LTC3864 behavior at high duty%

mcerisola on May 16, 2022

Category: Hardware

Product Number: LTC3864

I am simulating with LTSpice LTC3864 operated at high duty cycle, with V(output) close to V(input). I'm using the model for LTC3864 provided by LTSpice.

V(input) = 24V

V(output) = 22V

The output voltage is reached correctly.

Then, the pin RUN is pulled Low to disable the Controller.

Instead of pulling GATE to VCC to turn off the external PMOS, GATE keeps the PMOS permanently ON, at 100% duty cycle, and V(output) increases to 24V.

Is this incorrect behavior a bug in the simulation model, or this is the real behavior of the IC?

Attached is a ZIP with schematic and plot settings.

Thanks,

Mauro Cerisola

LTC3864_test_rev1.zip

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+1 ron.p

on May 18, 2022 2:00 AM
Hi Mauro,

The simulation works properly if you use Rload=2.4 ohms and below. What is your max output current?
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0 mcerisola on May 18, 2022 7:53 AM in reply to ron.p

Thanks Ron,

my output current ranges from 0A to 10A.

I ran the simulation with 2.3 ohm, and the output turns off, as you say.

But the waveforms at turn-off time are as in the attached screenshot.

Before turn-off, the GATE is switched at 300kHz with high duty cycle, the inductor current ripples around 10A.

When RUN goes Low, GATE stops switching and stays Active, the external PMOS stays On, the inductor current surges up to 18.6A, the output voltage increases from 22V to 23.2V.

Then Gate goes Inactive and the PMOS is switched off.

I guess what triggers the GATE going Inactive is the SENSE input tripping the 95mV threshold of Current Limit. I reduced the sense resistor to 4mOhm, and the GATE stayed Active forever because the 95mV was never reached.

With this behavior, it is not possible to turn off the buck converter at medium and light load, which is definitely undesirable.

Can anybody confirm that the real device turns off the GATE output even at light load?
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+1 ron.p

on May 20, 2022 1:18 AM
LTC3864 behavior at high duty cycle - Discussions - Power Management - EngineerZone (analog.com)
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