LM3402 datasheet(17/22 Pages) NSC

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Design Example 1: LM3402

(Continued)

size such as SOD-123 in a 40V, 0.5A Schottky diode at 350

mA is approximately 0.4V and the

JA is 206˚C/W. Power

dissipation and temperature rise can be calculated as:

D = 0.298 x 0.4 = 119 mW

RISE = 0.119 x 206 = 24.5˚C

According to these calculations the SOD-123 diode will meet

the requirements. Heating and dissipation are among the

factors most difficult to predict in converter design. If pos-

sible, 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

JA, reducing temperature rise.

B and CF

The bootstrap capacitor C

B should always be a 10 nF ce-

ramic capacitor with X7R dielectric. A 25V rating is appropri-

ate for all application circuits. The linear regulator filter ca-

pacitor 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. This term 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:

O =IF xVO = 0.35 x 3.7 = 1.295W

Conduction loss, P

C, in the internal MOSFET:

C =(IF

2 xR

DSON)xD= (0.35

2 x 1.5) x 0.154 = 28 mW

Gate charging and VCC loss, P

G, in the gate drive and linear

regulator:

G =(IIN-OP +fSW xQG)xVIN

G =(600x10

-6 + 468000x3x10-9)x24=48mW

Switching loss, P

S, in the internal MOSFET:

S =0.5xVIN xIF x(tR +tF)xfSW

S =0.5x24x0.35x(40x10

-9) x 468000 = 78 mW

AC rms current loss, P

CIN, in the input capacitor:

CIN =IIN(rms)

2 x ESR = (0.126)2 x 0.006 = 0.1 mW (negli-

gible)

DCR loss, P

L, in the inductor

L =IF

2 x DCR = 0.352 x 0.096 = 11.8 mW

Recirculating diode loss, P

D = 119 mW

Current Sense Resistor Loss, P

SNS =92mW

Electrical efficiency,

η =P

O /(PO + Sum of all loss terms) =

1.295 / (1.295 + 0.377) = 77%

DIE TEMPERATURE

LM3402 =(PC +PG +PS)x

LM3402 = (0.028 + 0.05 + 0.078) x 200 = 31˚C

Design Example 2: LM3402HV

The second example application is an RGB backlight for a

flat screen monitor. A separate boost regulator provides a

60V ±5% DC input rail that feeds three LM3402HV current

regulators to drive one series array each of red, green, and

blue 1W LEDs. The target for average LED current is 350

mA ±5% in each string. The monitor will adjust the color

temperature dynamically, requiring fast PWM dimming of

each string with external, parallel MOSFETs. 1W green and

blue InGaN LEDs have a typical forward voltage of 3.5V,

however red LEDs use AlInGaP technology with a typical

forward voltage of 2.9V. In order to match color properly the

design requires 14 green LEDs, twice as many as needed

for the red and blue LEDs. This example will follow the

design for the green LED array, providing the necessary

information to repeat the exercise for the blue and red LED

arrays. The circuit schematic for Design Example 2 is the

same as the Typical Application on the front page. The bill of

materials (green array only) can be found in Table 2 at the

end of this datasheet.

OUTPUT VOLTAGE

Green Array: V

O(G) = 14 x 3.5 + 0.2 = 49.2V

Blue Array: V

O(B) = 7 x 3.5 + 0.2 = 24.7V

Red Array: V

O(R) = 7 x 2.9 + 0.2 = 20.5V

ON and tON

A compromise in switching frequency is needed in this ap-

plication to balance the requirements of magnetics size and

efficiency. The high duty cycle translates into large conduc-

tion losses and high temperature rise in the IC. For best

response to a PWM dimming signal this circuit will not use an

output capacitor; hence a moderate switching frequency of

300 kHz will keep the inductance from becoming so large

that a custom-wound inductor is needed. This design will use

only surface mount components, and the selection of off-the-

shelf SMT inductors for switching regulators is poor at 1000

µH and above. R

ON is selected from the equation for switch-

ing frequency as follows:

ON = 49.2 / (1.34 x 10

-10 x3x105) = 1224 k

Ω

The closest 1% tolerance resistor is 1.21 M

Ω. The switching

frequency and on-time of the circuit can then be found using

the equations relating R

ON and tON to fSW:

SW = 49.2 / (1210000 x 1.34 x 10

-10) = 303 kHz

LM3402 Datasheet(PDF) 17 Page - National Semiconductor (TI)

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