LM2727, LM2737

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SNVS205D – AUGUST 2002 – REVISED MARCH 2013

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In the case of a desktop computer system, the input current slew rate is the system power supply or "silver box"

output current slew rate, which is typically about 0.1A/µs. Total input capacitor ESR is 9m

Ω, hence ΔV is

10*0.009 = 90 mV, and the minimum inductance required is 0.9µH. The input inductor should be rated to handle

the DC input current, which is approximated by:

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handle 8.2Arms, and has a DCR of 7m

Ω.

OUTPUT INDUCTOR

The output inductor forms the first half of the power stage in a Buck converter. It is responsible for smoothing the

square wave created by the switching action and for controlling the output current ripple. (

chosen by selecting between tradeoffs in efficiency and response time. The smaller the output inductor, the more

quickly the converter can respond to transients in the load current. As shown in the efficiency calculations,

however, a smaller inductor requires a higher switching frequency to maintain the same level of output current

ripple. An increase in frequency can mean increasing loss in the FETs due to the charging and discharging of the

gates. Generally the switching frequency is chosen so that conduction loss outweighs switching loss. The

equation for output inductor selection is:

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Plugging in the values for output current ripple, input voltage, output voltage, switching frequency, and assuming

a 40% peak-to-peak output current ripple yields an inductance of 1.5µH. The output inductor must be rated to

handle the peak current (also equal to the peak switch current), which is (Io + 0.5*

design. The Coilcraft D05022-152HC is 1.5µH, is rated to 15Arms, and has a DCR of 4m

Ω.

OUTPUT CAPACITOR

The output capacitor forms the second half of the power stage of a Buck switching converter. It is used to control

the output voltage ripple (

In this example the output current is 10A and the expected type of capacitor is an aluminum electrolytic, as with

the input capacitors. (Other possibilities include ceramic, tantalum, and solid electrolyte capacitors, however the

ceramic type often do not have the large capacitance needed to supply current for load transients, and tantalums

tend to be more expensive than aluminum electrolytic.) Aluminum capacitors tend to have very high capacitance

and fairly low ESR, meaning that the ESR zero, which affects system stability, will be much lower than the

switching frequency. The large capacitance means that at switching frequency, the ESR is dominant, hence the

type and number of output capacitors is selected on the basis of ESR. One simple formula to find the maximum

ESR based on the desired output voltage ripple,

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In this example, in order to maintain a 2% peak-to-peak output voltage ripple and a 40% peak-to-peak inductor

current ripple, the required maximum ESR is 6m

Ω. Three Sanyo 10MV5600AX capacitors in parallel will give an

equivalent ESR of 6m

Ω. The total bulk capacitance of 16.8mF is enough to supply even severe load transients.

Using the same capacitors for both input and output also keeps the bill of materials simple.

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