LM3405A_14 datasheet(10/31 Pages) TI1

Part #	LM3405A

Description	1.6MHz, 1A Constant Current Buck LED Driver with Internal Compensation in Tiny SOT and MSOP PowerPAD Packages

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LM3405A

SNVS508B – OCTOBER 2007 – REVISED OCTOBER 2008

www.ti.com

An alternate method is to place the zener diode D3 in a shunt configuration as shown in Figure 17. A small

350mW to 500mW, 5.1V zener in a SOT or SOD package can be used for this purpose. A small ceramic

capacitor such as a 6.3V, 0.1µF capacitor (C5) should be placed in parallel with the zener diode. When the

internal NMOS switch turns on, a pulse of current is drawn to charge the internal NMOS gate capacitance. The

0.1µF parallel shunt capacitor ensures that the VBOOST voltage is maintained during this time. Resistor R2 should

be chosen to provide enough RMS current to the zener diode and to the BOOST pin. A recommended choice for

the zener current (IZENER) is 1mA. The current IBOOST into the BOOST pin supplies the gate current of the NMOS

power switch. It reaches a maximum of around 3.6mA at the highest gate drive voltage of 5.5V over the

LM3405A operating range.

For the worst case IBOOST, increase the current by 50%. In that case, the maximum boost current will be:

IBOOST-MAX = 1.5 x 3.6mA = 5.4mA

(6)

R2 will then be given by:

R2 = (VIN - VZENER) / (IBOOST_MAX + IZENER)

(7)

For example, let VIN = 12V, VZENER = 5V, IZENER = 1mA, then:

R2 = (12V - 5V) / (5.4mA + 1mA) = 1.09k

Ω

(8)

Figure 17. VBOOST derived from VIN through a Shunt Zener

The fourth method can be used in an application which has an external low voltage rail, VEXT. C3 can be charged

through D2 from VEXT, independent of VIN and VOUT voltage levels. Again for best performance, ensure that the

gate drive voltage, VEXT - VD2 + VD1, falls in the range of 2.5V to 5.5V.

SETTING THE LED CURRENT

LM3405A is a constant current buck regulator. The LEDs are connected between VOUT and the FB pin as shown

in the Typical Application Circuit. The FB pin is at 0.205V in regulation and therefore the LED current IF is set by

VFB and resistor R1 from FB to ground by the following equation:

IF = VFB / R1

(9)

IF should not exceed the 1A current capability of LM3405A and therefore R1 minimum must be approximately

0.2

Ω. IF should also be kept above 200mA for stable operation, and therefore R1 maximum must be

approximately 1

Ω. If average LED currents less than 200mA are desired, the EN/DIM pin can be used for PWM

dimming. See LED PWM DIMMING section.

OUTPUT VOLTAGE

The output voltage is primarily determined by the number of LEDs (n) connected from VOUT to FB pin and

therefore VOUT can be written as :

VOUT = ((n x VF) + VFB)

(10)

where VF is the forward voltage of one LED at the set LED current level (see LED manufacturer datasheet for

forward characteristics curve).

10

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