DN445 - uModule LED Driver Integrates All Circuitry, Including the Inductor, in a Surface Mount Package

µModule LED Driver Integrates All Circuitry, Including the
Inductor, in a Surface Mount Package – Design Note 445
David Ng
Introduction
Once relegated to the hinterlands of low cost indicator
lights, the LED is again in the spotlight of the lighting
world. LED lighting is now ubiquitous, from car headlights
to USB-powered lava lamps. Car headlights exemplify
applications that capitalize on the LED’s clear advantages—unwavering high quality light output, tough-assteel robustness, inherent high efficiency—while a USB
lava lamp exemplifies applications where only LEDs work.
Despite these clear advantages, their requirement for
regulated voltage and current make LED driver circuits
more complex than the venerable light bulb, but some new
devices are closing the gap. For instance, the LTM ®8040
μModule™ LED driver integrates all the driver circuitry
into a single package, allowing designers to refocus their
time and effort on the details of lighting design critical
to a product’s success.
A Superior LED Driver
The LTM8040 is a complete step-down DC/DC switching converter system that can drive up to 1A through a
string of LEDs. Its 4V to 36V input voltage range makes
it suitable for a wide range of power sources, including
2-cell lithium-ion battery packs, rectified 12VAC and
industrial 24V. The LTM8040 features both analog and
PWM dimming, allowing a 250:1 dimming range. The
built-in 14V output voltage clamp prevents damage in
VIN
14V TO 36V
C1
2.2μF
VIN
LEDA
Easy to Use
The high level of integration in the LTM8040 minimizes
external components and simplifies board layout. As
shown in Figure 1, all that is necessary to drive an LED
string up to 1A is the LTM8040 and an input decoupling
capacitor. Even with all this built-in functionality, the
LTM8040 itself is small, measuring only 15mm × 9mm
× 4.32mm.
Rich Feature Set
The LTM8040 features an ADJ pin for precise LED current amplitude control. The ADJ pin accepts a full-scale
input voltage range of 0V to 1.25V, linearly adjusting the
output LED current from 0A to 1A. Figure 2 shows the
ratiometric response of the output LED current versus the
ADJ voltage. The ADJ pin is internally pulled up through
a 5.11k precision resistor to an internal 1.25V reference,
so the output LED current can also be adjusted by applying a single resistor from ADJ to ground, as shown
in Figure 3.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
μModule is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
1A
LPWR
SHDN
LTM8040
BIAS
ADJ
LED
CURRENT
500mA/DIV
PWM
RT
the case of an accidental open LED string. The default
switching frequency of the LTM8040 is 500kHz, but
switching frequencies to 2MHz can be set with a resistor
from the RT pin to GND.
0 AMPS
ADJ PIN
VOLTAGE
500mV/DIV
GND
0 VOLTS
5ms/DIV
DN445 F02
DN445 F01
Figure 1. Driving an LED String with the LTM8040 Is
Simple—Just Add the Input Capacitor and Connect the
LED String
01/09/445
Figure 2. Drive a 0V to 1.25V Voltage into the ADJ Pin to
Control the LED Current Amplitude
VIN
4V TO 36V
C1
2.2μF
LEDA
VIN
LPWR
SHDN
LTM8040
ADJ
BIAS
LED
CURRENT
500mA/DIV
PWM
5.11k
PWM
SIGNAL
5V/DIV
GND
20ms/DIV
DN445 F03
Figure 3. Control the LED Current with a Single
Resistor from ADJ to Ground
The PWM control pin allows high dimming ratios. With an
external MOSFET in series with the LED string as shown
in Figure 4, the LTM8040 can achieve dimming ratios in
excess of 250:1. As seen in Figure 5, there is little distortion of the PWM LED current, even at frequencies as low
as 10Hz. The 10Hz performance is shown to illustrate the
capabilities of the LTM8040—this frequency is too low
for practical pulse width modulation, being well within
the discrimination range of the human eye.
VIN
4V TO 36V
C1
2.2μF
VIN
LEDA
1A
SHDN
LPWR
LTM8040
ADJ
BIAS
DN445 F05
Figure 5. The LTM8040 Can PWM LED Current with
Minimal Distortion, Even at Frequencies as Low as 10Hz
The LTM8040 also features a low power shutdown state.
When the SHDN pin is active low, the input quiescent
current is less than 1μA.
Figure 6. Only 9mm × 15mm × 4.32mm, the LTM8040 LED
Driver is a Complete System in an LGA Package
Conclusion
The LTM8040 μModule LED driver makes it easy to drive
LEDs. Its high level of integration and rich feature set,
including open LED protection, analog and PWM dimming,
save significant design time and board space.
PWM
GND
DN445 F04
Figure 4. The LTM8040 Can PWM its LED String
with an External MOSFET
Data Sheet Download
www.linear.com, see also www.linear.com/micromodule
For applications help,
call (408) 432-1900, Ext. 2593
Linear Technology Corporation
dn445f LT/TP 0109 392K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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