Datasheet
ESR
MAX
=
'V
OUT
'I
OUT
LM2745, LM2748
www.ti.com
SNOSAL2E –APRIL 2005–REVISED APRIL 2013
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 trade-off 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 2 to 10A.
Losses in the high-side MOSFET can be broken down into conduction loss, gate charging loss, and switching
loss. Conduction, 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
where ‘n’ is the number of MOSFETs (if multiple devices have been placed in parallel), V
DD
is the driving voltage
(see MOSFET Gate Drivers section) and Q
GS
is the gate charge of the MOSFET. If different types of MOSFETs
are used, the ‘n’ term can be ignored and their gate charges simply summed to form a cumulative Q
G
. Gate
charge loss differs from conduction and switching losses in that the actual dissipation occurs in the LM2745/8,
and not in the MOSFET itself.
Switching loss occurs during the brief transition period as the high-side MOSFET turns on and off, during which
both current and voltage are present in the channel of the MOSFET. It can be approximated as:
P
SW
= 0.5 x V
IN
x I
O
x (t
r
+ t
f
) x f
SW
where t
r
and t
f
are the rise and fall times of the MOSFET. Switching loss occurs in the high-side MOSFET only.
For this example, the maximum drain-to-source voltage applied to either MOSFET is 3.6V. The maximum drive
voltage at the gate of the high-side MOSFET is 3.1V, and the maximum drive voltage for the low-side MOSFET
is 3.3V. Due to the low drive voltages in this example, a MOSFET that turns on fully with 3.1V of gate drive is
needed. For designs of 5A and under, dual MOSFETs in SO-8 provide a good trade-off between size, cost, and
efficiency.
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