LMR16030: Webench BOM ceramic cap sizes & duty cycle control

Frank De Stasi said:

Ceramic capacitors have low impedance out to very high frequencies

Gerat for output capacitors but these are input source capacitors. So an X5R should suffice any input ripple being chopped even snubbed by much higher PWM switch speed frequency. 

Frank De Stasi said:

Since many ceramic capacitors have very bad D.C. bias de-rating, we like to over-rate the size and voltage

to help mitigate this effect.

There is also the point of going overboard in a circuit design. Perhaps for space flight or aviation use all counter measures make sense. Webench choose different part list shows many different input capacitor values 100nF - 100µF but all were high price X7R C types. Many were not even listed in TDK datasheet specifications. There was a recent class action lawsuit against one vendor for price gouging. Then there are import tariffs added to the final cost caps made in China. Congress has done nothing to undo that unfair tax placed on US electronics manufactures.

The datasheet mentioned to also use electrolytic if LMR is several inches from the input voltage source. However when only a few centimeters 4.7-100µF but does not mention stacking them as shown in Tina model, doubles capacitance and divides the input impedance in half. Again BOM shows all X7R choices when X5R especially two stacked should be more than adequate as X7R does not gain any speed advantage over X5R ceramic. 

Another thing about the diode selection being 25% greater than input. Seemingly UVLO recovery with overshoot control diode cathode should never have full input voltage or the transient would blow up downstream regulators etc.. In that case we want the diode to short out stomp on FB loop shut down PWM drive blow a fuse or any other less costly counter measure. Hence derating diode break down voltage can add switching efficiency and cost benefit factor in circuit reliability when the MCU 3v3 rail is spared being shorted. So a 100V Schottky is not any more reliable than 60V and may lead to future disaster. Like to run Tina simulations on diode break down voltages to see the transient UVLO recovery operation.

8.2.2.6 Schottky Diode Selection The breakdown voltage rating of the diode is preferred to be 25% higher than the maximum input voltage.

9 Power Supply Recommendations: The LMR16030 is designed to operate from an input voltage supply range between 4.3 V and 60 V. This input supply must be able to withstand the maximum input current and maintain a stable voltage. The resistance of the input supply rail should be low enough that an input current transient does not cause a high enough drop at the LMR16030 supply voltage that can cause a false UVLO fault triggering and system reset. If the input supply is located more than a few inches from the LMR16030, additional bulk capacitance can be required in addition to the ceramic input capacitors. The amount of bulk capacitance is not critical, but a 47-μF or 100-μF electrolytic capacitor is a typical choice.

Selected aluminum polymer electrolytic 56µF 80v largest SMD being manufactured as 48v battery array will be several feet away from the input connector. Yet the soft start (SS) takes 300ms to startup LMR into DCM bucking, it should never overshoot inrush plugging. It seems plausible CCM will take place at some point well after hot plugging if ever though Tina shows 3.6A current peaks so 4.5A Schottky diode SOD128 makes good design sense.

8.2.2.7 Input Capacitor Selection The LMR16030 device requires high frequency input decoupling capacitor or capacitors and a bulk input capacitor, depending on the application. The typical recommended value for the high frequency decoupling capacitor is 4.7 μF to 10 μF. A high-quality ceramic capacitor type X5R or X7R with sufficiency voltage rating is recommended. To compensate the derating of ceramic capacitors, a voltage rating of twice the maximum input voltage is recommended. Additionally, some bulk capacitance can be required, especially if the LMR16030 circuit is not located within approximately 5 cm from the input voltage source. This capacitor is used to provide damping to the voltage spike due to the lead inductance of the cable or the trace. For this design, two 2.2-μF, X7R ceramic capacitors rated for 100 V are used. 0.1 μF for high-frequency filtering and place it as close as possible to the device pins.

Hi Collin,

The original PCB 24v input had 180µF electrolytic and 3x 4.7uF 50v ceramic capacitor each buck switcher 1.5MHz. The LMR frequency (760KHz) and Tina transient analysis has some kind of switched 24v DC supply built into the macro. So the macro it's not giving a good study of the input impedance though have past added ohms scope to graph output impedance of operational amplifiers.   

Agree, oddly the Tina LMR macro cannot produce correct input or output load via wattage meter. Input wattage peaks above 350 watts. That same circuit analysis works great with LM317KCB.