tON = 1 x 10

-11 x R

ON x

VO

VIN

fSW =

1

1 x 10

-11 x RON

RON =

1

fSW x 1 x 10

-11

GND

DIM

BOOT

SW

VOUT

RON

LM3406

VIN

D1

L1

CB

RSNS

CF

RON

CIN

VIN = 24V ±10%

IF = 1.5A ±5%

VCC

CS

COMP

CC

LED1

LEDn

One to

five

LEDs

CO

LM3406, LM3406HV, LM3406HV-Q1

www.ti.com

SNVS512E – SEPTEMBER 2008 – REVISED MARCH 2014

Transient Protection Considerations (continued)

8.2.4 GENERAL COMMENTS REGARDING OTHER PINS

Any pin that goes “off-board” through a connector should have series resistance of at least 1k

Ω to 10kΩ in series

with it to protect it from ESD or other transients. These series resistors limit the peak current that can flow (or

cause a voltage drop) during a transient event, thus protecting the pin and the device. Pins that are not used

should not be left floating. They should instead be tied to GND or to an appropriate voltage through resistance.

8.3 Design Example 1

The first example circuit uses the LM3406 to create a flexible LED driver capable of driving anywhere from one to

five white series-connected LEDs at a current of 1.5A ±5% from a regulated DC voltage input of 24V ±10%. In

addition to the ±5% tolerance specified for the average output current, the LED ripple current must be controlled

to 10%P-P of the DC value, or 150 mAP-P. The typical forward voltage of each individual LED at 1.5A is 3.9V,

hence the output voltage ranges from 4.1V to 19.7V, adding in the 0.2V drop for current sensing. A complete bill

of materials can be found in Table 1 at the end of this datasheet.

Schematic for Design Example 1

8.3.1 RON and tON

A moderate switching frequency of 500 kHz will balance the requirements of inductor size and overall power

efficiency. The LM3406 will allow some shift in switching frequency when VO changes due to the number of LEDs

in series, so the calculation for RON is done at the mid-point of three LEDs in series, where VO = 11.8V. Note that

the actual RON calculation is done with the high accuracy expression listed in the Appendix.

(20)

RON = 144 kΩ

(21)

The closest 1% tolerance resistor is 143 k

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

checked for one, three and five LEDs using the equations relating RON and tON to fSW. As with the RON

calculation, the actual fSW and tON values have been calculated using the high accuracy expressions listed in the

Appendix.

(22)

fSW(1 LED) = 362 kHz

(23)

fSW(3 LEDs) = 504 kHz

(24)

fSW(5 LEDs) = 555 kHz

(25)

(26)

tON(1 LED) = 528 ns

(27)

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