NSC LM3405

National Semiconductor
Application Note 1644
Vinit Jayaraj
January 17, 2008
The LM3405 is a 1A, constant-current buck regulator designed to provide a simple, highly-efficient solution for powering LEDs with the highest power density. The low-feedback
reference voltage of 205 mV (typical) allows the use of LEDs
with large forward voltages. The LM3405 LED driver comes
with an integrated 1A high-side power switch, internal current
limit, over-voltage protection and thermal shutdown. The
LM3405 driver utilizes current mode control with internal compensation offering ease-of-use and predictable, high-performance regulation over a wide range of operating conditions.
This article describes a few of the many application solutions
for the LM3405 LED driver. The LM3405 device is offered in
a 6-pin Thin-SOT (TSOT) package and operates at an internal
switching frequency of 1.6 MHz, allowing the use of smallvalue inductors, thereby saving board space.
Typical Application Circuit
Figure 1 shows the typical application circuit using the
LM3405 regulator driving a single LED. The boost voltage can
be derived from VIN or VOUT. The BOOST to SW voltage
(VBOOST-VSW) should never exceed the operating limit of
5.5V, and must be greater than 2.5V for optimum efficiency.
30022901
FIGURE 1. Typical Application Circuit
In the above circuit, the maximum VIN is 5V and therefore the
boost voltage is derived from VIN. The LM3405 data sheet
provides a detailed description of various circuits for boostvoltage generation. The LM3405 driver tightly regulates the
FB-pin voltage to 205 mV (typical) and this allows the LED
current (IF) to be set by the following equation:
off the internal power switch on a cycle-by-cycle basis. In case
of an over voltage (sensed at the FB pin), the internal power
switch is turned off. The LM3405 regulator also has a built-in
thermal shutdown (with hysteresis) which also turns off the
internal power switch if the die temperature exceeds a typical
value of 165°C.
PWM Dimming Using the EN/DIM Pin
The LEDs can be dimmed by applying a PWM-logic signal to
the EN/DIM pin of the LM3405 driver as shown in Figure 2.
R1 (min) is 200 mΩ such that IF (max) is limited to 1A. In case
of an over current, the internal current limit will trigger and turn
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Powering and Dimming High-Brightness LEDs with the LM3405 Constant-Current Buck
Regulator
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Powering and Dimming
High-Brightness LEDs with
the LM3405 ConstantCurrent Buck Regulator
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30022903
FIGURE 2. PWM Dimming of LEDs by Applying a PWM Signal to the EN/DIM Pin
A logic high at V_PWM enables the IC, and a logic low disables the IC. In this manner, the LED current is turned on and
off. In order to eliminate flicker, the lowest PWM-dimming frequency is normally chosen to be above 100 Hz. The upper
end of the PWM frequency is determined by the turn-on delay
of the LM3405. If the V_PWM signal has an ON-time of TON
and a period of TPER, the average IF is given approximately
by the equation below. Note that the delay in enabling and
disabling the IC is not included in the following equation. Typically the time delay from the instant where EN/DIM=1 to the
instant where IF is fully established is approximately 100 μs
(see Figure 4).
The startup delay of the LM3405 device is shown in Figure
4 for an LED-current setting of 1A. This is the delay from the
instant when EN/DIM=1 to the instant when IF settles to
900 mA (90% of the set LED current).
The average LED current is therefore controlled by TON,
TPER, or both and is perceived by the eye as a brightness
change. Figure 3 shows the measured average LED current
for varying duty cycles and frequencies.
30022906
FIGURE 4. Startup Response of the LM3405 to an EN/DIM
Signal with IF Set at 1A
Deriving a Self-Biased Boost
Voltage When Driving Two or More
LEDs in Series
In a typical application where two or more LEDs are driven in
a single string, the boost voltage is derived from a separate,
external low-voltage source so as to meet the (VBOOSTVSW) voltage requirement. This is because when powering
two or more LEDs in series, VIN will be greater than 5V and
therefore VIN cannot be used to provide VBOOST. In this
case, VOUT will also be high and cannot be used to derive
VBOOST.
Figure 5 shows another approach for deriving a self-biased
boost voltage from the LED string itself without the need for
generating a separate low-voltage supply.
30022905
FIGURE 3. Average LED Current versus Duty Cycle and
Frequency of V_PWM at the EN/DIM Pin
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30022907
FIGURE 5. Deriving a Self-Biased Boost Voltage from the LED String When Driving Two or More LEDs in Series
The anode of the boost diode D2 is connected to the anode
of LED3. The voltage at the anode of LED3 is approximately
equal to 205 mV plus the forward voltage VF of LED3. This
voltage is approximately in the range of 3V to 4V (depending
on the IF3 current setting) and meets the BOOST to SW voltage requirement. It must be noted that the current through
LED1 and LED2 will be slightly larger than the current in LED3
due to the fact that the average charging current for the boost
capacitor C3 will now be provided through LED1 and LED2.
Therefore, the LED currents are:
where IF1, IF2, and IF3 are the currents in LED1, LED2, and
LED3, respectively and IC3 is the average current charging C3
over one switching cycle. This approach can be used if exact
current matching (and hence brightness) is not required between IF1 and IF3 or between IF2 and IF3.
Driving Multiple LED Strings with
the LM3405 LED Driver
The LM3405 LED driver can be used to drive multiple LED
strings in parallel as shown in Figure 6. The current in the
primary branch (with LED1 and LED2) will be tightly regulated
by the feedback loop.
30022909
FIGURE 6. Driving Multiple LED Strings
The current in the secondary branch (with LED3) will be regulated based on VOUT, forward voltage of LED3, and R2. The
value of R2 can be adjusted to get the desired brightness for
LED3.
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If VBST is derived from the cathode of LED3, this VBST can
be written as (assuming VF1=VF2=VF3=VF=3.8V):
VBST = 205 mV + VF1 + VF2 - VF3 ≅ 205 mV + VF ≅ 4V
Note that the average current through LED3 will now be higher since the average current to charge C3 is supplied through
LED3 as discussed in the previous section.
where VF1 and VF2 are the respective forward voltages of
LED1 and LED2 at the primary current level, and VF3 is the
forward voltage of LED3 at the secondary current level. IF1,
IF2, and IF3 are the currents in LED1, LED2, and LED3, respectively.
In Figure 6, VBST can be connected to an external DC source
of 3V to 5V. VBST can also be derived from the cathode of
LED3.
Driving Parallel LEDs with the
LM3405 LED Driver
Multiple LEDs can be paralleled and connected between
VOUT and FB as shown in Figure 7.
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FIGURE 7. Driving Multiple LEDs in Parallel
The voltage at the FB pin is regulated to 205 mV by the control
loop and therefore the current in R1 is fixed. VOUT is determined by the LED with the highest forward voltage. This
solution has the advantage of having a single resistor set the
total currents in the LEDs but has no control of equal current
sharing between the LEDs.
highly-compact, driving and dimming solutions for high-brightness LEDs. The LM3405 data sheet may be referred to for a
detailed description of operation and component design
guidelines.
Conclusion
This article describes a few application solutions using the
LM3405 buck regulator. These solutions provide the user with
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Notes
5
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Powering and Dimming High-Brightness LEDs with the LM3405 ConstantCurrent Buck Regulator
Notes
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