TPS92640: VREF pin 5 is still 0V.

hi 

Do you get the "VREF pin 5 is still 0V" when you only apply 48V Vin? would you mind sharing start-up waveform for VREF, VCC, SW?

Besides, I notice that the gate drive resistor for HS is 43ohm, I do not think this is a reasonable value. I think 2.2ohm same as LS should be enough.

also, what is the purpose for R77 and VR15?

thank you for your reply.
VREF=VCC=SW remained at 0V when the power was turned on under the conditions of Tp26=1.541V and TP27=0V. Originally, SW should be a switching waveform, but it remains at 0V. Is it OK to input 0-2V? Also is the IC broken?
Thank you for pointing out 43Ω. R77 and VR15 are set based on Figure 18 in the datasheet.

hi 

Did you capture the waveforms by triggering function? If VREF=VCC=SW remained 0 all the time then I think start-up fault can be excluded. 

Besides, what do you mean by "input 0-2V"?

I think we can double check the start up again to confirm this and then replace a new IC to check if this has been destroyed. 

Thank you for your reply.
This is the waveform when the 48V power supply is turned on.　I still don't understand that VREF is 0V.

0-2V means the input voltage of the IADJ pin. Dimming is activated by applying 0-2V to this pin.

Please give me advice.

hi 

Normally, VCC will be generated by internal LDO as long as input power supply is applied on VIN pin. Vref will then be generated from VCC. But from the waveforms, both VCC and Vref keeps low even though Vin has ramped up to 48V. So some of my suggestions are:

1. try to increase input voltage 

2. check voltage rating of VCC capacitor, would be better to have greater than 15V to avoid too much capacitance derating

3. replace a new device

Thank you for your reply.
I tried the following based on your advice.
1. Power supply 12~32V input, but SW=VCC=VREF=0 no change
2. C70, C73, C71, C77: rating 50V C74: rating 25V is used. None of the capacitors are short-circuited, so they don't seem to be broken.
3. Used the same new board. When only 12V to 48V power supply is input, SW outputs a clock, but the waveform is not normal. Only in the case of 48V power supply, originally VREF=3V, but it drops to VREF=2V.

4. I checked the board pattern again, but there is no problem.

Can I judge that my circuit has no problem using your simulation tool?

If so, we can determine that there is a problem somewhere in the actual board.
In addition, using this IC, other LED circuits using 12V power supply are working normally.See below. 

Please give me some advice.

hello 

thanks for the information. 

for suggestion #1, have you tried to put the input voltage higher like 50V, 60V.

For you reply #3, do you mean it shows different results with different boards? In the first waveform, how VCC behaves? for the same operation, can you also try to remove R74 and see if Vref still keeps 2V?

thank you for your reply.
#1,C71 is rated at 50V, is it okay to input 50-60V power supply?
#3 gives the same result on different boards. Another circuit with the same IC on the board works fine. DC power supply 12V/48V and FET type are different.
Could it be that the DC power supply and the FETs are different?
When I removed R74, VREF output 3V.
Furthermore, the waveform when R77 is deleted is as follows.

It seems that the waveform is output, but it is not normal.
Please give me advice.

hi sir

I think so far the basic LDO works fine from VCC result. From the SW result, the device seems to shut down for around 270us and then restarts again. I suspect the triggering of OCP during start up. So pls monitor the DS voltage of high side Fet (protection limit is ~1.28V) and at the same time share the datasheet of this Fet. thanks

Thank you for your replay.

Is it correct to understand that OCP operates when 1.28V or higher is applied between Q2's FETGate and Drain?

The Q2 and Q3 FETs are NTD3055-150G (ON Semiconductor). I am using the same type of FET as in the TPS92641EVM user guide.

FET is less than Vds=0.5V when Id=1A. (NTD3055-150 Characteristic) The LED current I use is 1.5A max.

The last waveform I sent was with the LED not connected. Therefore, LED current = 0A.

I don't know what to do. Please give me advice.

hi 

Yes, the DS voltage will be monitored inside the device, once it exceed 1.28V, it triggers OCP.

Do you mean below waveforms were tested without LED? then I would like to know the result with LED.

hi

The result of connecting the LED is as follows. OVP seems to work.

Now I am connecting HG and Q2-Gate directly because D11 might be doing something wrong. This seems fine. Also, I changed R75 from 43Ω to 2.2Ω, but there was no effect.

I'm sorry again, but please give me some advice.

hi 

Pls provide waveforms of Vds of high side Fet as the timing seems match the OCP mechanism. we need to confirm first if there is current overshoot that triggers OCP during start up and then try to figure out more actions. I agree that stable 1.5A current will not trigger OCP. 

another thinking is that  boot capacitor C77 is 0.1uF while it is 0.22uF in EVM. You can check this action at the same time.

Waveforms for three cases were measured.
#1 VIN=48V UDIM=0V IADJ=0V　#1 does nothing.

#2 VIN=48V UDIM=0→1.5V IADJ=0V

#3 VIN=48V UDIM=0→1.5V IADJ=1V

#2 and #3 pulse a little each time the OCP trigger 270usc occurs. Can you infer anything from this?

Please let me know if there is another way to measure.

Please give me your advise.

hi sir

what does ch3 HG represent? Does it mean gate driving signal of high side Fet Q2? But it shows very high voltage spike. what we need to measure is the voltage between Vin and TP45, which is also the drain-source voltage of Q2. you can use a differential voltage probe to test.

for the operation condition, you can just leave UDIM floating without adding any signal to TP26 and apply 1V to IADJ, this will make the device work in analog dimming only. we can begin from this first.

at the same time, have you tried to increase C77 to 0.22uF?

Thank you for your reply.
CH3 is the signal for the HG pin of TPS92640 IC.
I don't have 0.22uF,so I changed C77 to 0.2uF. The schematic is below.

CH1 is the differential voltage waveform between VIN (48V) and the source of Q2 (TP45).

 pulse is generated every 270usec.OCP still seems to work. I don't know what is causing it.

Please give me advice.

hi sir

please follow the operation condition: Vin=48V, UDIM floating, IADJ=1V

please check the action: 1) increase C73 to 0.47uF; 2) decrease output capacitor from 22uF*2 to 0.1uF

Hi

The waveform I sent you last time does not input voltage to the UDIM. It is floating.
1) C73: 0.47uF was changed, but the waveform did not change.
2) Furthermore, the waveform changed to 0.1uF is as follows.

OCP seems to work. LED1 + / TP30 has signal fluctuation.

The operating circuit is as follows.

The differences from the EVM circuit are the values of R78, R72, R73, and L11. It is possible to replace the resistor, but is it necessary?
There are no replacement parts for the L11.

Please give me your advise.

hi sir

I've compared the configuration between the schematic and EVM's. Looks like no big difference. can you further provide waveforms of pin Ron, inductor current together with SW in the same waveform and also zoom in to see the details like below. I am just wondering how the inductor current behaves during start-up that causes OCP. 

thank you for your reply.
The requested waveform is as follows. CH3 is RON (IC-2 pin).

CH3 (RON) has a little noisy in sync with the CH1 signal.
Does this tell you anything?

From Sept. 28 to Oct. 2 will be maintenance, so please let me know how to contact you.

hi sir

Do you mind getting the start-up waveform rather than steady result? I would like to check the initial state of LED1+/TP30, pls also put SW1/TP45 and boot voltage (voltage of C77) in the same waveform.

I think this is due to that high side Fet is not properly turned on which leads to large voltage drop on drain-source. abnormal boot voltage is highly suspected.

1. pls ensure TP30 voltage is zero at initial state, this will help charge boot capacitor during start up.

2. short R75 

3. is the bootstrap diode D12 same as EVM?

you can contact  me by mason-wang@ti.com

Thank you for your reply.
The waveform when DC48V is turned on is as follows.
IADJ=1V is input from the external power supply before turning on the DC48V.

1) TP30 is 5V 2) R75 is shorted
3) D12 is different from DVM. The D12 I'm using is rated at 20V.
The DVM is rated at 100V. CH3( BOOT) is up to 48V, so I think this is the problem. What do you think about it?

this thread has been replied by mail

I will be replacing the D12 now . Please wait while I check the operation.

Hi Wang

The waveform after replacing D12 is as follows

The waveform is the same as the one you sent me.
Also, the LED can now be turned on.
I think this is fine, what do you think about it?

hi sir

I think the waveforms are fine and no more OCP observed. You can further check if LED current is the target value.

Hi Wang

Dimming problem has occurred.
As in the previous report, when IADJ=1V and the 48V power supply is turned on, the LED lights up.

 SW signal is working fine.

However, when IADJ is about 0V, the LED does not light when the 48V power supply is turned on,

and when IADJ is set to 1V, the LED does not light.

The waveform at this time is as follows.

The left is 48V power on, the right is followed by IADJ set to 1V. SW pulse signal does not work.
What is wrong? Please help me.

hello sir

I will verify this action on the EVM. Do you still keep the changes shown in the schematic and apply 1V or 0.2V DC supply at TP27? Also pls monitor VCC when you change the Iadj back to 1V. 

Hi Wang

Thank you for your reply.
The working circuit is the same as the schematica above.

The voltage( 0.2V,1V) is input to TP27 using an external power supply.
I will check VCC when setting IADJ=1V.

hello sir

The operation with IADJ=0.2V is normal on EVM, pls see below waveform: CH1 VIADJ, CH3 SW, CH4 LED current 

Do you mind checking if it helps after removing C72?

Hi Wang

I removed C72, but after turning on the 48V power supply with IADJ=0V, it does not light up even if IADJ=1V.

Therefore, I changed the circuit so that the IADJ voltage can be input to the VREF reference. The waveforms when IADJ = 1 V, 0.7 V, 0.5 V, and 0.2 V are as follows.

IADJ = 0.7, 0.5V disappears after 230usec interval pulse generation. 0.2V has no pulse waveform. 

Please give me advice.

hello sir

Do you mean you change the circuit by re-adding VR10 and R74 to adjust Iadj by Vref and under this condition 1V Iadj operation is available while directly applying 1V on Iadj pin will not work? 

Can you help check the LED current with Iadj=1V, 0.7V, 0.5V and see if the current follows Iled=VIadj/10/Rcs? calculated values 1V-->0.5A, 0.7V-->0.35A, 0.2V-->0.1A.

Can you help check if SW on time (high voltage level) with Iadj=0.5V is around 235ns, which is the min. on time?

Hi Wang

As with the EVM circuit, I tried connecting IADJ to VREF. Dimming can be done by inputting 0 to 2V to IADJ by adjusting VR10.The graph at the time of lighting is as follows.

Operates according to ILED = Vadj/10/Rcs.
The waveforms when IADJ=0.5V, 0V are as follows.

The SW1 ON signal is 1.63usec. It is not 235nsec.

What I noticed is that VREF=3V when it can be lit, and VREF=2.9V when it can't be lit at startup. If it does not turn on at startup, it is not possible to turn on even if IADJ is set to 1V or higher, so is it possible that an interlock such as UVLO or Shutdown is operating? I am concerned that C69 is 10uF larger than EVM. Please give me some advice.

hello sir

Actually my action on the EVM was removing R5 to disconnect Vref and apply DC source on IADJ. In this condition, EVM can also work well. So my thinking is that the problem is not related to whether connecting Iadj to Vref or not. 

From your description, this problem does not always happen but has a chance. Is my understanding correct?

Actually my concern is that VCC pin may not operate normally as a high voltage may be applied on this pin with original low voltage rating diode D12 before(if it was shoot through, high voltage directly applied on VCC.) can you check VCC performance for both normal (can light on and be adjusted between 0-1V) and abnormal (cannot light on or can only light on with large value Iadj)conditions? 

Have you replaced a new device after changing to the new diode D12?

Hi Wang

thank you for your answer.
If IADJ is set to 0.7V or more at startup, it lights up, and if it is less than that, it cannot light up. If it can be lit, it can be dimmed even if it is lowered to IADJ=0V. U16 has not been replaced after replacing D12. Exchange of U16 is impossible.

I have 2 more of the same board, one probably had a damaged U16 and one was powered on only once. The circuit configuration is in the initial state. So I'm worried about VCC damege. I don't want to use it because it's the last board. Should I try this?

hello Sir

For the last board, my thinking is that as long as you use the diode with enough voltage rating, the board should be safe. 

Before trying your last board, I think it would be better to double confirm the VCC performance for normal and abnormal conditions as I mentioned above.

Also I would like to confirm the symptom of this issue again: 

1. start up with Iadj>0.7V, can light up and be dimmed even lower to Iadj=0V; start up with Iadj<0.7V, cannot light up.

2. I notice you mentioned that after connecting Iadj to Vref, Dimming can be done by inputting 0 to 2V to IADJ by adjusting VR10. So under this condition, Iadj should also be larger than 0.7V during start-up, right?

Hi Wang

thank you for your answer.
Both 1 and 2 are correct.
Please give me advice.

After receiving your advise, I will decide whether to use the last board.

hello Sir

My advice is to check start-up waveform again with Iadj <0.7V and monitor signals of VCC, Vref, Vboot. Just want to confirm again if these regulated voltages work normally.

After that, I recommend to test on a new board. also you mentioned that there are two more boards, and one is possibly damaged. So this one also has this issue?

Hi Wang

Sorry for the delay in contacting you.
The waveforms when starting with IADJ=1V and IADJ=0.4V are as follows.

CH2 is the waveform of VCC, VREF and BOOT.

I can't figure out the cause by looking at these waveforms.
A damaged board does not work at all. The rest will be this board and the final new board.

Is the IC chip really broken?

Please give me advice.

hi sir

sorry for the delay.

thanks for the result. I guess sth is wrong with VCC as it does not ramp again. But have you tried the new board with new diode? I think the operation is safe with right voltage rating diode. Need the comparison result to figure out if VCC shoot through by wrong diode is the root cause.

Hi Wang

thank you for your answer.

The test results look good on the new board.

The circuit exchanged only D12. The circuit is as follows.

The waveform is as follows.

When starting with Iadj=0V, a pulse is generated in SW1. When IADJ was gradually increased to 0.5V and 1.2V, CH2:LED+ voltage increased and lit up. 
I think that the reason why IADJ=1.2V is the first lighting is due to the offset by R77 and VR15. Therefore,
try changing the resistance to an appropriate value.

hello sir

yes, this can be the reason. To verify it, can you reduce R77+VR15 to see if 1.2V first lighting will move to a smaller IADJ?