LM3406_09 Datasheet, PDF (16/26 Pages) National Semiconductor (TI) – 1.5A Constant Current Buck Regulator for Driving High Power LEDs

RECIRCULATING DIODE
The input voltage of 24V ±5% requires Schottky diodes with
a reverse voltage rating greater than 30V. The next highest
standard voltage rating is 40V. Selecting a 40V rated diode
provides a large safety margin for the ringing of the switch
node and also makes cross-referencing of diodes from differ-
ent vendors easier.
The next parameters to be determined are the forward current
rating and case size. The lower the duty cycle the more ther-
mal stress is placed on the recirculating diode. When driving
one LED the duty cycle can be estimated as:
D = 4.1 / 24 = 0.17
The estimated average diode current is then:
I_D = (1 - 0.17) x 1.54 = 1.28A
A 2A-rated diode will be used. To determine the proper case
size, the dissipation and temperature rise in D1 can be cal-
culated as shown in the Design Considerations section. V_D
for a case size such as SMB in a 40V, 2A Schottky diode at
1.5A is approximately 0.4V and the θ_JA is 75°C/W. Power dis-
sipation and temperature rise can be calculated as:
P_D = 1.28 x 0.4 = 512 mW
T_RISE = 0.51 x 75 = 38°C
C_B, C_C AND C_F
The bootstrap capacitor C_B should always be a 22 nF ceramic
capacitors with X7R dielectric. A 25V rating is appropriate for
all application circuits. The COMP pin capacitor C_C and the
linear regulator filter capacitor C_F should always be 100 nF
ceramic capacitors, also with X7R dielectric and a 25V rat-
ings.
EFFICIENCY
To estimate the electrical efficiency of this example the power
dissipation in each current carrying element can be calculated
and summed. Electrical efficiency, η, should not be confused
with the optical efficacy of the circuit, which depends upon the
LEDs themselves. One calculation will be detailed for three
LEDs in series, where V_O = 11.8V, and these calculations can
be repeated for other numbers of LEDs.
Total output power, P_O, is calculated as:
P_O = I_F x V_O = 1.54 x 11.8 = 18.2W
Conduction loss, P_C, in the internal MOSFET:
P_C = (I_F2 x R_DSON) x D = (1.54² x 0.75) x 0.5 = 890 mW
Gate charging and VCC loss, P_G, in the gate drive and linear
regulator:
P_G = (I_IN-OP + f_SW x Q_G) x V_IN
P_G = (600 x 10^-6 + 5 x 10⁵ x 9 x 10^-9) x 24 = 122 mW
Switching loss, P_S, in the internal MOSFET:
P_S = 0.5 x V_IN x I_F x (t_R + t_F) x f_SW
P_S = 0.5 x 24 x 1.54 x 40 x 10^-9 x 5 x 10⁵ = 370 mW
AC rms current loss, P_CIN, in the input capacitor:
P_CIN = I_IN(rms)2 x ESR = 0.75² 0.003 = 2 mW (negligible)
DCR loss, P_L, in the inductor
P_L = I_F2 x DCR = 1.54² x 0.06 = 142 mW
Recirculating diode loss, P_D = (1 - 0.5) x 1.54 x 0.4 = 300 mW
Current Sense Resistor Loss, P_SNS = 293 mW
Electrical efficiency, η = P_O / (P_O + Sum of all loss terms) =
18.2 / (18.2 + 2.1) = 89%
Temperature Rise in the LM3406 IC is calculated as:
T_LM3406 = (P_C + P_G + P_S) x θ_JA = (0.89 + 0.122 + 0.37) x 50 =
69°C
Design Example 2
The second example circuit uses the LM3406 to drive a single
white LED at 1.5A ±10% with a ripple current of 20%_P-P in a
typical 12V automotive electrical system. The two-wire dim-
ming function will be employed in order to take advantage of
the legacy 'theater dimming' method which dims and bright-
ens the interior lights of automobiles by chopping the input
voltage with a 200Hz PWM signal. As with the previous ex-
ample, the typical V_F of a white LED is 3.9V, and with the
current sense voltage of 0.2V the total output voltage will be
4.1V. The LED driver must operate to specifications over an
input range of 9V to 16V as well as operating without suffering
damage at 28V for two minutes (the 'double battery jump-
start' test) and for 300 ms at 40V (the 'load-dump' test). The
LED driver must also be able to operate without suffering
damage at inputs as low as 6V to satisfy the 'cold crank' tests.
A complete bill of materials can be found in Table 2 at the end
of this datasheet.
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