be set to HIGH. With a rising edge at any of the pins IL1 to IL3 or IH1 to
IH3, the diagnostic output DG1will be reset and the drivers can be switched on again.
3.9 Cross Conduction Timer
In order to prevent damage of the motor bridge due to peak currents a non-overlapping phase
for switching the power-NMOS transistors is mandatory. Therefore, a cross conduction timer has
been implemented to prevent switching on any output driver for a time t
CC
after any other driver
has been switched off. This also accounts for toggling any other driver after a short circuit was
detected. An external RC parallel combination defines the value for t
CC
and can be estimated as
follows:
t
CC
= K
CC
×
R
CC
[k
Ω
]
×
C
CC
[nF], K
CC
is specified in Section 8. “Electrical Characteristics” on
page 15 .
The RC combination is connected between CC and GND pins. When one of the drivers has
been switched off the RC combination is charged to 5V (VINT) and discharged with its time con-
stant. Any low to high transition at the control inputs will be masked out at the driver outputs until
the voltage at CC pin drops below 67% of its initial value (VINT). The timer will be re-triggered at
any time by any falling edge at the control inputs. This is shown in the following figure.
Figure 3-6. Timing Scheme of the Cross Conduction Timer
At least 5 k
Ω
minimum and 5 nF at maximum should be used as values for the RC combination.
10 k
Ω
is recommended. If the non-overlapping phase is controlled by the external microcon-
troller, it is possible to do without the external capacitor. The minimum time t
CC
is defined by the
parasitic capacitance at CC pin.
t
cc
t
cc t
cc
L1
IH1
IL3
CC
L3
H1
IL1
V
CC
= 67% V
VINT
V
CC
= V
VINT