551600003-001A National Semiconductor, 551600003-001A Datasheet - Page 22

• Manufacturer Part Number: 551600003-001A
• Description: BOARD WEBENCH MSOP LM3402
• Manufacturer: National Semiconductor
• Series: WEBENCH® Buildit Boardr

Specifications of 551600003-001A

• Utilized Ic / Part: LM3402
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Features: -
• Voltage - Output: -
• Voltage - Input: -
• Current - Output / Channel: -
• Outputs And Type: -

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To select the proper output capacitor the equation from Buck
Regulators with Output Capacitors is re-arranged to yield the
following:
The target tolerance for LED ripple current is ±5% or 10%
P
r
reduce the worst-case inductor ripple current of 258 mA
therefore:
A ceramic capacitor will be used and the required capacitance
is selected based on the impedance at 468 kHz:
This calculation assumes that impedance due to the equiva-
lent series resistance (ESR) and equivalent series inductance
(ESL) of C
value is 2.2 µF. The capacitor used should be rated to 10V or
more and have an X7R dielectric. Several manufacturers pro-
duce ceramic capacitors with these specifications in the 0805
case size. A typical value for ESR of 1 mΩ can be read from
the curve of impedance vs. frequency in the product
datasheet.
INPUT CAPACITOR
Following the calculations from the Input Capacitor section,
Δv
pacitance is:
In expectation that more capacitance will be needed to pre-
vent power supply interaction a 1.0 µF ceramic capacitor
rated to 50V with X7R dielectric in a 1206 case size will be
used. From the Design Considerations section, input rms cur-
rent is:
Ripple current ratings for 1206 size ceramic capacitors are
typically higher than 1A, more than enough for this design.
RECIRCULATING DIODE
The first parameter for D1 which must be determined is the
reverse voltage rating. Schottky diodes are available at re-
verse ratings of 30V and 40V, often in the same package, with
the same forward current rating. To account for ringing a 40V
Schottky will be used.
The next parameters to be determined are the forward current
rating and case size. In this example the low duty cycle (D =
3.7 / 24 = 15%) requires the recirculating diode D1 to carry
the load current much longer than the internal power MOS-
FET of the LM3402. The estimated average diode current is:
D
= 35 mA
of 1.0Ω at 350 mA. The required capacitor impedance to
IN(MAX)
I
C
C
IN-RMS
Z
O
will be 1%
C
IN(MIN)
P-P
O
= 1/(2 x
= [0.035 / (0.258 - 0.035] x 1.0 = 0.157Ω
is negligible. The closest 10% tolerance capacitor
, and the LED datasheet gives a typical value for
= 0.35 x Sqrt(0.154 x 0.846) = 126 mA
= (0.35 x 300 x 10
I
D
π
= 0.35 x 0.85 = 298 mA
P-P
x 0.157 x 4.68 x 10
= 240 mV. The minimum required ca-
-9
) / 0.24 = 438 nF
5
) = 2.18 µF
P-P
P-
is
22
Schottky diodes are available at forward current ratings of
0.5A, however the current rating often assumes a 25°C am-
bient temperature and does not take into account the appli-
cation restrictions on temperature rise. A diode rated for
higher current may be needed to keep the temperature rise
below 40°C.To determine the proper case size, the dissipa-
tion and temperature rise in D1 can be calculated as shown
in the Design Considerations section. V
such as SOD-123 in a 40V, 0.5A Schottky diode at 350 mA is
approximately 0.4V and the θ
tion and temperature rise can be calculated as:
According to these calculations the SOD-123 diode will meet
the requirements. Heating and dissipation are among the fac-
tors most difficult to predict in converter design. If possible, a
footprint should be used that is capable of accepting both
SOD-123 and a larger case size, such as SMA. A larger diode
with a higher forward current rating will generally have a lower
forward voltage, reducing dissipation, as well as having a
lower θ
C
The bootstrap capacitor C
capacitor with X7R dielectric. A 25V rating is appropriate for
all application circuits. The linear regulator filter capacitor C
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. This term should not be confused with the op-
tical efficacy of the circuit, which depends upon the LEDs
themselves.
Total output power, P
Conduction loss, P
Gate charging and VCC loss, P
regulator:
Switching loss, P
AC rms current loss, P
P
CIN
B
P
and C
P
C
P
= I
S
G
= (I
= 0.5 x 24 x 0.35 x (40 x 10
JA
IN(rms)
= (600 x 10
, reducing temperature rise.
F
F
2
x R
P
2
P
O
x ESR = (0.126)
S
DSON
T
P
= I
P
= 0.5 x V
RISE
S
G
D
, in the internal MOSFET:
F
C
-6
= (I
= 0.298 x 0.4 = 119 mW
, in the internal MOSFET:
) x D = (0.35
x V
+ 468000 x 3 x 10
= 0.119 x 206 = 24.5°C
O
IN-OP
CIN
, is calculated as:
O
B
= 0.35 x 3.7 = 1.295W
, in the input capacitor:
IN
should always be a 10 nF ceramic
+ f
x I
JA
2
SW
F
G
x 0.006 = 0.1 mW (negligible)
x (t
is 206°C/W. Power dissipa-
2
, in the gate drive and linear
x Q
x 1.5) x 0.154 = 28 mW
-9
R
) x 468000 = 78 mW
+ t
G
) x V
D
-9
F
for a small case size
) x f
) x 24 = 48 mW
IN
SW
F