Damaged TPL7407LA Transistor Array (*Updated With Scope Testing Images*)

Question

Application We have chosen the Texas Instruments TPL7407LA to drive groups of LEDs for an automotive lighting solution. The TPL7407LA inputs are connected to +12 VDC input signals to trigger the TPL7407LA’s outputs to turn on. These outputs are sinking current from groupings of automotive LEDs through the TPL7407LA.

INPUTS

BRAKE-IN +12VDC signal from vehicle's mechanical brake switch

TURN-IN +12VDC signal from vehicle's electronic flasher switch OR thermal hazard switch (depending which is enabled)

REVERSE-IN +12VDC signal from vehicle's mechanical reverse switch

PARK-IN +12VDC signal from vehicle's mechanical headlight switch

GND Vehicle's ground

COM Connected to 9V rail. 9V rail is also providing power to LED groupings.

OUTPUTS

BRAKE-PARK-LED-GND (O1/O2) LED common ground from brake light LED grouping

TURN-ON Connected to micrcontroller pin. Pullup resistor to 5V rail installed to preventing floating. 5V rail also connected to microcontroller power supply pin. REVERSE-LED-GND LED common ground from reverse light LED grouping

BRAKE-PARK-LED-GND (O6) LED common ground from brake light LED grouping. Resistor inline used to dissipate power and ultimately lower LED grouping's brightness.

Application We have chosen the Texas Instruments TPL7407LA to drive LEDs for an automotive lighting solution. The TPL7407LA inputs are connected to +12 VDC input signals to trigger the TPL7407LA’s outputs to turn on. These outputs are sinking current from groupings of automotive LEDs through the TPL7407LA.

Issue When applying power to the TPL7407LA on the test bench, the TPL7407LA inputs and outputs work just as expected. Power from the test bench is supplied by a generic adjustable power supply set at 12.5 VDC in this scenario. The TPL7407LA sinks a maximum of approximately 1.0 A total when all inputs into the TPL7407LA are activated with the given +12 VDC inputs.

When installing our product on a vehicle, testing goes mostly as expected except for the scenario where more than one of the +12 VDC inputs into the TPL7407LA goes high. The vehicle provides the switched +12 VDC power input triggers via mechanical OEM vehicle switches (brake light push button switch, turn signal electronic flasher, hazard thermal flasher, etc).

We notice that when multiple transistor are activated (typically from both the brake and the turn +12 VDC signal inputs), then after the lines are deactivated - upon the next activation of the lines, all outputs are activated no matter which singular transistor is activated. I believe this is a symptom of a damaged transistor array and we are unsure why this condition is occurring.

We have since ordered an automotive grade ULQ2003AQDRQ1 equivalent to see if this part is more resilient than the TPL7407LA for this application, but it is still very bizarre to us why this transistor array would be damaged in this type of environment.

TPL7407LA Datasheet: http://www.ti.com/lit/ds/symlink/tpl7407la.pdf

Update

We were able to purchase and make use of an oscilloscope and here were our findings:

TURN-IN Active (electronic flasher), No Diode On Input

TURN-IN Active (thermal flasher), No Diode On Input

TURN-IN Active (electronic flasher), Diode Added On Input

TURN-IN Active (thermal flasher), Diode Added On Input

TURN-IN Active (thermal flasher), BRAKE-IN Active, Diode Added On Input

PARK Active (Headlight Switch), Diode Added On Input

Answer 1

Unlike bipolar transistors, mosfet input stages are high impedance, which is usually an advantage but sometimes carries disadvantages as well. In noisy environments like automotive applications, there are often energetic transients coupled into lines, and the higher the impedance terminating the line, the more voltage produced by the energy of a transient.

What is needed is a means of dissipating short bursts of energy, and clamping diodes are excellent at the job. Diodes are quite robust and it's difficult to damage one in short spaces of time.

I suggest you try clamping each of your inputs with at least one diode, cathode to input and anode to ground. That will limit the negative voltage excursion to a diode drop below ground and allow the energy of any transient to dissipate in the diode.

As you found, trying to use a diode to block a transient presents it with a high impedance which cause high voltage spikes until the the transient finds a way to dissipate its energy.

If you want to be quite sure, you can also use another diode, anode to input and cathode to a positive rail to clamp the the inputs' positive excursions to one diode drop above that rail, although the TPL7407 datasheet indicates it has over-voltage protection on its inputs. In any case, you are usually better off allowing spikes to be dissipated in cheap, robust components like diodes, rather than inside an IC.

I understand that adding extra components partially defeats the point of using an IC for your low-side driver, but a few diodes in the right places can save you a lot of grief.

Answer 2

I suspect your problem is one of layout. If the chip ground bounces below MCU ground, you can easily exceed the -300mV absolute maximum input voltage. That can cause latchup with your relatively large currents.

I suggest series resistors on each input in the 1K range.

You may be a bit close on maximum total current, with all 7 outputs activated and a relatively modest 70°C Ta you're limited to a bit over 150mA per output in the PW package, but I don't think that's what's happening in your bench tests.