Dec 2005 High Efficiency, Monolithic Synchronous Buck-Boost LED Driver Drives up to 1A Continuous Current

DESIGN FEATURES
High Efficiency, Monolithic
Synchronous Buck-Boost LED Driver
Drives up to 1A Continuous Current
by Aspiyan Gazder
Introduction
The LTC3454 is a synchronous buckboost DC/DC converter, designed for
driving a single high power LED with
regulated currents up to 1A from a
single Li-Ion battery. Switching converters are typically used to regulate
a voltage, but LEDs require constant
current to generate predictable light
output. The LTC3454 uses an autozero
transconductance error amplifier in its
regulation loop to accurately control
LED current. The LED current can
be set to one of four levels, including
shutdown, using two external resistors
and dual enable pins. In shutdown no
current is drawn.
The wide VIN range of a Lithium-Ion
battery (2.7V to 4.2V) requires that a
converter be able to both step-up and
step-down the input voltage when
the LED forward voltage is within the
range of the battery discharge profile.
The LTC3454 LED driver efficiently
performs step-up and step-down
conversion via four internal switches.
The regulator operates in synchronous
buck, synchronous boost or buckboost mode, depending on VIN and
the LED forward voltage. Transitions
between modes are automatic and
smooth.
The LTC3454 operates at a high
fixed frequency of 1MHz, which re-
L2
SW1
VIN
2.7V TO 5.5V
C6
R9
10µF 20.5k 1%
C7
10µF
D1
LED
EN2
ISET2
VC
C8
D1: LXCL-LW3C
L2: CDRH6D28-5R0NC
EN1 EN2
0
1
0
1
0
0
1
1
ILED
0 (SHUTDOWN)
150mA
850mA
1000mA
Figure 1. LTC3454 used in a typical application
duces inductor size and eases output
filtering.
Application
Figure 1 shows the LTC3454 driving
a high power LED in torch and flash
modes. Only six external components
are required in this application. Efficiency, PLED/PIN, greater than 90% is
possible over the entire usable range
of a Li-Ion battery (see Figure 2).
The LTC3454 has two enable pins
that control two current setting amplifiers. A resistor connected from an
ISET pin to GND programs the LED
current to:
ILED = 3850 •
0.8
,
RISET
when the current setting amplifier
is enabled via its EN pin. When both
enable pins are asserted, the net LED
AI = 3850
+
INTERNAL CURRENT
SETTING
AMPLIFIER 1 I
SET1
I
ILED = 3850 • I
Σ
ISET2
–
ILED = 150mA
RISET1
90
EFFICIENCY (%)
ISET1
LTC3454
EXPOSED PAD
100
ISET1
85
80
VOUT
EN1
R8
3.65k 1%
800mV
95
SW2
VIN
ILED = 1mA
800mV
+
INTERNAL CURRENT
SETTING
AMPLIFIER 2
75
70
–
65 T = 25°C
A
EFFICIENCY = (VOUT – VLED)ILED/VIN • IIN
60
3.5
2.7 3.1
3.9 4.3 4.7 5.1
RISET2
5.5
VIN (V)
Figure 2. Efficiency for circuit of Figure 1
Linear Technology Magazine • December 2005
ISET2
Figure 3. Two current setting amplifiers give the user the
flexibility to choose more than one non-zero current level.
23
DESIGN FEATURES
VIN
COUT
CIN
ILED
AUTO ZERO gm
ISET
R
–
PWM AND GATE
MULTIPLEXING
LOGIC
+
–
R
C/D PAIR PWM
COMPARATOR
+
VC
CVC
+
–
A/B PAIR PWM
COMPARATOR
Figure 4. An auto-zeroing transconductance amplifier maintains loop regulation.
current is the sum of each individually
programmed current. Figure 3 shows
schematically how the LED current is
programmed.
Autozeroing
Transconductance-AmplifierBased Current Regulation
The LTC3454 employs an auto-zeroing transconductance amplifier in its
regulation loop, as shown in Figure 4.
The autozero amplifier topology nullifies any offset at its input, allowing an
accurate LED current to be achieved
with very low common mode input
voltage levels, resulting in high PLED/
PIN efficiency. The regulation voltage
present at the LED pin can be as low
as 100mV at 100mA of LED current.
Synchronous
Buck-Boost DC/DC Converter
The LTC3454 can drive an LED at up
to 1A continuous current. LEDs that
can be driven with such high current
typically have forward voltage drops
of 3.3V – 3.6V. When powered from a
single Li-Ion battery (2.7V to 4.2V),
as in the case of handheld battery
powered applications, neither a pure
buck nor a pure boost solution can
efficiently regulate the LED current.
A pure buck would dropout at lower
battery voltages, causing a lower than
programmed LED current to flow. At
high battery voltages, a pure boost
converter would regulate a higher
output voltage than necessary, result24
ing in low efficiency. The buck-boost
converter can efficiently regulate LED
current over the entire Li-Ion battery
range.
The autozero amplifier
topology nullifies any offset
at its input, allowing an
accurate LED current to
be achieved with very low
common mode input voltage
levels, resulting in high
PLED/PIN efficiency.
The control voltage, VC, determines
the region of operation of the buckboost converter. The gate drives of
the internal power switches A, B,
C and D are controlled by the logic
block (Figure 4). A patented gate drive
multiplexing scheme enables smooth
SW1
2V/DIV
transition between buck and boost
modes and through the four-switch
region. In buck mode, the duty
cycles on gate drives of switches A
and B are controlled while switch D
is turned on continuously. In boost
mode, duty cycles of switches C and
D are controlled, while switch A is on
continuously.
Using synchronous rectifier switches B and D instead of catch diodes
helps improve efficiency. This scheme
requires that the synchronous rectifier
switch and the main switch are not
turned on simultaneously. A cross
conduction delay prevents this condition from occurring. The LTC3454
has a break before make time of approximately 30ns. During this time
the current conduction path is completed through the body diodes of the
switches. In the case of forward current
flow from the SW1 pin to the SW2 pin
through the inductor, the body diode
of NMOS switch B conducts in buck
mode. The SW1 node is pulled a diode
drop below ground. Likewise, in boost
mode, the body diode of PMOS switch
D conducts during the switch C and
switch D switching, but node SW2 now
flys above VOUT by a diode drop. Body
diodes of the main switches A and C
conduct during reverse current flow.
Figure 5 shows the switch waveforms
in the buck-boost mode.
The LTC3454 has both forward and
reverse current limiting—requiring no
external sense resistors. If the peak
input current exceeds approximately
3.4A, forward current limit is tripped
and switches B and D are turned on
for the rest of the cycle. The reverse
current limit is tripped when current
flowing from switch D through the
inductor to the SW1 node exceeds
approximately 250mA and switches
A and C are turned on for the rest of
the cycle.
Robust Design:
Can Tolerate Open and
Shorted LED Conditions
0V
SW2
2V/DIV
0V
VIN = 3.6V
VOUT = 3.5V
Figure 5. Switching waveforms
in buck-boost mode
If the LED faults as an open circuit,
the regulation loop drives VC higher,
which has the effect of raising the
output voltage. A safety amplifier—a
continued on page 46
Linear Technology Magazine • December 2005
NEW DEVICE CAMEOS
Performance of 50µA CMOS
Amplifier Rivals Best Bipolar
Op Amps with 0.7µV/°C Drift
The LTC6078/LTC6079 are dual/
quad, low offset, low noise operational
amplifiers with low power consumption and rail-to-rail input/output
swing.
Input offset voltage is trimmed to
less than 25µV and the CMOS inputs
draw less than 50pA of bias current.
The low offset drift, excellent CMRR,
and high voltage gain make it a good
choice for precision signal conditioning.
Each amp draws only 54µA current on a 3V supply. The micropower,
rail-to-rail operation of the LTC6078/
LTC6079 is well suited for portable
instruments and single supply applications.
LTC2950/51, continued from page 4
ure 5 shows an actual ESD event. Note
the arc onto the PB pin. The ESD strike
fed directly onto the pin; there were no
series resistors or parallel capacitors.
This strike did not damage the pin, nor
did it generate any leakage.
LTC2950-1 and
LTC2950-2 Versions
The LTC2950-1 (high true EN) and
LTC2950-2 (low true EN) differ only
by the polarity of the EN/EN pin.
Both versions allow the user to extend
the amount of time that the PB must
be held low in order to begin a valid
power on/off sequence. An external
capacitor placed on the ONT pin adds
additional time to the turn-on time.
An external capacitor placed on the
LTC3454, continued from page 24
transconductance amplifier with sink
only capability—takes control of the
regulation loop and prevents VOUT
runaway. The VOUT threshold at which
this happens is approximately 5V.
If the LED faults as a short circuit, the regulation loop continues
to regulate the output current to its
programmed current level.
46
The LTC6078/LTC6079 are specified on power supply voltages of 3V
and 5V from –40°C to 125°C. The
dual amplifier LTC6078 is available in
8-lead MSOP and 10-lead DFN packages. The quad amplifier LTC6079 is
available in 16-lead SSOP and DFN
packages.
Dual and Quad,
1.8V, 13µA Precision
Rail-to-Rail Op Amps
The LT6001 and LT6002 are dual
and quad precision rail-to-rail input
and output operational amplifiers.
Designed to maximize battery life in
always-on applications, the devices
operate on supplies down to 1.8V while
drawing only 13µA quiescient current.
The low supply current and low voltage
operation is combined with precision
OFFT pin adds additional time to
the turn-off time. If no capacitor is
placed on the ONT (OFFT) pin, then
the turn on (off) duration is given by
an internally fixed 32ms timer. The
LTC2950 fixes the KILL turn off delay
time (tKILL(OFF DELAY)) at 1024ms (the
amount of time from interrupting the
µP to turning off power).
LTC2951-1 and
LTC2951-2 Versions
The LTC2951 fixes the turn on debounce time at 128ms. The turn off
debounce time is the same as the
LTC2950: 32ms internal plus the
optional additional external when a
capacitor is placed on the OFFT pin.
The KILLT pin in the LTC2951-1 and
LTC2951-2 provides extendable KILL
specifications—for instance, input
offset is guaranteed less than 500µV.
The performance on 1.8V supplies is
fully specified and guaranteed over
temperature. A shutdown feature in
the 10-lead dual version can be used
to extend battery life by allowing the
amplifiers to be switched off during
periods of inactivity.
The LT6001 is available in the 8-lead
MSOP package and a 10-lead version
with the shutdown feature in a tiny,
dual fine pitch leadless package (DFN).
The quad LT6002 is available in a 16lead SSOP package and a 16-lead DFN
package. These devices are specified
over the commercial and industrial
temperature range.
Authors can be contacted
at (408) 432-1900
turn off timer, tKILL(OFF DELAY, ADDITIONAL),
by connecting an optional external
capacitor on the KILLT pin. The default power down delay time is 128ms,
tKILL(OFF DELAY).
Conclusion
The LTC2950/LTC2951 is a family of
micro-power (6µA), wide input voltage
range (2.7V to 26.4V) push button controllers. The parts lower system cost
and preserve battery life by integrating
flexible push button timing, a high
voltage LDO, and a simple µP interface
that provides intelligent power up and
power down. The device is available in
space saving 8-lead 3mm × 2mm DFN
and ThinSOT™ packages.
Conclusion
The LTC3454 adds to Linear Technology’s family of LED drivers. High
efficiencies can be achieved over the
entire Li-Ion range with a minimal
number of external components. Additionally, it draws zero current when
in shutdown, helping conserve battery
life in hand held battery powered applications. The LTC3454 is available
in a low profile small footprint 3mm
× 3mm DFN package.
for
the latest information
on LTC products,
visit
www.linear.com
Information furnished herein by Linear Technology Corporation
is believed to be accurate and reliable. However, no responsibility
is assumed for its use. Linear Technology Corporation makes
no representation that the interconnection of its circuits, as
described herein, will not infringe on existing patent rights.
Linear Technology Magazine • December 2005
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