TL4242-Q1: General Design Questions

Are any heat issues expected?

- i think 6.6w is very severe for the theraml consumption of device even on aluminum MK2 PCB. But as you say you have a simliar case you i recommend you to double check  the device's power in your case.

The ST pin is not used, can we use a 4.7K pull-up resistor to Vin?

-it's open drain pin, so it's ok to pull up with 4.7 resistor.

The PWM is applied by an N-Channel MOSFET with a 10K pull-up resistor to Vin, is this OK?

-Becasue the absolute value is 40 V for the PWM pin, so the layout should be   

Is it OK to rely on the reverse polarity protection of the TL4242 and using only a bidirectional TVS to protect the input from spikes?

-i think you should series a diode in the input line as the reverse polarity protection.

What is the lowest PWM frequency recommended for automotive lamps?

-becasue the highest frequecny change that our eyes can notice is 60Hz so i recommend the lowest frequency is 100Hz

Are there any known electromagnetic emissions, radiated or conducted? There is no mentioning about this in the data sheet.

-we have the BCI reference design for TPS92611-Q1 you can reference that.

https://www.ti.com.cn/tool/cn/TIDA-050041

i have some concern of your design:

1) the thermal part: because the 6.6 comsumtion is too high for the device even in the aluminum MK2 PCB i suggest you use 2 device to share the thermal. We have a new device named TPS92613-q1 which is defined for the severe thermal condition. So i recommed you to use 2 pics of TPS92613-q1 in this case.

2)the usual Vin range is 18-36V in 24V system, i want to know why the range is 20-35V in your case?

3)i think 8 LED series in one string is too tough for you desgin becasue the max vf=2.4*8=19.2V and when Vin=20V only 0.8V for the device's headroom.

Hi Frank,

Thank you for the reply.

Regarding the Vin, 18-36 is indeed the regular range but we got the client's approval to use 20-35V.

20V will be a minimum in a worst case where all the LEDs will have Vf 2.4V. 

29V is the charging voltage and we set the cut-off to 35Vmax with some tolerance to be sure it will work between 34-35V, also approved by the customer.

Practically the functions of the lamp are not for long duration, one function is the brake and the other turn indicator.

Brake is not applied for long periods and the turn blinks at 50% duty cycle.

The test however requires each function to be active for 1 hour, and it is performed at 28V so the largest power to be dissipated by the IC will be 3W.

We can't add a series diode for reverse polarity protection because it will cause a large voltage drop. The data sheet mentions reverse polarity protection but if not absolutely sure we can consider an alternative solution like a MOSFET. 

We checked the new TPS92613, it could solve another problem we don't like, the PWM dimming of the tail light. We could use 2 IC one for tail at 50mA and another for brake at 300mA, something we can't do with the TL4242, but the thing is it has a much larger voltage drop, almost 1.5V@350mA while the TL4242 is 0.7V.