LM3402 datasheet(21/36 Pages) TI1

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LMIN =

VIN(MAX) - VO

'iL

x tON

RON =

tON x VIN

1.34 x 10

-10

GND

DIM

BOOT

RON

LM3402/02HV

VIN

RSNS

RON

CIN

VIN = 24V

IF = 350 mA

VCC

LED1

LM3402, LM3402HV

www.ti.com

SNVS450D – SEPTEMBER 2006 – REVISED FEBRUARY 2010

Design Example 1: LM3402

The first example circuit will guide the user through component selection for an architectural accent lighting

application. A regulated DC voltage input of 24V ±10% will power a single 1W white LED at a forward current of

350 mA ±5%. The typical forward voltage of a 1W InGaN LED is 3.5V, hence the estimated average output

voltage will be 3.7V. The objective of this application is to place the complete current regulator and LED in the

compact space formerly occupied by an MR16 halogen light bulb. (The LED will be on a separate metal-core

PCB.) Switching frequency will be as fast as the 300 ns tON limit allows, with the emphasis on space savings over

efficiency. Efficiency cannot be ignored, however, as the confined space with little air-flow requires a maximum

temperature rise of 40°C in each circuit component. A complete bill of materials can be found in Table 1 at the

end of this datasheet.

Figure 30. Schematic for Design Example 1

RON and tON

To select RON the expression relating tON to input voltage from the Controlled On-time Overview section can be

re-written as:

(18)

Minimum on-time occurs at the maximum VIN, which is 24V x 110% = 26.4V. RON is therefore calculated as:

RON = (300 x 10

-9 x 26.4) / 1.34 x 10-10 = 59105 Ω

(19)

The closest 1% tolerance resistor is 59.0 k

Ω. The switching frequency of the circuit can then be found using the

equation relating RON to fSW:

fSW = 3.7 / (59000 x 1.34 x 10

-10) = 468 kHz

(20)

USING AN OUTPUT CAPACITOR

The inductor will be the largest component used in this design. Because the application does not require any

PWM dimming, an output capacitor can be used to greatly reduce the inductance needed without worry of

slowing the potential PWM dimming frequency. The total solution size will be reduced by using an output

capacitor and small inductor as opposed to one large inductor.

OUTPUT INDUCTOR

Knowing that an output capacitor will be used, the inductor can be selected for a larger current ripple. The

desired maximum value for

ΔiL is ±30%, or 0.6 x 350 mA = 210 mAP-P. Minimum inductance is selected at the

maximum input voltage. Re-arranging the equation for current ripple selection yields the following:

(21)

LMIN = [(26.4 – 3.7) x 300 x 10

-9] / (0.6 x 0.35) = 32.4 µH

(22)

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