LM3402 datasheet(26/38 Pages) TI

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RON =

1.34 x 10

-10 x fSW

LM3402, LM3402HV

SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013

www.ti.com

Gate charging and VCC loss, PG, in the gate drive and linear regulator:

PG = (IIN-OP + fSW x QG) x VIN PG = (600 x 10

-6 + 468000 x 3 x 10-9) x 24 = 48 mW

(41)

Switching loss, PS, in the internal MOSFET:

PS = 0.5 x VIN x IF x (tR + tF) x fSW PS = 0.5 x 24 x 0.35 x (40 x 10

-9) x 468000 = 78 mW

(42)

AC rms current loss, PCIN, in the input capacitor:

PCIN = IIN(rms)

2 x ESR = (0.126)2 x 0.006 = 0.1 mW (negligible)

(43)

DCR loss, PL, in the inductor

PL = IF

2 x DCR = 0.352 x 0.096 = 11.8 mW

(44)

Recirculating diode loss, PD = 119 mW

Current Sense Resistor Loss, PSNS = 92 mW

Electrical efficiency,

η = PO / (PO + Sum of all loss terms) = 1.295 / (1.295 + 0.377) = 77%

DIE TEMPERATURE

TLM3402 = (PC + PG + PS) x θJA TLM3402 = (0.028 + 0.05 + 0.078) x 200 = 31°C

(45)

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: VO(G) = 14 x 3.5 + 0.2 = 49.2V

(46)

Blue Array: VO(B) = 7 x 3.5 + 0.2 = 24.7V

(47)

Red Array: VO(R) = 7 x 2.9 + 0.2 = 20.5V

(48)

RON and tON

A compromise in switching frequency is needed in this application to balance the requirements of magnetics size

and efficiency. The high duty cycle translates into large conduction 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. RON is selected from the equation for switching frequency as

follows:

(49)

RON = 49.2 / (1.34 x 10

-10 x 3 x 105) = 1224 kΩ

(50)

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 RON and tON to fSW:

fSW = 49.2 / (1210000 x 1.34 x 10

-10) = 303 kHz

(51)

tON = (1.34 x 10

-10 x 1210000) / 60 = 2.7 µs

(52)

USING AN OUTPUT CAPACITOR

This application is dominated by the need for fast PWM dimming, requiring a circuit without any output

capacitance.

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