Datasheet

L
1
=
V
IN
x D
f
SW
x 'i
L
LM5022
SNVS480G –JANUARY 2007–REVISED DECEMBER 2013 www.ti.com
Switching loss, P
SW
, occurs during the brief transition period as the MOSFET turns on and off. During the
transition period both current and voltage are present in the channel of the MOSFET. The loss can be
approximated as:
P
SW
= 0.5 x V
IN
x [I
O
/ (1 – D)] x (t
R
+ t
F
) x f
SW
where
• t
R
is the rise time of the MOSFET
• t
F
is the fall time of the MOSFET (6)
For this example, the maximum drain-to-source voltage applied across the MOSFET is V
O
plus the ringing due to
parasitic inductance and capacitance. The maximum drive voltage at the gate of the high side MOSFET is VCC,
or 7V typical. The MOSFET selected must be able to withstand 40V plus any ringing from drain to source, and
be able to handle at least 7V plus ringing from gate to source. A minimum voltage rating of 50V
D-S
and 10V
G-S
MOSFET will be used. Comparing the losses in a spreadsheet leads to a 60V
D-S
rated MOSFET in SO-8 with an
R
DSON
of 22 mΩ (the maximum vallue is 31 mΩ), a gate charge of 27 nC, and rise and falls times of 10 ns and
12 ns, respectively.
OUTPUT DIODE
The boost regulator requires an output diode D1 (see Figure 12) to carrying the inductor current during the
MOSFET off-time. The most efficient choice for D1 is a Schottky diode due to low forward drop and near-zero
reverse recovery time. D1 must be rated to handle the maximum output voltage plus any switching node ringing
when the MOSFET is on. In practice, all switching converters have some ringing at the switching node due to the
diode parasitic capacitance and the lead inductance. D1 must also be rated to handle the average output current,
I
O
.
The overall converter efficiency becomes more dependent on the selection of D1 at low duty cycles, where the
boost diode carries the load current for an increasing percentage of the time. This power dissipation can be
calculating by checking the typical diode forward voltage, V
D
, from the I-V curve on the diode's datasheet and
then multiplying it by I
O
. Diode datasheets will also provide a typical junction-to-ambient thermal resistance, θ
JA
,
which can be used to estimate the operating die temperature of the Schottky. Multiplying the power dissipation
(P
D
= I
O
x V
D
) by θ
JA
gives the temperature rise. The diode case size can then be selected to maintain the
Schottky diode temperature below the operational maximum.
In this example a Schottky diode rated to 60V and 1A will be suitable, as the maximum diode current will be
0.5A. A small case such as SOD-123 can be used if a small footprint is critical. Larger case sizes generally have
lower θ
JA
and lower forward voltage drop, so for better efficiency the larger SMA case size will be used.
BOOST INDUCTOR
The first criterion for selecting an inductor is the inductance itself. In fixed-frequency boost converters this value
is based on the desired peak-to-peak ripple current, Δi
L
, which flows in the inductor along with the average
inductor current, I
L
. For a boost converter in CCM I
L
is greater than the average output current, I
O
. The two
currents are related by the following expression:
I
L
= I
O
/ (1 – D) (7)
As with switching frequency, the inductance used is a tradeoff between size and cost. Larger inductance means
lower input ripple current, however because the inductor is connected to the output during the off-time only there
is a limit to the reduction in output ripple voltage. Lower inductance results in smaller, less expensive magnetics.
An inductance that gives a ripple current of 30% to 50% of I
L
is a good starting point for a CCM boost converter.
Minimum inductance should be calculated at the extremes of input voltage to find the operating condition with the
highest requirement:
(8)
By calculating in terms of amperes, volts, and megahertz, the inductance value will come out in micro henries.
In order to ensure that the boost regulator operates in CCM a second equation is needed, and must also be
evaluated at the corners of input voltage to find the minimum inductance required:
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