DRV8412_11 Datasheet, PDF (12/31 Pages) Texas Instruments – Dual Full Bridge PWM Motor Driver

DRV8412
DRV8432
SLES242D – DECEMBER 2009 – REVISED JULY 2011
Figure 6 illustrates cycle-by-cycle operation with high
side OC event and Figure 7 shows cycle-by-cycle
operation with low side OC. Dashed lines are the
operation waveforms when no CBC event is triggered
and solide lines show the waveforms when CBC
event is triggered. In CBC current limiting mode,
when low side FET OC is detected, devcie will turn
off the affected low side FET and keep the high side
FET at the same half brdige off until next PWM cycle;
when high side FET OC is detected, devcie will turn
off the affected high side FET and turn on the low
side FET at the half brdige until next PWM cycle.
In OC latching shut down mode, the CBC current limit
and error recovery circuitries are disabled and an
overcurrent condition will cause the device to
shutdown immediately. After shutdown, RESET_AB
and/or RESET_CD must be asserted to restore
normal operation after the overcurrent condition is
removed.
For added flexibility, the OC threshold is
programmable using a single external resistor
connected between the OC_ADJ pin and GND pin.
See Table 2 for information on the correlation
between programming-resistor value and the OC
threshold. The values in Table 2 show typical OC
thresholds for a given resistor. Assuming a fixed
resistance on the OC_ADJ pin across multiple
devices, a 20% device-to-device variation in OC
threshold measurements is possible. Therefore, this
feature is designed for system protection and not for
precise current control. It should be noted that a
properly functioning overcurrent detector assumes
the presence of a proper inductor or power ferrite
bead at the power-stage output. Short-circuit
protection is not guaranteed with direct short at the
output pins of the power stage.
For normal operation, inductance in motor (assume
larger than 10 µH) is sufficient to provide low di/dt
output (e.g. for EMI) and proper protection during
overload condition (CBC current limiting feature). So
no additional output inductors are needed during
normal operation.
However during a short condition, the motor (or other
load) is shorted, so the load inductance is not present
in the system anymore; the current in the device can
reach such a high level that may exceed the abs max
current rating due to extremely low impendence in the
short circuit path and high di/dt before oc detection
circuit kickes in. So a ferrite bead or inductor is
recommended to utilize the short circuit protection
feature in DRV8412/32. With an external inductance
or ferrite bead, the current will rise at a much slower
rate and reach a lower current level before oc
protection starts. The device will then either operate
CBC current limit or OC shut down automatically
(when current is well above the current limit
threshold) to protect the system.
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For a system that has limited space, a power ferrite
bead can be used instead of an inductor. The current
rating of ferrite bead has to be higher than the RMS
current of the system at normal operation. A ferrite
bead designed for very high frequency is NOT
recommended. A minimum impedance of 10 Ω or
higher is recommended at 10 MHz or lower frequency
to effectively limit the current rising rate during short
circuit condition.
The TDK MPZ2012S300A (with size of 0805 inch
type) have been tested in our system to meet a short
circuit condition in the DRV8412. But other ferrite
beads that have similar frequency characteristics can
be used as well.
For higher power applications, such as in the
DRV8432, there might be limited options to select
suitable ferrite bead with high current rating. If an
adequate ferrite bead cannot be found, an inductor
can be used.
The inductance can be calculated as:
Loc
_ min
=
PVDD ×Toc _ delay
Ipeak - Iave
(1)
Where Toc_delay = 250 nS, Ipeak = 15 A (below abs
max rating).
Because an inductor usually saturates quickly after
reaching its current rating, it is recommended to use
an inductor with a doubled value or an inductor with a
current rating well above the operating condition.
Table 2. Programming-Resistor Values and OC
Threshold
OC-ADJUST RESISTOR
VALUES (kΩ)
22⁽¹⁾
MAXIMUM CURRENT BEFORE
OC OCCURS (A)
11.6
24
10.7
27
9.7
30
8.8
36
7.4
39
6.9
43
6.3
47
5.8
56
4.9
68
4.1
82
3.4
100
2.8
120
2.4
150
1.9
200
1.4
(1) Recommended to use in OC Latching Mode Only
Overtemperature Protection
The DRV8412/32 have a two-level
Copyright © 2009–2011, Texas Instruments Incorporated