Datasheet

LM2727, LM2737
www.ti.com SNVS205D –AUGUST 2002–REVISED MARCH 2013
(8)
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 9mΩ, 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:
(9)
In this case I
IN-DC
is about 2.8A. One possible choice is the TDK SLF12575T-1R2N8R2, a 1.2µH device that can
handle 8.2Arms, and has a DCR of 7mΩ.
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. (ΔI
o
) The inductance is
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:
(10)
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*ΔI
o
). This is 12A for a 10A
design. The Coilcraft D05022-152HC is 1.5µH, is rated to 15Arms, and has a DCR of 4mΩ.
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 (ΔV
o
) and to supply load current during fast load transients.
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, ΔV
o
and the designed output current ripple, ΔI
o
, is:
(11)
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 6mΩ. Three Sanyo 10MV5600AX capacitors in parallel will give an
equivalent ESR of 6mΩ. 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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