NSC LM3405AXMKE 1.6mhz, 1a constant current buck led driver with internal compensation in tiny sot23 package Datasheet

LM3405A
1.6MHz, 1A Constant Current Buck LED Driver with Internal
Compensation in Tiny SOT23 Package
General Description
Features
The LM3405A is a 1A constant current buck LED driver designed to provide a simple, high efficiency solution for driving
high power LEDs. With a 0.205V reference voltage feedback
control to minimize power dissipation, an external resistor
sets the current as needed for driving various types of LEDs.
Switching frequency is internally set to 1.6MHz, allowing
small surface mount inductors and capacitors to be used. The
LM3405A utilizes current-mode control and internal compensation offering ease of use and predictable, high performance
regulation over a wide range of operating conditions. With a
maximum input voltage of 22V, it can drive up to 5 HighBrightness LEDs in series at 1A forward current, with the
single LED forward voltage of approximately 3.7V. Additional
features include user accessible EN/DIM pin for enabling and
PWM dimming of LEDs, thermal shutdown, cycle-by-cycle
current limit and over-current protection.
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VIN operating range of 3V to 22V
Drives up to 5 High-Brightness LEDs in series at 1A
Thin SOT23-6 package
1.6MHz switching frequency
EN/DIM input for enabling and PWM dimming of LEDs
300mΩ NMOS switch
40nA shutdown current at VIN = 5V
Internally compensated current-mode control
Cycle-by-cycle current limit
Input voltage UVLO
Over-current protection
Thermal shutdown
Applications
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LED Driver
Constant Current Source
Industrial Lighting
LED Flashlights
LED Lightbulbs
Typical Application Circuit
Efficiency vs LED Current (VIN = 12V)
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30015273
© 2007 National Semiconductor Corporation
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LM3405A 1.6MHz, 1A Constant Current Buck LED Driver with Internal Compensation
October 2007
LM3405A
Connection Diagrams
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6-Lead TSOT
NS Package Number MK06A
Pin 1 Identification
Ordering Information
Part Number
Package Type
NS Package Drawing
LM3405AXMKE
LM3405AXMK
TSOT-6
MK06A
LM3405AXMKX
Package Marking
Supplied As
SSEB
250 Units on Tape and Reel
SSEB
1000 Units on Tape and Reel
SSEB
3000 Units on Tape and Reel
*NOPB versions are available
Pin Descriptions
Pin(s)
Name
1
BOOST
2
GND
3
FB
4
EN/DIM
Application Information
Voltage at this pin drives the internal NMOS power switch. A bootstrap capacitor is
connected between the BOOST and SW pins.
Signal and Power ground pin. Place the LED current-setting resistor as close as possible
to this pin for accurate current regulation.
Feedback pin. Connect an external resistor from FB to GND to set the LED Current.
Enable control input. Logic high enables operation. Toggling this pin with a periodic logic
square wave of varying duty cycle at different frequencies controls the brightness of LEDs.
Do not allow this pin to float or be greater than VIN + 0.3V.
5
VIN
Input supply voltage. Connect a bypass capacitor locally from this pin to GND.
6
SW
Switch pin. Connect this pin to the inductor, catch diode, and bootstrap capacitor.
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If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN
SW Voltage
Boost Voltage
Boost to SW Voltage
FB Voltage
EN/DIM Voltage
Junction Temperature
-0.5V to 24V
-0.5V to 24V
-0.5V to 30V
-0.5V to 6.0V
-0.5V to 3.0V
-0.5V to (VIN + 0.3V)
150°C
Operating Ratings
2kV
-65°C to +150°C
220°C
(Note 1)
VIN
3V to 22V
-0.5V to (VIN + 0.3V)
2.5V to 5.5V
-40°C to +125°C
EN/DIM voltage
Boost to SW Voltage
Junction Temperature Range
Thermal Resistance θJA (Note 3)
118°C/W
Electrical Characteristics
Unless otherwise specified, VIN = 12V. Limits in standard type are for TJ = 25°C only;
limits in boldface type apply over the junction temperature (TJ) range of -40°C to +125°C. Minimum and Maximum limits are
guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm, and are
provided for reference purposes only.
Symbol
VFB
Parameter
Conditions
Feedback Voltage
Min
Typ
Max
Units
0.188
0.205
0.220
V
ΔVFB/(ΔVINxVFB) Feedback Voltage Line Regulation VIN = 3V to 22V
IFB
UVLO
Feedback Input Bias Current
Sink/Source
Under-voltage Lockout
VIN Rising
Under-voltage Lockout
VIN Falling
0.01
1.9
UVLO Hysteresis
%/V
10
250
nA
2.74
2.95
V
2.3
V
0.44
Switching Frequency
DMAX
Maximum Duty Cycle
VFB = 0V
RDS(ON)
Switch ON Resistance
VBOOST - VSW = 3V
Switch Current Limit
VBOOST - VSW = 3V, VIN = 3V
Quiescent Current
Switching, VFB = 0.195V
Quiescent Current (Shutdown)
VEN/DIM = 0V
Enable Threshold Voltage
VEN/DIM Rising
Shutdown Threshold Voltage
VEN/DIM Falling
EN/DIM Pin Current
Sink/Source
0.01
µA
Switch Leakage
VIN = 22V
0.1
µA
ICL
IQ
VEN/DIM_TH
IEN/DIM
ISW
1.2
85
1.2
1.6
V
fSW
1.9
MHz
300
600
mΩ
2.0
2.8
A
1.8
2.8
mA
94
%
0.3
µA
1.8
V
0.4
V
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings define the conditions under which the device
is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: Human body model, 1.5kΩ in series with 100pF.
Note 3: Thermal shutdown will occur if the junction temperature (TJ) exceeds 165°C. The maximum allowable power dissipation (PD) at any ambient temperature
(TA) is PD = (TJ(MAX) – TA)/θJA . This number applies to packages soldered directly onto a 3" x 3" PC board with 2oz. copper on 4 layers in still air. For a 2 layer
board using 1 oz. copper in still air, θJA = 204°C/W.
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LM3405A
ESD Susceptibility (Note 2)
Storage Temperature
Soldering Information
Infrared/Convection Reflow (15sec)
Absolute Maximum Ratings (Note 1)
LM3405A
Typical Performance Characteristics
Unless otherwise specified, VIN = 12V, VBOOST - VSW = 5V and
TA = 25°C.
Efficiency vs LED Current (VIN=5V)
Efficiency vs Input Voltage (IF = 1A)
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Efficiency vs Input Voltage (IF = 0.7A)
Efficiency vs Input Voltage (IF = 0.35A)
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VFB vs Temperature
Oscillator Frequency vs Temperature
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RDS(ON) vs Temperature (VBOOST - VSW = 3V)
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Quiescent Current vs Temperature
Startup Response to EN/DIM Signal
(VIN = 15V, IF = 0.2A)
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LM3405A
Current Limit vs Temperature
LM3405A
Block Diagram
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FIGURE 1. Simplified Block Diagram
Application Information
THEORY OF OPERATION
The LM3405A is a PWM, current-mode control switching buck
regulator designed to provide a simple, high efficiency solution for driving LEDs with a preset switching frequency of
1.6MHz. This high frequency allows the LM3405A to operate
with small surface mount capacitors and inductors, resulting
in LED drivers that need only a minimum amount of board
space. The LM3405A is internally compensated, simple to
use, and requires few external components.
The following description of operation of the LM3405A will refer to the Simplified Block Diagram (Figure 1) and to the
waveforms in Figure 2. The LM3405A supplies a regulated
output current by switching the internal NMOS power switch
at constant frequency and variable duty cycle. A switching
cycle begins at the falling edge of the reset pulse generated
by the internal oscillator. When this pulse goes low, the output
control logic turns on the internal NMOS power switch. During
this on-time, the SW pin voltage (VSW) swings up to approximately VIN, and the inductor current (IL) increases with a linear
slope. IL is measured by the current sense amplifier, which
generates an output proportional to the switch current. The
sense signal is summed with the regulator’s corrective ramp
and compared to the error amplifier’s output, which is proportional to the difference between the feedback voltage and
VREF. When the PWM comparator output goes high, the internal power switch turns off until the next switching cycle
begins. During the switch off-time, inductor current discharges through the catch diode D1, which forces the SW pin
to swing below ground by the forward voltage (VD1) of the
catch diode. The regulator loop adjusts the duty cycle (D) to
maintain a constant output current (IF) through the LED, by
forcing FB pin voltage to be equal to VREF (0.205V).
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FIGURE 2. SW Pin Voltage and Inductor Current
Waveforms of LM3405A
BOOST FUNCTION
Capacitor C3 and diode D2 in Figure 1 are used to generate
a voltage VBOOST. The voltage across C3, VBOOST - VSW, is
the gate drive voltage to the internal NMOS power switch. To
properly drive the internal NMOS switch during its on-time,
VBOOST needs to be at least 2.5V greater than VSW. Large
value of VBOOST - VSW is recommended to achieve better efficiency by minimizing both the internal switch ON resistance
(RDS(ON)), and the switch rise and fall times. However,
VBOOST - VSW should not exceed the maximum operating limit
of 5.5V.
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placing a zener diode D3 in series with D2 as shown in Figure
4. When using a series zener diode from the input, the gate
drive voltage is VIN - VD3 - VD2 + VD1.
VBOOST - VSW = VIN - VD2 + VD1
FIGURE 4. VBOOST derived from VIN through a Series
Zener
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When the NMOS switch turns on (TON), the switch pin rises
to:
An alternate method is to place the zener diode D3 in a shunt
configuration as shown in Figure 5. A small 350mW to
500mW, 5.1V zener in a SOT-23 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:
VSW = VIN – (RDS(ON) x IL)
Since the voltage across C3 remains unchanged, VBOOST is
forced to rise thus reverse biasing D2. The voltage at
VBOOST is then:
VBOOST = 2VIN – (RDS(ON) x IL) – VD2 + VD1
Depending on the quality of the diodes D1 and D2, the gate
drive voltage in this method can be slightly less or larger than
the input voltage VIN. For best performance, ensure that the
variation of the input supply does not cause the gate drive
voltage to fall outside the recommended range:
2.5V < VIN - VD2 + VD1 < 5.5V
The second method for deriving the boost voltage is to connect D2 to the output as shown in Figure 3. The gate drive
voltage in this configuration is:
VBOOST - VSW = VOUT – VD2 + VD1
IBOOST-MAX = 1.5 x 3.6mA = 5.4mA
Since the gate drive voltage needs to be in the range of 2.5V
to 5.5V, the output voltage VOUT should be limited to a certain
range. For the calculation of VOUT, see OUTPUT VOLTAGE
section.
R2 will then be given by:
R2 = (VIN - VZENER) / (IBOOST_MAX + IZENER)
For example, let VIN = 12V, VZENER = 5V, IZENER = 1mA, then:
R2 = (12V - 5V) / (5.4mA + 1mA) = 1.09kΩ
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FIGURE 3. VBOOST derived from VOUT
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The third method can be used in the applications where both
VIN and VOUT are greater than 5.5V. In these cases, C3 cannot
be charged directly from these voltages; instead C3 can be
charged from VIN or VOUT minus a zener voltage (VD3) by
FIGURE 5. VBOOST derived from VIN through a Shunt Zener
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LM3405A
When the LM3405A starts up, internal circuitry from VIN supplies a 20mA current to the BOOST pin, flowing out of the
BOOST pin into C3. This current charges C3 to a voltage sufficient to turn the switch on. The BOOST pin will continue to
source current to C3 until the voltage at the feedback pin is
greater than 123mV.
There are various methods to derive VBOOST:
1. From the input voltage (VIN)
2. From the output voltage (VOUT)
3. From a shunt or series zener diode
4. From an external distributed voltage rail (VEXT)
The first method is shown in the Simplified Block Diagram of
Figure 1. Capacitor C3 is charged via diode D2 by VIN. During
a normal switching cycle, when the internal NMOS power
switch is off (TOFF) (refer to Figure 2), VBOOST equals VIN minus the forward voltage of D2 (VD2), during which the current
in the inductor (L1) forward biases the catch diode D1 (VD1).
Therefore the gate drive voltage stored across C3 is:
LM3405A
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.
with respect to VIN on the LED current is shown in Figure 7.
For a fast rising input voltage (200µs for example), there is no
need to delay the EN/DIM signal since soft-start can smoothly
bring up the LED current as shown in Figure 8.
SETTING THE LED CURRENT
LM3405A 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
IF should not exceed the 1A current capability of LM3405A
and therefore R1 minimum must be approximately 0.2Ω. IF
should also be kept above 200mA for stable operation, and
therefore R1 maximum must be approximately 1Ω. 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 :
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FIGURE 6. Startup Response to VIN with 5ms rise time
VOUT = ((n x VF) + VFB)
where VF is the forward voltage of one LED at the set LED
current level (see LED manufacturer datasheet for forward
characteristics curve).
ENABLE MODE / SHUTDOWN MODE
The LM3405A has both enable and shutdown modes that are
controlled by the EN/DIM pin. Connecting a voltage source
greater than 1.8V to the EN/DIM pin enables the operation of
LM3405A, while reducing this voltage below 0.4V places the
part in a low quiescent current (0.3µA typical) shutdown
mode. There is no internal pull-up on EN/DIM pin, therefore
an external signal is required to initiate switching. Do not allow
this pin to float or rise to 0.3V above VIN. It should be noted
that when the EN/DIM pin voltage rises above 1.8V while the
input voltage is greater than UVLO, there is a finite delay before switching starts. During this delay the LM3405A will go
through a power on reset state after which the internal softstart process commences. The soft-start process limits the
inrush current and brings up the LED current (IF) in a smooth
and controlled fashion. The total combined duration of the
power on reset delay, soft-start delay and the delay to fully
establish the LED current is in the order of 100µs (refer to
Figure 10).
The simplest way to enable the operation of LM3405A is to
connect the EN/DIM pin to VIN which allows self start-up of
LM3405A whenever the input voltage is applied. However,
when an input voltage of slow rise time is used to power the
application and if both the input voltage and the output voltage
are not fully established before the soft-start time elapses, the
control circuit will command maximum duty cycle operation of
the internal power switch to bring up the output voltage rapidly. When the feedback pin voltage exceeds 0.205V, the duty
cycle will have to reduce from the maximum value accordingly, to maintain regulation. It takes a finite amount of time for
this reduction of duty cycle and this will result in a spike in LED
current for a short duration as shown in Figure 6. In applications where this LED current overshoot is undesirable, EN/
DIM pin voltage can be separately applied and delayed such
that VIN is fully established before the EN/DIM pin voltage
reaches the enable threshold. The effect of delaying EN/DIM
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30015277
FIGURE 7. Startup Response to VIN with EN/DIM delayed
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FIGURE 8. Startup Response to VIN with 200µs rise time
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LM3405A
LED PWM DIMMING
The LED brightness can be controlled by applying a periodic
pulse signal to the EN/DIM pin and varying its frequency and/
or duty cycle. This so-called PWM dimming method controls
the average light output by pulsing the LED current between
the set value and zero. A logic high level at the EN/DIM pin
turns on the LED current whereas a logic low level turns off
the LED current. Figure 9 shows a typical LED current waveform in PWM dimming mode. As explained in the previous
section, there is approximately a 100µs delay from the EN/
DIM signal going high to fully establishing the LED current as
shown in Figure 10. This 100µs delay sets a maximum frequency limit for the driving signal that can be applied to the
EN/DIM pin for PWM dimming. Figure 11 shows the average
LED current versus duty cycle of PWM dimming signal for
various frequencies. The applicable frequency range to drive
LM3405A for PWM dimming is from 100Hz to 5kHz. The dimming ratio reduces drastically when the applied PWM dimming frequency is greater than 5kHz.
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FIGURE 11. Average LED Current versus Duty Cycle of
PWM Dimming Signal at EN/DIM Pin
UNDER-VOLTAGE LOCKOUT
Under-voltage lockout (UVLO) prevents the LM3405A from
operating until the input voltage exceeds 2.74V (typical). The
UVLO threshold has approximately 440mV of hysteresis, so
the part will operate until VIN drops below 2.3V (typical). Hysteresis prevents the part from turning off during power up if
VIN is non-monotonic.
CURRENT LIMIT
The LM3405A uses cycle-by-cycle current limit to protect the
internal power switch. During each switching cycle, a current
limit comparator detects if the power switch current exceeds
2.0A (typical), and turns off the switch until the next switching
cycle begins.
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OVER-CURRENT PROTECTION
The LM3405A has a built in over-current comparator that
compares the FB pin voltage to a threshold voltage that is
60% higher than the internal reference VREF. Once the FB pin
voltage exceeds this threshold level (typically 328mV), the internal NMOS power switch is turned off, which allows the
feedback voltage to decrease towards regulation. This
threshold provides an upper limit for the LED current. LED
current overshoot is limited to 328mV/R1 by this comparator
during transients.
FIGURE 9. PWM Dimming of LEDs using the EN/DIM Pin
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off
the internal power switch when the IC junction temperature
exceeds 165°C. After thermal shutdown occurs, the power
switch does not turn on until the junction temperature drops
below approximately 150°C.
Design Guide
INDUCTOR (L1)
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (VOUT) to input voltage (VIN):
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FIGURE 10. Startup Response to EN/DIM with IF = 1A
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LM3405A
selection, refer to Circuit Examples and Recommended Inductance Range in Table 1.
The catch diode (D1) forward voltage drop and the voltage
drop across the internal NMOS must be included to calculate
a more accurate duty cycle. Calculate D by using the following
formula:
TABLE 1. Recommended Inductance Range
IF
Inductance Range and Inductor Current Ripple
6.8µH-15µH
1.0A
Inductance
ΔiL / IF*
VSW can be approximated by:
6.8µH
10µH
15µH
51%
36%
24%
10µH
15µH
22µH
58%
39%
26%
10µH-22µH
VSW = IF x RDS(ON)
0.6A
Inductance
ΔiL / IF*
The diode forward drop (VD1) can range from 0.3V to 0.7V
depending on the quality of the diode. The lower VD1 is, the
higher the operating efficiency of the converter.
The inductor value determines the output ripple current (ΔiL,
as defined in Figure 2). Lower inductor values decrease the
size of the inductor, but increases the output ripple current.
An increase in the inductor value will decrease the output ripple current. The ratio of ripple current to LED current is
optimized when it is set between 0.3 and 0.4 at 1A LED current. This ratio r is defined as:
15µH-27µH
0.2A
Inductance
15µH
22µH
27µH
ΔiL / IF*
116%
79%
65%
*Maximum over full range of VIN and VOUT.
INPUT CAPACITOR (C1)
An input capacitor is necessary to ensure that VIN does not
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, voltage
rating, RMS current rating, and ESL (Equivalent Series Inductance). The input voltage rating is specifically stated by
the capacitor manufacturer. Make sure to check any recommended deratings and also verify if there is any significant
change in capacitance at the operating input voltage and the
operating temperature. The input capacitor maximum RMS
input current rating (IRMS-IN) must be greater than:
One must also ensure that the minimum current limit (1.2A)
is not exceeded, so the peak current in the inductor must be
calculated. The peak current (ILPK) in the inductor is calculated
as:
ILPK = IF + ΔiL/2
When the designed maximum output current is reduced, the
ratio r can be increased. At a current of 0.2A, r can be made
as high as 0.7. The ripple ratio can be increased at lighter
loads because the net ripple is actually quite low, and if r remains constant the inductor value can be made quite large.
An equation empirically developed for the maximum ripple
ratio at any current below 2A is:
It can be shown from the above equation that maximum RMS
capacitor current occurs when D = 0.5. Always calculate the
RMS at the point where the duty cycle D, is closest to 0.5. The
ESL of an input capacitor is usually determined by the effective cross sectional area of the current path. A large leaded
capacitor will have high ESL and a 0805 ceramic chip capacitor will have very low ESL. At the operating frequency of the
LM3405A, certain capacitors may have an ESL so large that
the resulting inductive impedance (2πfL) will be higher than
that required to provide stable operation. It is strongly recommended to use ceramic capacitors due to their low ESR and
low ESL. A 10µF multilayer ceramic capacitor (MLCC) is a
good choice for most applications. In cases where large capacitance is required, use surface mount capacitors such as
Tantalum capacitors and place at least a 1µF ceramic capacitor close to the VIN pin. For MLCCs it is recommended to use
X7R or X5R dielectrics. Consult capacitor manufacturer
datasheet to see how rated capacitance varies over operating
conditions.
r = 0.387 x IOUT-0.3667
Note that this is just a guideline.
The LM3405A operates at a high frequency allowing the use
of ceramic output capacitors without compromising transient
response. Ceramic capacitors allow higher inductor ripple
without significantly increasing LED current ripple. See the
output capacitor and feed-forward capacitor sections for more
details on LED current ripple.
Now that the ripple current or ripple ratio is determined, the
inductance is calculated by:
OUTPUT CAPACITOR (C2)
The output capacitor is selected based upon the desired reduction in LED current ripple. A 1µF ceramic capacitor results
in very low LED current ripple for most applications. Due to
the high switching frequency, the 1µF capacitor alone (without
feed-forward capacitor C4) can filter more than 90% of the
inductor current ripple for most applications where the sum of
LED dynamic resistance and R1 is larger than 1Ω. Since the
internal compensation is tailored for small output capacitance
with very low ESR, it is strongly recommended to use a ceramic capacitor with capacitance less than 3.3µF.
where fSW is the switching frequency and IF is the LED current.
When selecting an inductor, make sure that it is capable of
supporting the peak output current without saturating. Inductor saturation will result in a sudden reduction in inductance
and prevent the regulator from operating correctly. Because
of the operating frequency of LM3405A, ferrite based inductors are preferred to minimize core losses. This presents little
restriction since the variety of ferrite based inductors is huge.
Lastly, inductors with lower series resistance (DCR) will provide better operating efficiency. For recommended inductor
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LM3405A
Given the availability and quality of MLCCs and the expected
output voltage of designs using the LM3405A, there is really
no need to review other capacitor technologies. A benefit of
ceramic capacitors is their ability to bypass high frequency
noise. A certain amount of switching edge noise will couple
through the parasitic capacitances in the inductor to the output. A ceramic capacitor will bypass this noise. In cases where
large capacitance is required, use Electrolytic or Tantalum
capacitors with large ESR, and verify the loop performance
on bench. Like the input capacitor, recommended multilayer
ceramic capacitors are X7R or X5R. Again, verify actual capacitance at the desired operating voltage and temperature.
Check the RMS current rating of the capacitor. The maximum
RMS current rating of the capacitor is:
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FIGURE 13. PWM Dimming with a 1µF Feed-Forward
Capacitor
One may select a 1206 size ceramic capacitor for C2, since
its current rating is typically higher than 1A, more than enough
for the requirement.
CATCH DIODE (D1)
The catch diode (D1) conducts during the switch off-time. A
Schottky diode is required for its fast switching time and low
forward voltage drop. The catch diode should be chosen such
that its current rating is greater than:
FEED-FORWARD CAPACITOR (C4)
The feed-forward capacitor (designated as C4) connected in
parallel with the LED string is required to provide multiple
benefits to the LED driver design. It greatly improves the large
signal transient response and suppresses LED current overshoot that may otherwise occur during PWM dimming; it also
helps to shape the rise and fall times of the LED current pulse
during PWM dimming thus reducing EMI emission; it reduces
LED current ripple by bypassing some of inductor ripple from
flowing through the LED. For most applications, a 1µF ceramic capacitor is sufficient. In fact, the combination of a 1µF
feed-forward ceramic capacitor and a 1µF output ceramic capacitor leads to less than 1% current ripple flowing through
the LED. Lower and higher C4 values can be used, but bench
validation is required to ensure the performance meets the
application requirement.
Figure 12 shows a typical LED current waveform during PWM
dimming without feed-forward capacitor. At the beginning of
each PWM cycle, overshoot can be seen in the LED current.
Adding a 1µF feed-forward capacitor can totally remove the
overshoot as shown in Figure 13.
ID1 = IF x (1-D)
The reverse breakdown rating of the diode must be at least
the maximum input voltage plus appropriate margin. To improve efficiency, choose a Schottky diode with a low forward
voltage drop.
BOOST DIODE (D2)
A standard diode such as the 1N4148 type is recommended.
For VBOOST circuits derived from voltages less than 3.3V, a
small-signal Schottky diode is recommended for better efficiency. A good choice is the BAT54 small signal diode.
BOOST CAPACITOR (C3)
A 0.01µF ceramic capacitor with a voltage rating of at least
6.3V is sufficient. The X7R and X5R MLCCs provide the best
performance.
POWER LOSS ESTIMATION
The main power loss in LM3405A includes three basic types
of loss in the internal power switch: conduction loss, switching
loss, and gate charge loss. In addition, there is loss associated with the power required for the internal circuitry of IC.
The conduction loss is calculated as:
If the inductor ripple current is fairly small (for example, less
than 40%) , the conduction loss can be simplified to:
PCOND = IF2 x RDS(ON) x D
The switching loss occurs during the switch on and off transition periods, where voltage and current overlap resulting in
power loss. The simplest means to determine this loss is to
empirically measure the rise and fall times (10% to 90%) of
the voltage at the switch pin.
Switching power loss is calculated as follows:
30015269
FIGURE 12. PWM Dimming without Feed-Forward
Capacitor
PSW = 0.5 x VIN x IF x fSW x ( TRISE + TFALL )
11
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LM3405A
The gate charge loss is associated with the gate charge QG
required to drive the switch:
PCB Layout Considerations
When planning layout there are a few things to consider when
trying to achieve a clean, regulated output. The most important consideration when completing the layout is the close
coupling of the GND connections of the input capacitor C1
and the catch diode D1. These ground ends should be close
to one another and be connected to the GND plane with at
least two through-holes. Place these components as close to
the IC as possible. The next consideration is the location of
the GND connection of the output capacitor C2, which should
be near the GND connections of C1 and D1.
There should be a continuous ground plane on the bottom
layer of a two-layer board except under the switching node
island.
The FB pin is a high impedance node and care should be
taken to make the FB trace short to avoid noise pickup that
causes inaccurate regulation. The LED current setting resistor R1 should be placed as close as possible to the IC, with
the GND of R1 placed as close as possible to the GND of the
IC. The VOUT trace to LED anode should be routed away from
the inductor and any other traces that are switching.
High AC currents flow through the VIN, SW and VOUT traces,
so they should be as short and wide as possible. Radiated
noise can be decreased by choosing a shielded inductor.
The remaining components should also be placed as close
as possible to the IC. Please see Application Note AN-1229
for further considerations and the LM3405A demo board as
an example of a four-layer layout.
PG = fSW x VIN x QG
The power loss required for operation of the internal circuitry:
PQ = IQ x VIN
IQ is the quiescent operating current, and is typically around
1.8mA for the LM3405A.
The total power loss in the IC is:
PINTERNAL = PCOND + PSW + PG + PQ
An example of power losses for a typical application is shown
in Table 2:
TABLE 2. Power Loss Tabulation
Conditions
Power loss
VIN
12V
VOUT
3.9V
IOUT
1.0A
VD1
0.45V
RDS(ON)
300mΩ
fSW
1.6MHz
PCOND
108mW
PSW
288mW
TRISE
18ns
TFALL
12ns
IQ
1.8mA
PQ
22mW
QG
1.4nC
PG
27mW
D is calculated to be 0.36
Σ ( PCOND + PSW + PQ + PG ) = PINTERNAL
PINTERNAL = 445mW
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12
LM3405A
LM3405A Circuit Examples
30015242
FIGURE 14. VBOOST derived from VIN
( VIN = 5V, IF = 1A )
Bill of Materials for Figure 14
Part ID
Part Value
Part Number
Manufacturer
U1
1A LED Driver
LM3405A
National Semiconductor
C1, Input Cap
10µF, 6.3V, X5R
C3216X5R0J106M
TDK
C2, Output Cap
1µF, 10V, X7R
GRM319R71A105KC01D
Murata
C3, Boost Cap
0.01µF, 16V, X7R
0805YC103KAT2A
AVX
C4, Feedforward Cap
1µF, 10V, X7R
GRM319R71A105KC01D
Murata
D1, Catch Diode
Schottky, 0.37V at 1A, VR = 10V MBRM110LT1G
ON Semiconductor
D2, Boost Diode
Schottky, 0.36V at 15mA
CMDSH-3
Central Semiconductor
L1
4.7µH, 1.6A
SLF6028T-4R7M1R6
TDK
R1
0.2Ω, 0.5W, 1%
WSL2010R2000FEA
Vishay
LED1
1.5A, White LED
LXK2-PW14
Lumileds
13
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LM3405A
30015243
FIGURE 15. VBOOST derived from VOUT
( VIN = 12V, IF = 1A )
Bill of Materials for Figure 15
Part ID
Part Value
Part Number
Manufacturer
U1
1A LED Driver
LM3405A
National Semiconductor
C1, Input Cap
10µF, 25V, X5R
ECJ-3YB1E106K
Panasonic
C2, Output Cap
1µF, 10V, X7R
GRM319R71A105KC01D
Murata
C3, Boost Cap
0.01µF, 16V, X7R
0805YC103KAT2A
AVX
C4, Feedforward Cap
1µF, 10V, X7R
GRM319R71A105KC01D
Murata
D1, Catch Diode
Schottky, 0.5V at 1A, VR = 30V SS13
Vishay
D2, Boost Diode
Schottky, 0.36V at 15mA
CMDSH-3
Central Semiconductor
L1
4.7µH, 1.6A
SLF6028T-4R7M1R6
TDK
R1
0.2Ω, 0.5W, 1%
WSL2010R2000FEA
Vishay
LED1
1.5A, White LED
LXK2-PW14
Lumileds
www.national.com
14
LM3405A
30015244
FIGURE 16. VBOOST derived from VIN through a Shunt Zener Diode (D3)
( VIN = 18V, IF = 1A )
Bill of Materials for Figure 16
Part ID
Part Value
Part Number
Manufacturer
U1
1A LED Driver
LM3405A
National Semiconductor
C1, Input Cap
10µF, 25V, X5R
ECJ-3YB1E106K
Panasonic
C2, Output Cap
1µF, 10V, X7R
GRM319R71A105KC01D
Murata
C3, Boost Cap
0.01µF, 16V, X7R
0805YC103KAT2A
AVX
C4, Feedforward Cap
1µF, 10V, X7R
GRM319R71A105KC01D
Murata
C5, Shunt Cap
0.1µF, 16V, X7R
GRM219R71C104KA01D
Murata
D1, Catch Diode
Schottky, 0.5V at 1A, VR = 30V SS13
Vishay
D2, Boost Diode
Schottky, 0.36V at 15mA
CMDSH-3
Central Semiconductor
D3, Zener Diode
4.7V, 350mW, SOT-23
BZX84C4V7
Fairchild
L1
6.8µH, 1.5A
SLF6028T-6R8M1R5
TDK
R1
0.2Ω, 0.5W, 1%
WSL2010R2000FEA
Vishay
R2
1.91kΩ, 1%
CRCW08051K91FKEA
Vishay
LED1
1.5A, White LED
LXK2-PW14
Lumileds
15
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LM3405A
30015249
FIGURE 17. VBOOST derived from VIN through a Series Zener Diode (D3)
( VIN = 15V, IF = 1A )
Bill of Materials for Figure 17
Part ID
Part Value
Part Number
Manufacturer
U1
1A LED Driver
LM3405A
National Semiconductor
C1, Input Cap
10µF, 25V, X5R
ECJ-3YB1E106K
Panasonic
C2, Output Cap
1µF, 10V, X7R
GRM319R71A105KC01D
Murata
C3, Boost Cap
0.01µF, 16V, X7R
0805YC103KAT2A
AVX
C4, Feedforward Cap
1µF, 10V, X7R
GRM319R71A105KC01D
Murata
D1, Catch Diode
Schottky, 0.5V at 1A, VR = 30V SS13
Vishay
D2, Boost Diode
Schottky, 0.36V at 15mA
CMDSH-3
Central Semiconductor
D3, Zener Diode
11V, 350mW, SOT-23
BZX84C11
Fairchild
L1
6.8µH, 1.5A
SLF6028T-6R8M1R5
TDK
R1
0.2Ω, 0.5W, 1%
WSL2010R2000FEA
Vishay
LED1
1.5A, White LED
LXK2-PW14
Lumileds
www.national.com
16
LM3405A
30015250
FIGURE 18. VBOOST derived from VOUT through a Series Zener Diode (D3)
( VIN = 18V, IF = 1A )
Bill of Materials for Figure 18
Part ID
Part Value
Part Number
Manufacturer
U1
1A LED Driver
LM3405A
National Semiconductor
C1, Input Cap
10µF, 25V, X5R
ECJ-3YB1E106K
Panasonic
C2, Output Cap
1µF, 16V, X7R
GRM319R71A105KC01D
Murata
C3, Boost Cap
0.01µF, 16V, X7R
0805YC103KAT2A
AVX
C4, Feedforward Cap
1µF, 16V, X7R
GRM319R71A105KC01D
Murata
D1, Catch Diode
Schottky, 0.5V at 1A, VR = 30V SS13
Vishay
D2, Boost Diode
Schottky, 0.36V at 15mA
CMDSH-3
Central Semiconductor
D3, Zener Diode
3.6V, 350mW, SOT-23
BZX84C3V6
Fairchild
L1
6.8µH, 1.5A
SLF6028T-6R8M1R5
TDK
R1
0.2Ω, 0.5W, 1%
WSL2010R2000FEA
Vishay
LED1
1.5A, White LED
LXK2-PW14
Lumileds
LED2
1.5A, White LED
LXK2-PW14
Lumileds
17
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LM3405A
30015251
FIGURE 19. LED MR16 Lamp Application
( VIN = 12V AC, IF = 0.75A )
Bill of Materials for Figure 19
Part ID
Part Value
Part Number
Manufacturer
U1
1A LED Driver
LM3405A
National Semiconductor
C1, Input Cap
10µF, 25V, X5R
ECJ-3YB1E106K
Panasonic
C2, Output Cap
1µF, 10V, X7R
GRM319R71A105KC01D
Murata
C3, Boost Cap
0.01µF, 16V, X7R
0805YC103KAT2A
AVX
C5, Input Cap
220µF, 25V, electrolytic
ECE-A1EN221U
Panasonic
D1, Catch Diode
Schottky, 0.5V at 1A, VR = 30V SS13
Vishay
D2, Boost Diode
Schottky, 0.36V at 15mA
CMDSH-3
Central Semiconductor
D3, Rectifier Diode
Schottky, 0.385V at 500mA
CMHSH5-2L
Central Semiconductor
D4, Rectifier Diode
Schottky, 0.385V at 500mA
CMHSH5-2L
Central Semiconductor
D5, Rectifier Diode
Schottky, 0.385V at 500mA
CMHSH5-2L
Central Semiconductor
D6, Rectifier Diode
Schottky, 0.385V at 500mA
CMHSH5-2L
Central Semiconductor
L1
6.8µH, 1.5A
SLF6028T-6R8M1R5
TDK
R1
0.27Ω, 0.33W, 1%
ERJ8BQFR27
Panasonic
LED1
1A, White LED
LXHL-PW09
Lumileds
www.national.com
18
LM3405A
Physical Dimensions inches (millimeters) unless otherwise noted
6-Lead TSOT Package
NS Package Number MK06A
19
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LM3405A 1.6MHz, 1A Constant Current Buck LED Driver with Internal Compensation
Notes
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