MP2480 Temperature issues

Hello,

I designed a LED driver with the MP2480, with the following specs:

Vin: 24V
Vout: 20V (6 LEDs @3.3V)
Iout: 2A
Inductor: 47uH @3A

FR-4 board, with vias and solder mask free area under the IC. Under operation, the IC temperature reaches 110°C (measured with a thermocouple at the area under the chip).

Increasing the inductor value would help reduce the temperature? What can be done?

Regards,

Well have you tried using a larger inductor? What diode are you using for a catch diode? The data sheet doesn’t provide much guidance, but I would be using a 30V 3A Schottky type .
This design should be workable.

Plus there is this

External Bootstrap Diode
An external bootstrap diode may enhance the
efficiency of the LED driver. For the cases listed
below, add an external BST diode is from a 5V
rail to the BST pin:
z There is a 5V rail available in the system;
z VIN ≤ 5V;
z 3.3V < VOUT < 5V;
Add a diode for high duty cycle operations
(when VOUT/VIN > 65%), and for very-high–

from the datasheet

Thanks for the reply.

Yeah, I saw that in the datasheet but for some reason the duty cycle case slipped my mind.
I´m gonna add the diode and check if it helps.

Another thing I noticed is related to the solder paste: in a second board, with exaclty the same circuit and components, the MP2480 does not heat up as much, in the same conditions and same LED string. I think the solder under the IC is better in this case, maybe there was more solder paste, I don´t know, will investigate that too.

Hi Thiago:

If you can increase the inductor value then that will decrease the switching frequency. This would reduce the switching losses.

Thanks,

ACDCGuy

Hello ACDCguy,

Thanks for your reply.

I was trying to evaluate the power loss on the MP2480, but some parameters are not informed in the datasheet - I mean - for the switching losses. With the Rds I can calculate the resistive loss of course, but you are right, the switching losses on a MOSFET depend on the switching frequency. Maybe I could go from 47uH to 68uH to decrease the switching frequency by 30%.

Regards,

What would be a good value for input capacitor to the MP2480 ?
I´m currently using 10uF 24V ceramic only (X7R) and I noticed a strange behavior when the output current is set to 2A. I don´t think it is temperature related. At 1A it works fine. Also, I had used the bootstrap diode (1N4148) and the driver didn´t work. Removed the diode and it worked. Could be input cap related ? Too small? Really need an electrolytic ?

Hello,

I switched the indutor from 47uH to 100uH (3.5A) and still, can´t make it work at 3A, the temperature increases rather rapidly, and it starts blinking the LEDs due to the turn off/turn on when it reaches aprox. 90°C (measured via thermocouple).
I have used a 100uF capacitor at VIN and the boostrap diode (1N4148).
Lots of vias under the MP2480 connected to the ground plane.

I´m out of ideas.

Maybe you should try a 100uH 5A inductor, at 3A there is some ripple current in the inductor so the peak current is higher than 3A also the 3.5A spec of the inductor is usually at 25C that limit decreases with temperature

Hello,

Sorry to reopen this discussion, but something really strange is going on. I´m using 3 resistors in parallel of 0.2 ohm each to set the current to 3A. Inductors 100uH 6A. But the current never reaches 3A, maximum 2.4A. I have tried adding the bootstrap diode, changed the inductor and nothing. Besides, the temperature is still high, reaching over 100° C and turning off. The tracks are wide and short, so no relevant resistance there.

With 2 resistors only, everything works fine (2A).

Any ideas?

Hi Thiago,

Could you share your latest schematic? It would help us understand better on what might be happening.

Hello Sunandhitha,

Sure. Please find attached, the schematic and a picture of an assembled PCB. It shows only 2 channels. I have added the 100uF capacitor manually to make it more stable in higher currents. VIN = 24V, bootstrap diode not mounted.