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LM3405 Arkusz danych(PDF) 9 Page - Texas Instruments

Numer części LM3405
Szczegółowy opis  Constant Current Buck Regulator
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LM3405 Arkusz danych(HTML) 9 Page - Texas Instruments

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LM3405
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SNVS429B – OCTOBER 2006 – REVISED MAY 2013
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 LM3405
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
(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
LM3405 is a constant current buck regulator. The LEDs are connected between VOUT and 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 the resistor R1 from FB to ground by the following equation:
IF = VFB / R1
(9)
IF should not exceed the 1A current capability of LM3405 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).
Copyright © 2006–2013, Texas Instruments Incorporated
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