Datasheet

ESR
MAX
=
'V
OUT
'I
OUT
V
IN(MAX)
- V
O
'I
OUT
=
F
SW
x L
ACTUAL
x D
LM2743
SNVS276G –APRIL 2004–REVISED MARCH 2013 www.ti.com
Here we have plugged in the values for output current ripple, input voltage, output voltage, switching frequency,
and assumed a 40% peak-to-peak output current ripple. This yields an inductance of 1.6 µH. The output inductor
must be rated to handle the peak current (also equal to the peak switch current), which is (I
OUT
+ (0.5 x ΔI
OUT
)) =
4.8A, for a 4A design. The Coilcraft DO3316P-222P is 2.2 µH, is rated to 7.4A peak, and has a direct current
resistance (DCR) of 12 mΩ.
After selecting an output inductor, inductor current ripple should be re-calculated with the new inductance value,
as this information is needed to select the output capacitor. Re-arranging the equation used to select inductance
yields the following:
V
IN(MAX)
is assumed to be 10% above the steady state input voltage, or 3.6V. The actual current ripple will then
be 1.2A. Peak inductor/switch current will be 4.6A.
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
OUT
) and to supply load current during fast load transients.
In this example the output current is 4A 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 the 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
OUT
and the designed output current ripple, ΔI
OUT
,
is:
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 20 mΩ. The Sanyo 4SP560M electrolytic capacitor will give an
equivalent ESR of 14 mΩ. The capacitance of 560 µF is enough to supply energy even to meet severe load
transient demands.
MOSFETs
Selection of the power MOSFETs is governed by a tradeoff between cost, size, and efficiency. One method is to
determine the maximum cost that can be endured, and then select the most efficient device that fits that price.
Breaking down the losses in the high-side and low-side MOSFETs and then creating spreadsheets is one way to
determine relative efficiencies between different MOSFETs. Good correlation between the prediction and the
bench result is not specified, however. Single-channel buck regulators that use a controller IC and discrete
MOSFETs tend to be most efficient for output currents of 2A to 10A.
Losses in the high-side MOSFET can be broken down into conduction loss, gate charging loss, and switching
loss. Conduction loss, or I
2
R loss, is approximately:
P
C
= D ((I
O
)
2
x R
DSON-HI
x 1.3) (High-Side MOSFET)
P
C
= (1 - D) x ((I
O
)
2
x R
DSON-LO
x 1.3) (Low-Side MOSFET)
In the above equations the factor 1.3 accounts for the increase in MOSFET R
DSON
due to heating. Alternatively,
the 1.3 can be ignored and the R
DSON
of the MOSFET estimated using the R
DSON
Vs. Temperature curves in the
MOSFET datasheets.
Gate charging loss results from the current driving the gate capacitance of the power MOSFETs, and is
approximated as:
P
GC
= n x (V
DD
) x Q
G
x f
SW
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