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Voltage rating of the bootstrap capacitor for LM5104

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powersys

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bootstrap capacitor

Wish to construct a 12V to 5V, 1 A, synchronous buck converter as shown in Figure 1.

Specifications
----------------
Vin = 12V
Vout = 5V
L = 100uH
Cin = 4.7uF (input capacitor)
Co = 22uF (output capacitor)
CBOOT = 0.1uF
PWM: 100kHz and 50% duty-cycle
Mosfet: IRF540
R1 = R2 = 28 ohm
R3 = 82kohm

[Q1]
Based on the specifications given above, what is the voltage rating of boostrap capacitor CBOOT that should be used?

[Q2]
Simulation based on the specificitions given above was carried out and it worked as expected. However, the constructed synchronous buck converter did not work. The measured output was very low, approaching zero. What are the possible causes to this problem?

[Q2]
What is the best value of VCC in this application?

Figure 1



Figure 2
 

v_c

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lm5104

You are probably not generating a high enough gate-source voltage to turn on the highside MOSFET. The voltage from HO to HS should be high enough to exceed the threshold voltage of the MOSFET. If this device is not turning on, then the output would indeed be very close to zero. Try using Vcc=20V to see if you get a high enough Vgs to turn the highside MOSFET on.

Best regards,
v_c
 

powersys

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capacitor voltage ratings

v_c said:
You are probably not generating a high enough gate-source voltage to turn on the highside MOSFET. The voltage from HO to HS should be high enough to exceed the threshold voltage of the MOSFET. If this device is not turning on, then the output would indeed be very close to zero. Try using Vcc=20V to see if you get a high enough Vgs to turn the highside MOSFET on.
Do u think the voltage rating of the boostrap capacitor CBOOT is important for the circuit to operate correctly? If I'm not mistaken, the voltage rating of the CBOOT used in the circuit is 6V or 10V... Pls advise.
 

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bootstrap capacitor buck

I don't think that is a big concern but just to be safe you can choose something with a larger rating. The main point with the rating is that you don't want to overstress the part. So go with something larger for the rating and it should work.

Best regards,
v_c
 

E-design

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lm5104 problem

Vcc should be no more than 14V

Bootstrap capacitor of voltage rating 2 x HV should be safe.
 

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boostrap capacitor design

Did I go over the Vddmax for the IC? I thought this part was rated for 30V :-(
 

E-design

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what does the voltage rating on a capacitor mean

Looks like it is advised that Vcc should be limited at 14V and that the absolute max is 18V(where it is about to blow up) 8O
 

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lm5104 application notes

E-design said:
Vcc should be no more than 14V

Bootstrap capacitor of voltage rating 2 x HV should be safe.
What does HV mean? Would u pls advise the value of HV for this application? Thanks.
 

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boostrap mosfet

On the datasheet I looked at, it is labeled HV. On your drawing it is Vin
 

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