DN325 - High Efficiency ThinSOT White LED Driver Features Internal Switch and Schottky Diode

High Efficiency ThinSOT White LED Driver Features Internal
Switch and Schottky Diode – Design Note 325
David Kim
Introduction
The LT®3465 white LED driver is ideal for backlight
circuits in small, battery-powered portable devices—
such as cellular phones, PDAs and digital cameras. The
LT3465 includes important features such as automatic
soft-start to prevent large inrush current, open LED
protection and an integrated Schottky diode in a low
profile (<1mm) ThinSOT™ package to save space. The
LT3465 is optimized for color display backlight applications with two to four white LEDs and a Li-Ion battery
input. Even so, its internal 30V switch is capable of
driving up to six LEDs in series.
The LT3465 uses a constant-current, step-up architecture which directly regulates the LED current and guarantees a consistent light intensity and color in each LED,
regardless of the differences in their forward voltage
drops. The constant 1.2MHz switching frequency allows
the use of tiny external components and minimizes both
input and output ripple for applications requiring low
input and output noise. The internal compensation of
the LT3465 reduces output capacitor requirements to
a single 0.22μF ceramic capacitor—a significant space
and cost savings over compensation schemes that have
more strict output capacitance requirements.
The 200mV feedback voltage, high efficiency internal
power switch and internal Schottky diode minimize
power losses in the LT3465, resulting a typical efficiency
of 80%. The LT3465 also comes in a 2.7MHz switching
frequency version (LT3465A), allowing the use of even
smaller components, such as chip inductors.
Li-Ion-Powered Driver for Four White LEDs
The compact white LED driver circuit shown in Figure 1
is designed to fit into small wireless devices such as
cellular phones or PDAs. The efficiency of this circuit
is higher than that of switched-capacitor based drivers
in a parallel architecture due to the constant current
step-up series LED architecture.
This design supplies 15mA of constant current, driving
four LEDs in series from a Li-Ion battery or a 5V adapter
input. The integrated Schottky diode, internal softstart and open LED protection simplify the circuit and
improve performance. The 1.2MHz constant frequency
and integrated optimized compensation allow the use
of small components, including tiny 0603-size ceramic
input and output capacitors and a tiny inductor (an even
smaller chip inductor can be used with the LT3465A).
The LED current is programmed with resistor R1 at the
feedback pin by the simple formula:
ILED = 200mV/R1 or R1 = 200mV/ILED (see Table 1).
Precision resistors (1%) are recommended for applications that require highly accurate LED current.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
82
L1
22μH
3V TO 5V
SW
SHUTDOWN
AND DIMMING
CONTROL
C1
1μF
VOUT
VIN
C2
0.22μF
LT3465
CTRL
FB
R1
10Ω
GND
EFFICIENCY (%)
80
78
76
74
72
C1, C2: X5R OR X7R DIELECTRIC
L1: MURATA LQH32CN220
DN325 F01a
70
0
10
5
15
20
LED CURRENT (mA)
DN325 F01b
Figure 1. Li-Ion Powered Driver for Four White LEDs
11/03/325_conv
Table 1. R1 Resistor Value Selection
FULL ILED (mA)
R1 (Ω)
5
40.2
10
20.0
15
13.3
20
10.0
Dimming Control
The LT3465 features single pin shutdown and dimming
control. By applying a DC voltage (Figure 2a) of 0.2V
to ~1.5V at the CTRL pin (Pin 4), the feedback voltage
changes from 25mV to 200mV. For a 20mA LED current
application (RFB = 10Ω), changing the CTRL pin voltage from 0.2V ~ 1.5V yields LED current from 2.5mA
to 20mA. A CTRL voltage below 50mV will shut down
the device. The curve in Figure 3 shows the correlation
between VFB vs VCTRL.
Three dimming methods are shown in Figure 2 via a DC
voltage, a filtered PWM signal and a logic signal. The first
two methods adjust LED brightness by varying voltage
at the CTRL pin and the logic signal method adjusts the
brightness by changing the feedback voltage directly.
when a logic signal turns on the MOSFET. The value
of R1 sets the minimum LED current and the parallel
combination of RINC and R1 sets the higher LED current.
This simple circuit allows for a single dimming step.
For more than two LED current levels, add additional
MOSFET-RINC circuits in parallel. Keep in mind that a
separate logic signal is needed for each current setting and that the LED current calculation involves the
parallel resistances of each RINC(n). With n switches
on, ILED(n) = 200mV [R1||RINC(1)...||RINC(n)]. The CTRL
pin can still be used as different resistors are switched
in to provide greater control of LED current.
Conclusion
The LT3465 is a white LED driver optimized for driving two to four LEDs from a Li-Ion input. It features a
36V, 1.2MHz internal power switch, internal Schottky
diode, automatic soft-start, open LED protection and
optimized internal compensation. The LT3465 is ideal
for wireless devices requiring small circuit size, high
efficiency and matching LED brightness.
250
FEEDBACK VOLTAGE (mV)
The filtered PWM dimming shown in Figure 2b works
similar to DC voltage dimming except the VDC input
comes from a filtered PWM signal. The filter is the
5kΩ, 100nF RC circuit which filters the PWM signal
to a DC voltage proportional to the duty cycle of the
PWM signal. In this case, the LED current increases
proportionally with the duty cycle of the PWM signal. A
100% duty cycle corresponds to full LED current, and
a 0% duty cycle corresponds to zero LED current. The
frequency recommended for filtered PWM dimming is
3kHz with a magnitude of 2V.
TA = 25°C
200
150
100
50
0
0
1
0.5
1.5
CONTROL VOLTAGE (V)
2
DN325 F03
The logic signal dimming method (Figure 2c) uses an
N-channel MOSFET and RINC to reduce the value of R1
Figure 3. VFB vs VCTRL Correlation Curve
LT3465
FB
LT3465
R1
5k
PWM
CTRL
VDC
LT3465
C1
100nF
CTRL
DN325 F02b
RINC
LOGIC
SIGNAL
2N7002
Figure 2a. DC Voltage Dimming
R1
DN325 F02c
DN325 F02a
Figure 2b. Filtered PWM Dimming
Data Sheet Download
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Linear Technology Corporation
Figure 2c. Logic Signal Dimming
For applications help,
call (408) 432-1900
dn325f_conv LT/TP 1109 351.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2003
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