LM3406_09 Datasheet, PDF (19/26 Pages) National Semiconductor (TI) – 1.5A Constant Current Buck Regulator for Driving High Power LEDs

RECIRCULATING DIODE
To survive an input voltage transient of 40V the Schottky
diode must be rated to a higher voltage. 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 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 / 13.8 = 0.3
The estimated average diode current is then:
I_D = (1 - 0.3) x 1.54 = 1.1A
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 60V, 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 = 1.1 x 0.4 = 440 mW
T_RISE = 0.44 x 75 = 33°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. One calculation will be detailed for the nominal
input voltage of 13.8V, and these calculations can be repeat-
ed for other numbers of LEDs.
Total output power, P_O, is calculated as:
P_O = I_F x V_O = 1.54 x 4.1 = 6.3W
Conduction loss, P_C, in the internal MOSFET:
P_C = (I_F2 x R_DSON) x D = (1.54² x 0.75) x 0.3 = 530 mW
Gate charging and VCC loss, P_G, in the gate drive and linear
regulator:
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 0.05 = 120 mW
Recirculating diode loss, P_D = (1 - 0.3) x 1.54 x 0.4 = 430 mW
Current Sense Resistor Loss, P_SNS = 293 mW
Electrical efficiency, η = P_O / (P_O + Sum of all loss terms) =
6.3 / (6.3 + 1.6) = 80%
Temperature Rise in the LM3406 IC is calculated as:
T_LM3406 = (P_C + P_G + P_S) x θ_JA = (0.53 + 0.06 + 0.19) x 50 =
39°C
Thermal Considerations During
Input Transients
The error amplifier of the LM3406 ensures that average LED
current is controlled even at the transient load-dump voltage
of 40V, leaving thermal considerations as a primary design
consideration during high voltage transients. A review of the
operating conditions at an input of 40V is still useful to make
sure that the LM3406 die temperature is not exceeded.
Switching frequency drops to 325 kHz, the on-time drops to
350 ns, and the duty cycle drops to 0.12. Repeating the cal-
culations for conduction, gate charging and switching loss
leads to a total internal loss of 731 mW, and hence a die tem-
perature rise of 37°C. The LM3406 should operate properly
even if the ambient temperature is as high a 85°C.
Layout Considerations
The performance of any switching converter depends as
much upon the layout of the PCB as the component selection.
The following guidelines will help the user design a circuit with
maximum rejection of outside EMI and minimum generation
of unwanted EMI.
COMPACT LAYOUT
Parasitic inductance can be reduced by keeping the power
path components close together and keeping the area of the
loops that high currents travel small. Short, thick traces or
copper pours (shapes) are best. In particular, the switch node
(where L1, D1, and the SW pin connect) should be just large
enough to connect all three components without excessive
heating from the current it carries. The LM3406/06HV oper-
ates in two distinct cycles whose high current paths are shown
in Figure 11:
P_G = (I_IN-OP + f_SW x Q_G) x V_IN
P_G = (600 x 10^-6 + 4.5 x 10⁵ x 9 x 10^-9) x 13.8 = 64 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 13.8 x 1.54 x 40 x 10^-9 x 4.5 x 10⁵ = 190 mW
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