TPS92640: Reference design values and calculated values do not match

Thanks. So far here are the values I have:

Rvout1    11.6K
Rvout2    10K
Con    1nF
Ron    4.22K
Rcs    25mR
L    33uH
Cout    100nF
Cin    47uH
Rudim1    100K
Rudim2    14.7K
Rudim3    17.8K
MOSFET    FDMS86183

The specs are here:

Vin-min    11.1
Vin(nom) 24
Vin(max) 75
Vled 5.2
Iled 10
Fsw 500KHz
Vcs 0.2V
ΔIled-pp 0.2A
ΔVin-pp 1.5
Vturn-on 10V
Vhys 5V
Efficiency (η)    0.85
# of LEDs 1

I calculated the MOSFET Pd as about 1.5W and conduction losses of about 4.5W. If I reduce the frequency, I'll need a bigger inductor and those at higher current values are really expensive. At the same time, I wanted to keep the ripple current as minimal as possible.

Would this design work?

Irwin Nederbragt said:

Fsw 500KHz, I get 511 KHz, Ron at 4.32K gives 500 KHz

Sure. I will update this.

Irwin Nederbragt said:

Vcs 0.2V, If Rcs is 25 mohm current will be 8A, if you have Viadj at 2.0 volts, if you have it high then it will be close to 10A, 2.54V/10=0.254V

I plan to use a 2.5V voltage reference here for analog dimming. So yes it will be closed to 10A then.

Irwin Nederbragt said:

MOSFETs will need good heatsinking to dissipate that much power.

Yes I was thinking about this. I really could have done without onboard heatsinks and I was hoping to use copper planes itself for the heatsinking.

Irwin Nederbragt said:

at 75 volts input 5.2 volts output and 500 KHz the minimum on-time will be less than the TPS92640 spec which means it will pulse skip.

How did you calculate ton and toff?

Thanks again for all the help on this. I really appreciate it.

Thank you.

Irwin Nederbragt said:

Vhys 5V, I get turn-off at 4.94V, I would lower the hysteresis because you don't want it running this low something like 10V to 9V or 10V to 8V depending on what the input ripple looks like.
Efficiency (η) 0.85

Hi, do you mean lowering the Vhys value for calculation to 2V or increasing it to about 10V?

Hello Irwin,

I was able to put this together and do some testing but it does not seem to be working correctly. The driver is putting out 28A instead of the designed 10A. I've attached the schematic here and the specs and BOM is listed here. I'm not sure what seems to be wrong.

A couple of notes about the schematic:

1. C2, C3, C4, C5, C8 and C9 are simply footprint placeholders and are not actually populated. CIN1 and CIN2 are 100uF.
2. Same goes for R6 and R7 where only R6 is populated and Cout1, 2, 3 and 4 where only Cout1 is populated.
3. Not sure if the gate resistors are needed and hence I've chosen to place 0R resistors there.
4. There is a LM4040 5V reference that is being stepped down to 0-2.5V at IADJ pin for analog output current control.

Specs:

• Vin-min 11.1V
• Vin(nom) 24V
• Vin(max) 60V
• Vled 5.2V
• Iled 10A
• Fsw 500KHz
• Vcs 0.2V
• ΔIled-pp = ΔIL-pp 0.2V
• ΔVin-pp 1.5V
• Vturn-on 10V
• Vhys 3V
• Efficiency (η) 0.85
• # of LEDs 1

BOM:

• Rvout1 11.5K
• Rvout2 10K
• Ron 4.3K
• Rcs 25mR (100mR x 4 in parallel)
• L 33uH
• Rudim1 51K
• Rudim2 7.5K
• Rudim3 11.8K
• Rf 1.5R
• MOSFET TK22A10N1
• Cout 100nF
• Cin 47uF
• Con 1nF
• Ccomp 470nF
• Cvcc 2.2uF
• Cboot 470nF
• Cvref 100nF
• D1 BAS316
• D2 FERD20S100SH

PWM_CC_laserdriver_v9-alt.pdf

Hi,

I can answer some of these right away. I'll come back with answers to signals on the pins you've suggested. Another point I missed mentioning earlier was. The heatsinks on the top two MOSFETs seem to be getting hot while the lower ones are stone cold. This probably suggests that the lower ones are not being turned on at all? 

Can you verify that the current sense resistors are actually 0.025 ohm X 4? - These are actually 100mR x 4 in parallel. I don't have a milliohm meter so I'm not sure how to measure these.

Can you verify that the current measurement is accurate (can check with a lab supply shorted and see if it matches what it's putting out)? - This was done using a clamp meter. The power supply is a 24V 15A brick. 

Viadj needs to be 2.5V to get 10A so 0.2 will only give 8A. Also check Viadj. - Before connecting the load I set this to 2.5V using the potentiometer. 

If you have a potentiometer on Viadj does the output current change when you lower the resistance of the pot? - No, it does not. 

Can you check to make sure the MOSFETs are connected correct, Left pin is gate, middle is the drain and right is the source? - Yes this is connected correctly. In fact, this was the first thing I checked. 

Hello,

Sorry, I meant 0.100 ohms X4, just used the wrong number.

Okay, if the potentiometer is not changing the current and CS is at 0.960 volts something is wrong.  Can you check if it is even switching?  It could be hitting the upper current limit and cycling but I think you'll find that it's just trying to be on and not regulating which would explain why the top MOSFETs are hot and the bottom  are not.  CS should be 0.25 volts if it is above it's not working.  I would check a few things, RT, the upper gate (this is a bit tough since it's floating) to see if it's almost always on.  Comp seems low for that high of a CS, makes me thing that the upper MOSFETs are damaged or connected in a way that they can be on when they are not supposed to be, I don't think I have a schematic but make sure the gate is connected to the correct pin.

Regards,

Hi,

Sorry about the delayed response. I'll try some of the troubleshooting methods you've suggested.