LM5118: pmp7851 and component choice

Jason Ciou

Part Number: LM5118

Hi team,

My customer want to use LM5118 to do a buck-boost converter, and his design is based on the pmp7851. and he want to know how to select the Schottky and MOSFET.

Here is the circuit specification:

PMP7851.1
Vin (Min) (V)	30
Vin (Max) (V)	60
Vout (Nom) (V)	54
Iout (Max) (A)	1.25
Switching Frequency	300k
Isolated/Non-Isolated	Non-Isolated
Input Type	DC

Here are my questions:

1.) According to the datasheet, I can find the voltage stress of Schottky and MOSFET. but I do not understand how to find the current stress, because this circuit has two mode buck and buck-boost mode.

Could you tell me how to calculate the maximum current on D1, D2, Q1, Q2?

D1 max voltage	Vin_max
D2 max voltage	Vo
Q1 max voltage	Vin_max
Q2 max voltage	Vo

2.) In this case I choose 100V Schottky and MOSFET. is this make sense?

3.) I use Tina TI to do the simulation based on the pmp7851's specification, but the result is not accurate, could you help me to check the file.

1667.LM5118_STEADY_STATE_MOD.TSC

over 4 years ago

0 Stefan Schauer over 4 years ago

TI__Guru 58661 points

Hi Jason,

let me look into this into more detail. I will come back with an answer within the next two days.

Thanks for your patience.

Best regards,

Stefan

+1 Stefan Schauer over 4 years ago in reply to Stefan Schauer

TI__Guru 58661 points

Hallo Jason,

for a Buck-Boost you basically need to consider the two modes separate, so evaluate the Buck section and the Boost section and the take the more critical value from both. There are two very good reports discussing that in detail:

2003 Power Seminar - Under the Hood of Low-Voltage DC/DC Converters (ti.com)

Topic_3_Lynch.pdf (ti.com)

Another great tool is the Power Stage Designer which give you an easy access to the currents and voltages on the different components within the SMPS.

So using this it will give you for the Diode D1 in the Boost condition:

I max = 3.31A

I RMS = 1.74

V reverse max = -54 V

For the Buck condition you can use the current through the inductor:

I max = 1.66A

I RMS = 1.27

Using a 100V FET should be good choice.

For the TINA simulation - may you can provide more details on what you expect and what you get. So it would be easier to focus on that.

Best regards,

Stefan

0 Jason Ciou over 4 years ago in reply to Stefan Schauer

TI__Expert 3600 points

Hi Stefan,

Thanks for your help.

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LM5118: pmp7851 and component choice