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Problem with LM3478 in SEPIC configuration in low voltages

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Hi,

I have designed a SEPIC converter using the LM3478 with the following requirements:

Input Voltage : 3.1VDC - 16 VDC

Output Voltage : 5.3 VDC

Maximum Output Current: 1.1 A

I'm using uncoupled L1 and L2.

However I have a problem with current I can draw from the circuit especially in the lower voltages. Please see below summary table with various measurements:

Input Voltage

Output Voltage

Draw current

Below the 3.3 V now working properly (output voltage is too low)

Less than 4 V

200 mA

3.3 V

5.07 V

200mA

3.3 V

4.4 V

210 mA

 

 

 

3.7 V

5.07 V

210 mA

3.7 V

5.07 V

310 mA

3.7 V

5.07 V

410 mA

3.7 V

4.37 V

500 mA

3.7 V

3.90 V

550 mA

 

 

 

4.7 V

5.07 V

500 mA

4.7 V

5.07 V

610 mA

4.7 V

4.32 V

700 mA

4.7 V

3.89 V

760 mA

 

 

 

6.7 V

5.07 V

610 mA

6.7 V

5.07 V

765 mA

6.7 V

5.07 V

815 mA

6.7 V

4.43 V

910 mA

6.7 V

3.86 V

1010 mA

 

 

 

10 V

5.07 V

720 mA

10 V

5.07 V

820 mA

10 V

5.07 V

920 mA

10 V

4.83 V

1020mA

10 V

4.20 V

1130 mA

10 V

3.92 V

1170 mA

 

 

 

13.3 V

5.07 V

870 mA

13.3 V

5.04 V

1000 mA

13.3 V

4.81 V

1100 mA

13.3 V

4.12 V

1220 mA

 

 

 

16 V

5.04 V

1000 mA

16 V

4.88 V

1100 mA

 

 

 

You can see from the table that for low voltages it can not provide sufficient current (1 A) but only 200 mA. 

Please see attached schematic. I have very good level of ripple. The only problem is that it can not provide requested current with low input voltages. You help would be very appreciated.

Thank you in advance

Grazyna


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