LM46000: Output voltage not stable (hiccup) when slowly increasing input voltage (with 0.5A load)

Robin Maeder

Part Number: LM46000

Hello All

As described in the title. When I have attached a resistive load of ca. 20R and I start increasing the input voltage from 0V the output is in hiccup mode until the input voltage reaches about 30V. The goal is that the output voltage is stable when Vin >= 27V. When I apply the load after the DC/DC is powered up then it works down to Vin = 20V.

This is the circuit (all mentioned voltages are referenced to GND_H_1):

How can the circuit be improved to start correctly under high laod and low input voltage?

Thanks for your help.

Best regards

over 3 years ago

0 Andy Chen over 3 years ago

TI__Mastermind 27045 points

Hi Robin,

Pls download below Application Note on TI.com for better understanding the operation of inverter Buck/boost.

SNVA856–February 2019 Working with inverting buck-boost converters

The output ceramic cap rating should be 35V or higher, No use C122 220pF.

This circuit should be able to startup with VIN down to 15V, startup with Vin below 10V may trig OCP and enter Hiccup mode.

B R

Andy

0 Robin Maeder over 3 years ago in reply to Andy Chen

Intellectual 450 points

Hi Andy

Thanks for your reply. Why do the output capacitors need to be 35V?

I tried to remove C122 but behaviour did not change.

Regards

0 Andy Chen over 3 years ago in reply to Robin Maeder

TI__Mastermind 27045 points

Hi Robin,

The ceramic cap effective capacitance will decrease with bias voltage, the cap voltage rating should be 25V or 35V if output is 10V.

Pls upload PCB layout if problem still existing.

B R

Andy

0 Robin Maeder over 3 years ago in reply to Andy Chen

Intellectual 450 points

Hi Andy

The current capacitors are 25V.

Here the layout:

Best regards

0 Andy Chen over 3 years ago in reply to Robin Maeder

TI__Mastermind 27045 points

Hi Robin,

I will check and update you soon.

B R

Andy

0 Andy Chen over 3 years ago in reply to Robin Maeder

TI__Mastermind 27045 points

Hi Robin,

The layout looks good.

The inductor saturation current should be >1A

This circuit should be able to startup with VIN >15V and 20R load.

B R

Andy

0 Robin Maeder over 3 years ago in reply to Andy Chen

Intellectual 450 points

Hi Andy

The inductor has an Isat of 1.2A. Maybe I could try one with a bit more current. When you say VIN > 15V you mean 15V from 30V_H_1 to GND_H_1?

Regads

+1 Robin Maeder over 3 years ago in reply to Andy Chen

Intellectual 450 points

Hi Andy

If I calculate Ivalley according to SNVA856 equation 5, I get 0.5A with Vin = 40V. With lower Vin the currents get higher. The Ils-limit of the LM46000 is 0.46A in worstcase. Isn't that the reason for the hiccup mode? So to test the behaviour I changed the inductor to 95uH and the frequency to 180kHz. Then the circuit starts with VIN = 19V and works down to VIN = 16.5V. So is the current limit of the LM46000 too low for Iout = 0.5A and VIN = 40V?

Regards

+1 Andy Chen over 3 years ago in reply to Robin Maeder

TI__Mastermind 27045 points

Hi Robin,

You are right, for lower Vin and heavy load, the inductor current valley maybe higher than 0.6A typical, which will trig Hiccup OCP. on the other hand, the HS peak current 1.35A also limits the max load current at low Vin condition.

B R

Andy

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LM46000: Output voltage not stable (hiccup) when slowly increasing input voltage (with 0.5A load)