LMR23610: inrush current due to input capacitors characteristics?

Hi Ethan,

Attached are the schematic (PNG), BOM (CSV format), and Webench simulation measurements (CSV).

Carl

The CSV BOM didn't seem to upload properly. See the plain text: I used the part manufacturers and part numbers Webench recommended except for the Cff feedforward cap. For Cff I used the AVX 04023A221JAT2A instead, 220PF 25V 5% C0G 0402, which has the same value and specs as the Samsung cap Webench recommended.

Part Manufacturer Part Number Quantity Price ($) Footprint (mm²) Description
Cff Samsung Electro-Mechanics CL05C221JA5NNNC 1 0.01 3 Cap: 220 pF  Total Derated Cap: 220 pF  VDC: 25 V  ESR: 0 ?  Package: 0402 
      
Cin TDK C3225X7R1H106M250AC 2 0.5 14.7 Cap: 10 µF  Total Derated Cap: 6.7 µF  VDC: 50 V  ESR: 1 m?  Package: 1210 
      
Cout Panasonic 6TPE150MAPB 1 0.5 17.1 Cap: 150 µF  Total Derated Cap: 150 µF  VDC: 6.3 V  ESR: 25 m?  Package: 3528-20 
      
U1 Texas Instruments LMR23610ADDAR 1 1.3 55.2 
      
Cvcc Kemet C0603C225K8PACTU 1 0.04 4.68 Cap: 2.2 µF  Total Derated Cap: 2.2 µF  VDC: 10 V  ESR: 1 m?  Package: 0603 
      
L1 Bourns SRN6045-150M 1 0.2 64 L: 15 µH  DCR: 95.8 m?  IDC: 1.9 A 
      
Cboot MuRata GRM155R71C104KA88D 1 0.01 3 Cap: 100 nF  Total Derated Cap: 100 nF  VDC: 16 V  ESR: 1 m?  Package: 0402 
      
Rfbb Vishay-Dale CRCW040222K1FKED 1 0.01 3 Resistance: 22.1 k?  Tolerance: 1.0%  Power: 63 mW 
      
Rfbt Vishay-Dale CRCW040253K6FKED 1 0.01 3 Resistance: 53.6 k?  Tolerance: 1.0%  Power: 63 mW 

Here are the operating values provided by Webench for the design. Sorry again, the CSV files don't seem to upload properly.

Name Value Category Description
Phase Marg 116.28 ° System Information Bode Plot Phase Margin
Cross Freq 54 kHz System Information Bode plot crossover frequency
Low Freq Gain 77.22 dB System Information Gain at 1Hz
Gain Marg -24.76 dB System Information Bode Plot Gain Margin
Vout 3.4 V System Information Operational Output Voltage
Cin IRMS 392.71 mA Capacitor Input capacitor RMS ripple current
Cin Pd 77.11 µW Capacitor Input capacitor power dissipation
Cout IRMS 141.24 mA Capacitor Output capacitor RMS ripple current
Cout Pd 498.74 µW Capacitor Output capacitor power dissipation
Duty Cycle 18.46% System Information Duty cycle
Efficiency 88.90% System Information Steady state efficiency
Frequency 400 kHz System Information Switching frequency
IC Tj 55.84 °C IC IC junction temperature
L Ipp 489.28 mA Inductor Peak-to-peak inductor ripple current
L Pd 97.71 mW Inductor Inductor power dissipation
IC Pd 324.37 mW IC IC power dissipation
Pout 3.4 W System Information Total output power
Iin Avg 191.14 mA IC Average input current
Mode CCM System Information Conduction Mode
Vout p-p 12.23 mV System Information Peak-to-peak output ripple voltage
ICThetaJA Effective 18 °C/W IC Effective IC Junction-to-Ambient Thermal Resistance
IC Iq Pd 2.7 µW IC IC Iq Pd
Total Pd 422.8 mW Power Total Power Dissipation
FootPrint 182 mm² System Information Total Foot Print Area of BOM components
Vin 20 V System Information Vin operating point
Iout 1:00 AM System Information Iout operating point
Cin Pd 77.11 µW Power Input capacitor power dissipation
Cout Pd 498.74 µW Power Output capacitor power dissipation
L Pd 97.71 mW Power Inductor power dissipation
IC Pd 324.37 mW Power IC power dissipation
Vout Actual 3.43 V System Information Vout Actual calculated based on selected voltage divider resistors
Vout Tolerance 3.46% System Information Vout Tolerance based on IC Tolerance (no load) and voltage divider resistors if applicable
Total BOM $3.08  System Information Total BOM Cost
BOM Count 10 System Information Total Design BOM count

Ethan,

every place in this circuit where I used a ceramic cap, it has been an X7R characteristic. Are you recommending that I change Cout to a ceramic cap of 88uF or 66uF, or to continue using tantalum but change the value to 88 or 66?

Thanks

Carl

Hi Carl,

You can reduce the size to observe any changes in inrush current. Usually we recommend ceramic capacitors for lower ESR (lower vout ripple).

Regards,

Ethan

I replaced the output capacitor, a 150uF/6.3V tantalum cap, with a 47uF/6.3V tantalum cap (what I had on hand). The noise and ripple on the 3.4V supply increased but there was no reduction in the inrush.

Hi Carl,

Please try using ceramic capacitors then. Is there a load already connected to DC converter?

Regards,

Ethan

Hi Ethan,

As a reminder, this problem is one of high inrush (current into the main input of my overall circuit). I measure over 15A with a few microseconds duration. This was a good working design to which I added the LMR23610. The revised design shows this high inrush current. In comparison, before adding the LMR23610 the inrush peak was about 2A. The circuit has more than one voltage regulator. It also uses an LM2734 that steps 12V down to 3.3V.

A few changes I tried:

1. 3.4V regulator Cout, changed from 150uF tantalum to 47uF tantalum --> no significant change in the inrush current 

2. Disconnected the 3.4V regulator output from its load (3.4 regulator output is now unloaded) --> the inrush current at the input looks the same.

3. The Cin for 3.4V regulator is two 10uF ceramic caps in parallel, physically close to the regulator. I removed one of these caps --> inrush is about the same.

4. Disabled the output of the 3.3V regulator by tying the EN pin to GND --> no inrush reduction.

5. Disabled the output of the 3.4V regulator by tying the EN pin to GND --> possible slight inrush reduction (but did not test thoroughly)

Next I can disconnect the supply input to the LMR23610. What do you think?

Thanks

Carl

Hi Carl,

That would be a good next step. Have you tried ceramic output capacitors for the LMR23610 as I suggested?

Regards,

Ethan