LM5156-Q1: modulating the output voltage by injecting the control voltage to the FB network

Hi Haider,

Thanks for using E2E.

Could you please share your schematic, as well as the simulation files and your measurements of the issue you see?

What are the specs of your design (Vin, Vout(range), Iout)?

I am looking forward for your response.

Best regards

Moritz

hi 

is ti ok if i send you the simulation files in Cadence?

Vin = 24V with input PI filter

VOUT range = 8 - 35V

total load is 500mA

control signal is 1-5V with an fall time and rise time of 50usec, i have to stick to these numbers as this is driving a transmission line to overcome line ringing.

on the dropping edge am placing a sink load to decrease the discharge time of the output capacitor, its working, but i dont feel this is an efficient way

will send you the schematics, the circuit is under design.

also, for another issue, 

the switching MOSFET has a leading pulse of a very high current which i think its due to the capacitance of the schottkey diode, is there a way to reduce that.

i tried series resistor with the gate to slow down the switching time of the MOSFET but it does not help if its in the range of 1-5 Ohms, if you go higher the switching gets skewed and corrupted.

is this a diode problem? can you suggest a diode with lower capacitance? if that was the problem!

cheers

Haider

hi Attached the schematic in PDF am running a simulation now will send the simulation once it ends

how can i upload a file? 

LM5156_Haider_E2E.pdf

attached the simulationLM5156-Q1_TRANS_Modulated_out.zipVout_and currents_modulated.pdf

Hi Haider,

Thanks for the files.

To reduce the large current spike, you can try to change the diode to an ultra fast silicon type. These have less junction capacitance.

From the schematic i see that there is a resistor connected to the SS pin. This should be a capacitor.

So what is the other issue related to the softstart? Im not sure if i fully understand what you mean.

Best regards

Moritz

Hi Moritz

so when the SS is active and there is the SS capacitor, when the signal falls from 35V to 8 V the SS gets reset and the switching halts which disturbs the signal.

also it happens when it climbs up and the it reaches its final 35 V level the SS resets and switching halts which causes big ripple.

So to disable the SS I put a 300K resistor so even if the SS resets it only trips for 32uSec which does not effect a lot the signal shape and causes less distortion.

my question is why the SS is resetting when there is a light charging to the capacitors?

if I decrease the capacitors I have a large ripple.

i Tried various fast recovery diodes but still the same response, is that the simulation? Or in fact it happens?

cheers

haider