Texas Instruments LM3404 Evaluation Boards LM3404HVEVAL/NOPB LM3404HVEVAL/NOPB Benutzerhandbuch
Produktcode
LM3404HVEVAL/NOPB
R
SNS
=
2
V
IN
- V
O
0.2 x L
I
F
x L + V
O
x t
SNS
-
x t
ON
SNVS465F – OCTOBER 2006 – REVISED MAY 2013
R
SNS
A preliminary value for R
SNS
was determined in selecting
Δ
i
L
. This value should be re-evaluated based on the
calculations for
Δ
i
F
:
(67)
t
SNS
= 220 ns, R
SNS
= 0.43
Ω
(68)
Sub-1
Ω
resistors are available in both 1% and 5% tolerance. A 1%, 0.43
Ω
device is the closest value, and a
0.25W, 0805 size device will handle the power dissipation of 110 mW. With the resistance selected, the average
value of LED current is re-calculated to ensure that current is within the ±10% tolerance requirement. From the
expression for average LED current:
value of LED current is re-calculated to ensure that current is within the ±10% tolerance requirement. From the
expression for average LED current:
I
F
= 0.2 / 0.33 – (7.1 x 2.2 x 10
-7
) / 47 x 10
-6
+ 0.266 / 2 = 505 mA
(69)
INPUT CAPACITOR
Following the calculations from the Input Capacitor section,
Δ
v
IN(MAX)
will be 48V x 2%
P-P
= 960 mV. The
minimum required capacitance is:
C
IN(MIN)
= (0.5 x 3.3 x 10
-6
) / 0.96 = 1.7 µF
(70)
To provide additional safety margin a 2.2 µF ceramic capacitor rated to 100V with X7R dielectric in an 1812 case
size will be used. From the Design Considerations section, input rms current is:
size will be used. From the Design Considerations section, input rms current is:
I
IN-RMS
= 0.5 x Sqrt(0.73 x 0.27) = 222 mA
(71)
Ripple current ratings for 1812 size ceramic capacitors are typically higher than 2A, more than enough for this
design, and the ESR is approximately 3 m
design, and the ESR is approximately 3 m
Ω
.
RECIRCULATING DIODE
The input voltage of 48V requires Schottky diodes with a reverse voltage rating greater than 50V. The next
highest standard voltage rating is 60V. Selecting a 60V rated diode provides a large safety margin for the ringing
of the switch node and also makes cross-referencing of diodes from different vendors easier.
highest standard voltage rating is 60V. Selecting a 60V rated diode provides a large safety margin for the ringing
of the switch node and also makes cross-referencing of diodes from different vendors easier.
The next parameters to be determined are the forward current rating and case size. In this example the high duty
cycle (D = 35.2 / 48 = 73%) places a greater thermal stress on the internal power MOSFET than on D1. The
estimated average diode current is:
cycle (D = 35.2 / 48 = 73%) places a greater thermal stress on the internal power MOSFET than on D1. The
estimated average diode current is:
I
D
= 0.5 x 0.27 = 135 mA
(72)
A Schottky with a forward current rating of 0.5A would be adequate, however reducing the power dissipation is
critical in this example. Higher current diodes have lower forward voltages, hence a 1A-rated diode will be used.
To determine the proper case size, the dissipation and temperature rise in D1 can be calculated as shown in the
Design Considerations section. V
critical in this example. Higher current diodes have lower forward voltages, hence a 1A-rated diode will be used.
To determine the proper case size, the dissipation and temperature rise in D1 can be calculated as shown in the
Design Considerations section. V
D
for a case size such as SMA in a 60V, 1A Schottky diode at 0.5A is
approximately 0.35V and the
θ
JA
is 75°C/W. Power dissipation and temperature rise can be calculated as:
P
D
= 0.135 x 0.35 = 47 mW T
RISE
= 0.047 x 75 = 3.5°C
(73)
C
B
AND C
F
The bootstrap capacitor C
B
should always be a 10 nF ceramic capacitor with X7R dielectric. A 25V rating is
appropriate for all application circuits. The linear regulator filter capacitor C
F
should always be a 100 nF ceramic
capacitor, also with X7R dielectric and a 25V rating.
EFFICIENCY
To estimate the electrical efficiency of this example the power dissipation in each current carrying element can
be calculated and summed. Electrical efficiency,
be calculated and summed. Electrical efficiency,
η
, should not be confused with the optical efficacy of the circuit,
which depends upon the LEDs themselves.
Total output power, P
O
, is calculated as:
P
O
= I
F
x V
O
= 0.5 x 35.2 = 17.6W
(74)
Copyright © 2006–2013, Texas Instruments Incorporated
29
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