WEBENCH® Tools/LM5088-Q1: LM5088 for -60V design

Hi Roger,

How do you apply Enable signal to LM5088?

EN pin to GND voltage must be below 14V, you need to use level shift circuit or zener clamp circuit to limit EN pin to GND pin voltage below 14V, otherwise device maybe damaged once output reach -60V !! 

the SW node should swing from 5V to -60V at normal operation. 

Pls also current sense resistor to 100mohm for debugging.

B R

Andy

Hi Andy,

The control voltage of EN pin is 3.3V. 

We modified current sense resistor from 30m ohm to 100m ohm, the result is the same.

we can not get -60V. 

Regards,

Roger

Hi Roger,

the EN control voltage 3.3V  is referred to input 5V GND? or referred to output -60V (same as LM5088 GND)?

Hi Andy,

EN control voltage 3.3V  is referred to input 5V GND and 5V GND is the same LM5088 GND.

Regards,

Roger

Hi sir,

LM5088 seems to be in Hiccup Mode Restart as below, right? If yes, what is the cause? In addition, the output is no loading.

Hi Andy,

In our schematic, NU is mean no components. So, I try R1553 NC and R1544 & R1545 mount components for EN pin.

I measured EN pin= 1.56V, but there is no signal as SW pin. 

Hi Andy,

In our schematic, NU is mean no components. So, I try R1553 NC and R1544 & R1545 mount components for EN pin.

I measured EN pin= 1.56V, but there is no signal as SW pin. 

Hi Andy,

I correct my answer about EN pin control issue.

EN pin control voltage 3.3V  is referred to input 5V GND. 

Regards,

Roger

Hi Roger,

Pls try below:

1. current sense resistor to 100mohm

2. R1553 is NC and R1544=10K & R1545=4.3K mount components

3. Add 100uF 16V Electrolytic cap on 5V input.

4. replace LM5088 with new unused sample.

5. carefully check external component soldering

6. If circuit still not work properly, pls provide waveform of Vin, SS, SW, Vout, and PCB layout.

Hi Andy,

We still can not get -60V....Here are waveform of Vin, SS, SW, Vout and PCB layout fyr.

please help to check.

Thank you.

Hi Roger,

I review your circuit design, Pls make below changes to see if any improvements:

1. R1545=24.9K for Fsw=250KHz, 

2. L1514=10uH 3A

3. R1548=33 mohm

4. C1543=0.1uF

5. R1547=10K

6. C1542=150pF

Hi Roger,

The duty cycle is about 93% for 5V to -60V conversion, the Fw should be set below 200KHz, pls make below changes:

1. R1545=30.1K for Fsw=200KHz

2. L1514=15uH >2.5A

3. R1548=33 mohm

4. C1543=0.1uF

Hi Roger,

The duty cycle is about 93% for 5V to -60V conversion, the Fw should be set below 200KHz, pls make below changes:

1. R1545=30.1K for Fsw=200KHz

2. L1514=15uH >2.5A

3. R1548=33 mohm

4. C1543=0.1uF

5. R1553 is NC and R1544=10K & R1545=4.3K mount components

6. Add 100uF 16V Electrolytic cap on 5V input.

Make sure to replace LM5088 with new unused sample.

Let me know if problem can be solved by above.

B R

Andy

Hi Andy,

The test result is same as before. Here is Fsw=200KHz, but we still can not get -60V.

BR,

Roger

Hi Roger,

How about your PCB layout? did you design layout by referring to LM5088 EVM?

I strongly suggest you to apply one LM5088EVM, then modify the circuit and verify your design on EVM.

B R

Andy

Hi Roger,

Pls send PCB layout in seperated layer to me at andy.chen@ti.com.

we can discuss it offline later.

B R

Andy 

Hello Andy,

May I ask 1 question? How calculate max. duty in this case? I don't find Max. duty in LM5088 electronic table.

Best Regards,

Kami Huang