LM3404MASLASHNOPB datasheet(27/39 Pages) TI1

27 / 39 page

LMIN =

VIN - VO

'iL

x tON

RON =

1.34 x 10

-10 x fSW

GND

BOOT

RON

LM3404HV

VIN

RSNS

RON

CIN

VIN = 48V ±10%

IF = 0.5A

VCC

LED1

LED10

LM3404, LM3404HV

www.ti.com

SNVS465F – OCTOBER 2006 – REVISED MAY 2013

Design Example 2: LM3404HV

The second example circuit will guide the user through component selection for an outdoor general lighting

application. A regulated DC voltage input of 48V ±10% will power ten series-connected LEDs at 500 mA ±10%

with a ripple current of 50 mAP-P or less. The typical forward voltage of the LED module in thermal steady state is

35V, hence the average output voltage will be 35.2V. A complete bill of materials can be found in Table 2 at the

end of this datasheet.

Figure 31. Schematic for Design Example 2

RON and tON

A low switching frequency, 225 kHz, is needed in this application, as high efficiency and low power dissipation

take precedence over the solution size. RON is selected from the equation for switching frequency as follows:

(50)

RON = 35.2 / (1.34 x 10

-10 x 2.25 x 105) = 1.16 MΩ

(51)

The next highest 1% tolerance resistor is 1.18 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 = 35.2 / (1.18 x 10

6 x 1.34 x 10-10) = 223 kHz

(52)

tON = (1.34 x 10

-10 x 1.18 x 106) / 48 = 3.3 µs

(53)

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 30%P-P makes a good trade-off between the current ripple and

the size of the inductor:

ΔiL = 0.3 x 0.5 = 0.15A

(54)

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

(55)

LMIN = [(48 – 35.2) x 3.3 x 10

-6] / (0.15) = 281 µH

(56)

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