LM3404MASLASHNOPB datasheet(24/39 Pages) TI1

24 / 39 page

LMIN =

VIN - VO

'iL

x tON

RON =

1.34 x 10

-10 x fSW

GND

DIM

BOOT

RON

LM3404

VIN

RSNS

RON

CIN

VIN = 24V

IF = 700 mA

VCC

LED1

LM3404, LM3404HV

SNVS465F – OCTOBER 2006 – REVISED MAY 2013

www.ti.com

Figure 30. Schematic for Design Example 1

RON and tON

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

efficiency. RON is selected from the equation for switching frequency as follows:

(17)

RON = 7.1 / (1.34 x 10

-10 x 4 x 105) = 132.5 kΩ

(18)

The closest 1% tolerance resistor is 133 k

Ω. The switching frequency and on-time of the circuit can then be

found using the equations relating RON and tON to fSW:

fSW = 7.1 / (1.33 x 10

5 x 1.34 x 10-10) = 398 kHz

(19)

tON = (1.34 x 10

-10 x 1.33 x 105) / 24 = 743 ns

(20)

OUTPUT INDUCTOR

Since an output capacitor will be used to filter some of the AC ripple current, the inductor ripple current can be

set higher than the LED ripple current. A value of 40%P-P is typical in many buck converters:

ΔiL = 0.4 x 0.7 = 0.28A

(21)

With the target ripple current determined the inductance can be chosen:

(22)

LMIN = [(24 – 7.1) x 7.43 x 10

-7] / (0.28) = 44.8 µH

(23)

The closest standard inductor value is 47 µH. The average current rating should be greater than 700 mA to

prevent overheating in the inductor. Separation between the LM3404 drivers and the LED arrays means that heat

from the inductor will not threaten the lifetime of the LEDs, but an overheated inductor could still cause the

LM3404 to enter thermal shutdown.

The inductance of the standard part chosen is ±20%. With this tolerance the typical, minimum, and maximum

inductor current ripples can be calculated:

ΔiL(TYP) = [(24 - 7.1) x 7.43 x 10

-7] / 47 x 10-6 = 266 mA

P-P

(24)

ΔiL(MIN) = [(24 - 7.1) x 7.43 x 10

-7] / 56 x 10-6 = 223 mA

P-P

(25)

ΔiL(MAX) = [(24 - 7.1) x 7.43 x 10

-7] / 38 x 10-6 = 330 mA

P-P

(26)

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