LTC3454
1A Synchronous Buck-Boost
High Current LED Driver
FEATURES
DESCRIPTION
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The LTC®3454 is a synchronous buck-boost DC/DC
converter optimized for driving a single high power LED
at currents up to 1A from a single cell Li-Ion battery input. The regulator operates in either synchronous buck,
synchronous boost, or buck-boost mode depending on
input voltage and LED forward voltage. PLED/PIN efficiency
greater than 90% can be achieved over the entire usable
range of a Li-Ion battery (2.7V to 4.2V).
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High Efficiency: >90% Typical in Torch Mode,
>80% in Flash Mode
Wide VIN Range: 2.7V to 5.5V
Up to 1A Continuous Output Current
3.5% LED Current Programming Accuracy
Internal Soft-Start
Open/Shorted LED Protection
Constant Frequency 1MHz Operation
Zero Shutdown Current
Overtemperature Protection
Small Thermally Enhanced 10-Lead (3mm × 3mm)
DFN Package
LED current is programmable to one of four levels, including shutdown, with dual external resistors and dual enable
inputs. In shutdown no supply current is drawn.
A high constant operating frequency of 1MHz allows the
use of small external components. The LTC3454 is offered
in a low profile (0.75mm) thermally enhanced 10-lead
(3mm × 3mm) DFN package.
APPLICATIONS
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Cell Phone Camera Flash
Cell Phone Torch Lighting
Digital Cameras
PDAs
Misc Li-Ion LED Drivers
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
High Efficiency Torch/Flash LED Driver
VIN
2.7V TO 4.2V
1-CELL Li-Ion
LED Power Efficiency vs VIN
L1
5μH
10μF
VIN
SW1
SW2
10μF
VOUT
100
ILED
95
VC
D
B
C
LED
LED
EN2 EN1 ILED
0
0
0 (SHUTDOWN)
0
1
150mA
EN1 (TORCH) 1
0
850mA
1
1
1A
ISET1
ISET2
0.1μF
LTC3454
LED: LUMILEDS LXL-PWF1
L1: SUMIDA CDRH6D28-5RONC
GND (EXPOSED PAD)
RISET2
3.65k
1%
3454 TA01a
85
ILED = 1A
80
75
70
EN2 (FLASH)
1MHz
BUCK-BOOST
ILED = 150mA
90
EFFICIENCY (%)
A
RISET1
20.5k
1%
65 TA = 25°C
EFFICIENCY = (VOUT – VLED)ILED/VINIIN
60
2.7 3.1 3.5 3.9 4.3 4.7 5.1
VIN (V)
5.5
3454 TA01b
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LTC3454
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
VIN, SW1, SW2, VOUT Voltage ...................... –0.3V to 6V
VC, EN1, EN2, ISET1, ISET2
Voltage.............................–0.3V to (VIN + 0.3V) or 6V
LED Peak Current ...................................................1.25A
Storage Temperature Range...................–65°C to 125°C
Operating Temperature Range (Note 2)....–40°C to 85°C
Junction Temperature (Note 3) ............................. 125°C
TOP VIEW
10 SW1
EN1
1
EN2
2
ISET1
3
ISET2
4
7 VOUT
LED
5
6 SW2
9 VIN
11
8 VC
DD PACKAGE
10-LEAD (3mm s 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 40°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3454EDD#PBF
LTC3454EDD#TRPBF
LBQX
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V, RISET = 20.5k unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
l
Input Supply Voltage (VIN)
Input DC Supply Current
Normal Operation
Shutdown
UVLO
(Typicals at VIN = 3.6V, RISET1 = RISET2 = 20.5k)
2.7V ≤ VIN ≤ 5.5V (Note 4)
2.7V ≤ VIN ≤ 5.5V, VEN1 = VEN2 = 0V
VIN < UVLO Threshold, VEN1 = VEN2 = VIN
l
Undervoltage Lockout Threshold
VIN Rising
VIN Falling
l
TYP
2.7
1.75
VEN1, VEN2 DC Threshold for Normal Operation (VIH)
l
VEN1, VEN2 DC Threshold for Shutdown (VIL)
l
0.2
VEN1, VEN2 Input Current
l
–1
MAX
UNITS
5.5
V
825
0
5
1200
1
10
μA
μA
μA
2.05
1.90
2.3
V
V
0.68
1.2
V
1
μA
0.66
V
ISET1 and ISET2 Servo Voltage
3.08k ≤ RISET1||RISET2 ≤ 20.5k
l
780
788
800
800
812
812
mV
mV
LED Output Current to Programming Current Ratio
ILED/(IISET1 + IISET2), ILED = 500mA (Note 5)
l
3725
3775
3850
3850
3975
3925
mA/mA
mA/mA
LED Pin Voltage
ILED = 1A
Regulated Maximum VOUT
LED Pin Open, Programmed ILED = 1A
PMOS Switch RON
Switches A and D (VOUT = 3.6V)
170
mΩ
NMOS Switch RON
Switches B and C
130
mΩ
Forward Current Limit
Switch A
Reverse Current Limit
Switch D (VOUT = 3.6V)
PMOS Switch Leakage
Switches A, D
–1
1
NMOS Switch Leakage
Switches B, C
–1
1
105
l
4.95
2.5
5.15
mV
5.35
V
3.4
A
275
mA
μA
μA
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LTC3454
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V, RISET = 20.5k unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
Oscillator Frequency
MIN
TYP
MAX
UNITS
0.9
1.0
1.15
MHz
Soft-Start Time
200
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC3454 is guaranteed to meet specifications from 0°C to
70°C. Specifications over the –40°C to 85°C operating temperature range
are assured by design, characterization and correlation with statistical
process controls.
μs
Note 3: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
TJ = TA + (PD • θJA °C/W).
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5: This parameter is tested using a feedback loop which servos VC
to 1.8V.
TYPICAL PERFORMANCE CHARACTERISTICS
Undervoltage Lockout Threshold
vs Temperature
2.4
1200
2.3
1100
VIN RISING
2.0
1.9
1.8
VIN FALLING
1.7
1.6
Enable Thresholds vs VIN
1200
VIN = 3.6V
1000
900
800
700
VIH
600
VIL
500
400
1.5
300
1.4
–55 –35 –15
200
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
800
800
500
400
200
2.7
TA = 25°C
RISET1 = RISET2 = 15k
808
808
804
804
800
3.9 4.3
VIN (V)
4.7
5.1
792
792
792
788
2.7
5.5
VIN = 3.6V
TA = 25°C
800
796
3454 G04
3.5
ISET1,2 Servo Voltage vs RISET
812
796
5 25 45 65 85 105 125
TEMPERATURE (°C)
3.1
3454 G03
796
788
–55 –35 –15
VIL
600
5 25 45 65 85 105 125
TEMPERATURE (°C)
VISET1,2 (mV)
VIN = 3.6V
RISET1,2 = 15k
804
VIH
700
ISET1,2 Servo Voltage vs VIN
812
VISET1,2 (mV)
VISET1,2 (mV)
900
3454 G02
ISET1,2 Servo Voltage
vs Temperature
808
1000
300
3454 G01
812
TA = 25°C
1100
ENABLE THRESHOLDS (mV)
2.1
ENABLE THRESHOLDS (mV)
2.2
UVLO THRESHOLD (V)
Enable Thresholds
vs Temperature
788
3.1
3.5
3.9 4.3
VIN (V)
4.7
5.1
5.5
3454 G05
3
7
11
19
15
RISET (kΩ)
23
27
31
3454 G06
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LTC3454
TYPICAL PERFORMANCE CHARACTERISTICS
LED Current Programming Ratio
vs VIN
4050
VIN = 3.6V
4000
4000
3950
3950
3900
3900
RATIO
RATIO
4050
3850
3800
150
PROGRAMMED ILED = 500mA
TA = 25°C
PROGRAMMED
ILED = 1A
3850
90
PROGRAMMED
ILED = 500mA
60
3750
3650
–55 –35 –15
30
PROGRAMMED ILED = 1A
PROGRAMMED ILED = 500mA
PROGRAMMED ILED = 150mA
3700
PROGRAMMED
ILED = 100mA
3700
5 25 45 65 85 105 125
TEMPERATURE (°C)
3650
2.7
3.1
3.5
3.9 4.3
VIN (V)
4.7
3454 G07
5.1
0
–55 –35 –15
5.5
3454 G09
Maximum Regulated VOUT
vs Temperature
60
Maximum Regulated VOUT
vs VIN
PROGRAMMED ILED = 500mA
58 TA = 25°C
5.40
5.40
56
5.30
5.30
54
5.25
5.25
52
5.20
5.20
PROGRAMMED ILED = 1A
5.35 TA = 25°C
48
VOUT (V)
VOUT (V)
PROGRAMMED ILED = 1A
5.35 VIN = 3.6V
50
5.15
5.15
5.10
5.10
46
5.05
5.05
44
5.00
5.00
42
4.95
4.95
40
4.90
–55 –35 –15
4.90
2.7
2.7
3.1
3.5
3.9 4.3
VIN (V)
4.7
5.1
5.5
5 25 45 65 85 105 125
TEMPERATURE (°C)
3454 G10
VIN = 3.6V
5.35 TA = 25°C
MEASURED AT 500mA
RDS (mΩ)
210
VIN = 2.7V
VIN = 3.6V
180
150
5.00
VIN = 5.5V
5.5
MEASURED AT 500mA
90
–55 –35 –15
140
VIN = 3.6V
VIN = 5.5V
120
VIN = 4.2V
100
80
VIN = 4.2V
120
3454 G13
5.1
VIN = 2.7V
RDS (mΩ)
5.25
4.90
100 200 300 400 500 600 700 800 900 1000
PROGRAMMED ILED (mA)
4.7
160
240
4.95
4.3
VIN (V)
180
5.30
5.05
3.9
NMOS RDS(ON) vs Temperature
200
270
5.10
3.5
3454 G12
PMOS RDS(ON) vs Temperature
300
5.40
5.15
3.1
3454 G11
Maximum Regulated VOUT
vs Programmed LED Current
5.20
5 25 45 65 85 105 125
TEMPERATURE (°C)
3454 G08
VLED vs VIN
VOUT (V)
VIN = 3.6V
120
3800
3750
VLED (mV)
VLED vs Temperature
VLED (mV)
LED Current Programming Ratio
vs Temperature
60
5 25 45 65 85 105 125
TEMPERATURE (°C)
3454 G14
40
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
3454 G15
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LTC3454
TYPICAL PERFORMANCE CHARACTERISTICS
Oscillator Frequency vs Temperature
1080
100
VOUT = 3V
95
FREQUENCY (kHz)
1060
VIN = 5.5V
1040
VIN = 4.2V
1020
1000
VIN = 2.7V
980
VIN = 3.6V
960
90
EFFICIENCY (%)
1100
LED Power Efficiency vs LED Current
85
80
75
70
940
920
900
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
VIN = 3.6V
TA = 25°C
65 EFFICIENCY = (VOUT – VLED)ILED/VINIIN
FRONT PAGE APPLICATION
60
100 200 300 400 500 600 700 800 900 1000
ILED (mA)
3454 G17
3454 G16
Output Voltage Ripple
Back Page Application
Start-Up Transient
Back Page Application
CH1, VOUT
1V/DIV
CH2, ILED
500mA
FINAL VALUE
20mV/DIV
0V, 0A
CH3, VEN1
1V/DIV
0V
VIN = 3.6V
ILED = 500mA
500ns/DIV
3454 G19
VIN = 3.6V
ILED = 500mA
5ms/DIV
3454 G19
PIN FUNCTIONS
EN1 (Pin 1): Enable Input Pin for ISET1 Current.
EN2 (Pin 2): Enable Input Pin for ISET2 Current.
ISET1 (Pin 3): LED Current Programming Pin. A resistor
to ground programs the current through the LED to ILED
= 3850(0.8V/RISET1). This amount of current adds to any
amount set by EN2/ISET2 if used.
ISET2 (Pin 4): LED Current Programming Pin. A resistor
to ground programs the current through the LED to ILED
= 3850(0.8V/RISET2). This amount of current adds to any
amount set by EN1/ISET1 if used.
LED (Pin 5): Low Dropout Output for LED Current Biasing.
Connect the LED between VOUT and the LED pin.
SW2 (Pin 6): Switching Node. External inductor connects between SW1 and SW2. Recommended value is
4.7μH/5μH.
VOUT (Pin 7): Buck-Boost Output Rail. Bypass to GND with
a ceramic capacitor. Recommended value is 10μF.
VC (Pin 8): Compensation Point for the Internal Error
Amplifier Output. Connect a ceramic capacitor from VC
to GND. Recommended value is 0.1μF.
VIN (Pin 9): Voltage Input Supply Pin (2.7V ≤ VIN ≤ 5.5V).
Bypass to GND with a ceramic capacitor. Recommended
value is 10μF.
SW1 (Pin 10): Switching Node. External inductor connects between SW1 and SW2. Recommended value is
4.7μH/5μH.
Exposed Pad (Pin 11): Ground Pin. Solder to PCB ground
for electrical contact and optimal thermal performance.
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LTC3454
BLOCK DIAGRAM
OPTIONAL
10
SW1
SWITCH A
GATE
DRIVERS
AND
ANTICROSSCONDUCTION
SWITCH B
UNDERVOLTAGE
LOCKOUT
UV
7
REVERSE
CURRENT
LIMIT
OT
BANDGAP
REFERENCE
–
–
3.4A
275mA
1.23V
LOGIC
+
–
–
AB PWM
COMPARATOR
CD PWM
COMPARATOR
UV
8
VOUT
SWITCH D
SWITCH C
FORWARD
CURRENT
LIMIT
+
OVERTEMP
PROTECTION
6
SW2
+
9
VIN
+
VIN
2.7V TO 5.5V
OPTIONAL
OT
1MHz
OSCILLATOR
VC
VOUT
1.23V
–
SAFETY
ERROR AMP
AUTOZEROING
ERROR AMP
LED
–
5
377k
R
+
+
123k
SOFTSTART
CLAMP
1.23V
800mV
+
LED CURRENT
SETTING AMP 1
CURRENT
MIRROR
IISET1
–
3
∑
ISET1
I
RISET1
800mV
+
3850 I
R
LED CURRENT
SETTING AMP 2
IISET2
–
4
RISET2
ISET2
EN1
1
EN2
2
SHUTDOWN
11
EXPOSED PAD (GND)
3454 BD
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LTC3454
OPERATION
Buck-Boost DC/DC Converter
The LTC3454 employs an LTC proprietary buck-boost
DC/DC converter to generate the output voltage required to
drive a high current LED. This architecture permits highefficiency, low noise operation at input voltages above,
below or equal to the output voltage by properly phasing
four internal power switches. The error amp output voltage
on the VC pin determines the duty cycle of the switches.
Since the VC pin is a filtered signal, it provides rejection
of frequencies well below the factory trimmed switching
frequency of 1MHz. The low RDS(ON), low gate charge
synchronous switches provide high frequency pulse width
modulation control at high efficiency. Schottky diodes
across synchronous rectifier switch B and synchronous
rectifier switch D are not required, but if used do provide
a lower voltage drop during the break-before-make time
(typically 20ns), which improves peak efficiency by typically 1% to 2% at higher loads.
Figure 1 shows a simplified diagram of how the four internal
power switches are connected to the inductor, VIN, VOUT
and GND. Figure 2 shows the regions of operation of the
buck-boost as a function of the control voltage VC. The
output switches are properly phased so transitions between
regions of operation are continuous, filtered and transparent to the user. When VIN approaches VOUT, the buck-boost
region is reached where the conduction time of the four
switch region is typically 150ns. Referring to Figures 1
and 2, the various regions of operation encountered as VC
increases will now be described.
Buck Mode (VIN > VOUT)
In buck mode, switch D is always on and switch C is
always off. Referring to Figure 2, when the control
voltage VC is above voltage V1, switch A begins to turn on
VIN
VOUT
9
7
SW1
10
NMOS B
DCBUCK|Max = 100% – DC4SW
where DC4SW equals the duty cycle in % of the “four
switch” range.
DC4SW = (150ns • f) • 100%
where f is the operating frequency in Hz.
Beyond this point the “four switch” or buck-boost region
is reached.
Buck-Boost or 4-Switch Mode (VIN ≈ VOUT)
Referring to Figure 2, when the control voltage VC is above
voltage V2, switch pair AD continue to operate for duty
cycle DCBUCK|max, and the switch pair AC begins to phase
in. As switch pair AC phases in, switch pair BD phases out
accordingly. When the VC voltage reaches the edge of the
buck-boost range at voltage V3, switch pair AC completely
phases out switch pair BD and the boost region begins at
duty cycle DC4SW. The input voltage VIN where the four
switch region begins is given by:
VIN = VOUT/[1 – (150ns • f)]
and the input voltage VIN where the four switch region
ends is given by
VIN = VOUT • (1 – DC4SW) = VOUT • [1 – (150ns • f)]
75%
DMAX
BOOST
V4 (2.1V)
A ON, B OFF
BOOST REGION
PWM CD SWITCHES
DMIN
BOOST
PMOS D
PMOS A
each cycle. During the off time of switch A, synchronous
rectifier switch B turns on for the remainder of the cycle.
Switches A and B will alternate conducting similar to a
typical synchronous buck regulator. As the control voltage increases, the duty cycle of switch A increases until
the maximum duty cycle of the converter in buck mode
reaches DCBUCK|Max given by:
SW2
DMAX
BUCK
V3 (1.65V)
FOUR SWITCH PWM
BUCK/BOOST REGION
V2 (1.55V)
D ON, C OFF
PWM AB SWITCHES BUCK REGION
6
NMOS C
3454 F01
Figure 1. Simplified Diagram of Internal Power Switches
V1 (0.9V)
0%
DUTY
CYCLE
3454 F02
CONTROL
VOLTAGE, VC
Figure 2. Switch Control vs Control Voltage, VC
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LTC3454
APPLICATIONS INFORMATION
Boost Mode (VIN < VOUT)
In boost mode, switch A is always on and switch B is always
off. Referring to Figure 2, when the control voltage VC is
above voltage V3, switches C and D will alternate conducting similar to a typical synchronous boost regulator. The
maximum duty cycle of the converter is limited to 88%
typical and is reached when VC is above V4.
Forward Current Limit
If the current delivered from VIN through PMOS switch A
exceeds 3.4A (typical), switch A is shut off immediately.
Switches B and D are turned on for the remainder of the
cycle in order to safely discharge the forward inductor
current at the maximum rate possible.
Reverse Current Limit
If the current delivered from VOUT backwards through
PMOS switch D exceeds 275mA (typical), switch D is
shut off immediately. Switches A and C are turned on for
the remainder of the cycle in order to safely discharge the
reverse inductor current at the maximum rate possible.
Undervoltage Lockout
To prevent operation of the power switches at high RDS(ON),
an undervoltage lockout is incorporated on the LTC3454.
When the input supply voltage drops below approximately
1.90V, the four power switches and all control circuitry are
turned off except for the undervoltage block, which draws
a few microamperes.
Overtemperature Protection
If the junction temperature of the LTC3454 exceeds 130°C
for any reason, all four switches are shut off immediately.
The overtemperature protection circuit has a typical hysteresis of 11°C.
Soft-Start
The LTC3454 includes an internally fixed soft-start which
is active when powering up or coming out of shutdown.
The soft-start works by clamping the voltage on the VC
node and gradually releasing it such that it requires 200μs
to linearly slew from 0.9V to 2.1V. This has the effect of
limiting the rate of duty cycle change as VC transitions
from the buck region through the buck-boost region into
the boost region. Once the soft-start times out, it can only
be reset by entering shutdown, or by an undervoltage or
overtemperature condition.
Autozero Error Amp
The error amplifier is an autozeroing transconductance
amp with source and sink capability. The output of this
amplifier drives a capacitor to GND at the VC pin. This
capacitor sets the dominant pole for the regulation loop.
(See the Applications Information section for selecting
the capacitor value). The feedback signal to the error
amp is developed across a resistor through which LED
current flows.
Safety Error Amp
The safety error amplifier is a transconductance amplifier
with sink only capability. In normal operation, it has no
effect on the loop regulation. However, if the LED pin opencircuits, the output voltage will keep rising, and the safety
error amp will eventually take over control of the regulation
loop to prevent VOUT runaway. The VOUT threshold at which
this occurs is approximately 5.15V.
LED Current Programming and Enable Circuit
Two enable pins work in conjunction with dual external
resistors to program LED current to one of three nonzero
settings. The table below explains how the current can
be set.
EN1
EN2
ILOAD (A)
GND
GND
0 (SHUTDOWN)
VIN
GND
3850 • 0.8V/RISET1
GND
VIN
3850 • 0.8V/RISET2
VIN
VIN
3850 • (0.8V/RISET1 + 0.8V/RISET2)
With either enable pin pulled high, the buck-boost will
regulate the output voltage at the current programmed
by RISET1 and/or RISET2.
With both enable pins pulled to GND, the LTC3454 is in
shutdown and draws zero current. The enable pins are
high impedance inputs and should not be floated.
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LTC3454
APPLICATIONS INFORMATION
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3454 allows the
use of small surface mount inductors. The inductor current ripple is typically set to 20% to 40% of the maximum
average inductor current. For a given ripple the inductance
term in boost mode is:
L>
VIN(MIN)2 •
( VOUT – VIN(MIN) ) • 100%
f • IOUT(MAX ) • %Ripple • VOUT 2
and in buck mode is:
( VIN(MAX) – VOUT ) • VOUT • 100%
L>
f • VIN(MAX ) • %Ripple • IOUT
where f = operating frequency, Hz
%Ripple = allowable inductor current ripple, %
Input Capacitor Selection
Since the VIN pin is the supply voltage for the IC it is recommended to place at least a 2.2μF, low ESR bypass capacitor
to ground. See Table 2 for a list of component suppliers.
Table 2. Capacitor Vendor Information
SUPPLIER
WEB SITE
AVX
www.avxcorp.com
Sanyo
www.sanyovideo.com
Taiyo Yuden
www.t-yuden.com
TDK
www.component.tdk.com
Output Capacitor Selection
The bulk value of the capacitor is set to reduce the ripple
due to charge into the capacitor each cycle. The steadystate ripple due to charge is given by:
%Ripple _ Boost =
VIN(MIN) = minimum input voltage, V
VIN(MAX) = maximum input voltage, V
VOUT = output voltage, V
IOUT(MAX) = maximum output load current
For high efficiency, choose an inductor with a high frequency core material, such as ferrite, to reduce core loses.
The inductor should have low ESR (equivalent series
resistance) to reduce the I2R losses, and must be able
to handle the peak inductor current without saturating.
Molded chokes or chip inductors usually do not have
enough core to support peak inductor currents >1A. To
minimize radiated noise, use a toroid, pot core or shielded
bobbin inductor. For white LED application, a 4.7μH/5μH
inductor value is recommended. See Table 1 for a list of
component suppliers.
Table 1. Inductor Vendor Information
SUPPLIER
WEB SITE
Coilcraft
www.coilcraft.com
%Ripple _ Buck =
(
)
IOUT(MAX ) • VOUT – VIN(MIN) • 100%
%
COUT • VOUT 2 • f
( VIN(MAX) – VOUT ) • 100%
8 • VIN(MAX ) • f 2 • L • COUT
where COUT = output filter capacitor, F
The output capacitance is usually many times larger in
order to handle the transient response of the converter. For
a rule of thumb, the ratio of operating frequency to unitygain bandwidth of the converter is the amount the output
capacitance will have to increase from the above calculations in order to maintain desired transient response.
The other component of ripple is due to ESR (equivalent
series resistance) of the output capacitor. Low ESR capacitors should be used to minimize output voltage ripple.
For surface mount applications, Taiyo Yuden, TDK, AVX
ceramic capacitors, AVX TPS series tantalum capacitors
or Sanyo POSCAP are recommended. For the white LED
application, a 10μF capacitor value is recommended. See
Table 2 for a list of component suppliers.
Cooper/Coiltronics
www.cooperet.com
Optional Schottky Diodes
Murata
www.murata.com
Sumida
www.japanlink.com/sumida
Toko
www.toko.com
Vishay-Dale
www.vishay.com
Schottky diodes across the synchronous switches B and
D are not required, but provide a lower drop during the
break-before-make time (typically 20ns) of the NMOS to
PMOS transition, improving efficiency. Use a Schottky
3454fa
9
LTC3454
TYPICAL APPLICATIONS
diode such as an MBRM120T3 or equivalent. Do not use
ordinary rectifier diodes, since the slow recovery times
will compromise efficiency.
and slower transient response. To ensure proper phase
margin, the loop is required to be crossed over a decade
before the LC double pole.
In applications in which VIN is greater than 4V and VOUT to
GND short-circuit protection is needed, a Schottky diode
such as MBRM120T3 or equivalent may be used from
GND to SW1 and/or a 2Ω/1nF series snubber from SW1
to GND. The Schottky diode should be added as close
to the pins as possible. Neither of these is required for
shorted LED protection.
The unity-gain frequency of the error amplifier with the
Type I compensation is given by:
gm
fUG =
2 • π • CVC
In applications in which VIN is greater than 4.5V, a Schottky
diode such as MBRM120T3 or equivalent may be required
from SW1 to VIN if the LTC3454 is enabled with an output
voltage already present. The Schottky diode should be
added as close to the pins as possible.
Closing the Feedback Loop
The LTC3454 incorporates voltage mode PWM control.
The control to output gain varies with operation region
(buck, boost, buck/boost), but is usually no greater than
15. The output filter exhibits a double pole response
given by:
1
fFILTER _ POLE =
Hz
2 • π • L • COUT
where COUT is the output filter capacitor.
The output filter zero is given by:
fFILTER _ ZERO =
1
2 • π • RESR • COUT
Hz
where RESR is the capacitor equivalent series resistance.
A troublesome feature in boost mode is the right-half plane
zero (RHP), and is given by:
2
fRHPZ
VIN
=
Hz
2 • π • IOUT • L • VOUT
The loop gain is typically rolled off before the RHP zero
frequency.
A simple Type I compensation network can be incorporated
to stabilize the loop but at a cost of reduced bandwidth
where gm is the error amp transconductance (typically
1/5.2k) and CVC is the external capacitor to GND at the
VC pin. For the white LED application, a 0.1μF or greater
capacitor value is recommended.
Maximum LED Current
As described in the Operation section, the output LED
current with both enable pins logic high is equal to
ILED = 3850 [0.8V/(RISET1 || RISET2)]
Since the maximum continuous output current is limited to
1A, this sets a minimum limit on the parallel combination
of RISET1 and RISET2 equal to
RMIN = (RISET1 || RISET2)|MIN = 3850(0.8V/1A)
= 3080Ω
Although the LTC3454 can safely provide this current
continuously, the external LED may not be rated for this
high a level of continuous current. Higher current levels
are generally reserved for pulsed applications, such as
LED camera flash. This is accomplished by programming
a high current with one of the RISET resistors and pulsing
the appropriate enable pin.
Varying LED Brightness
Continuously variable LED brightness control can be
achieved by interfacing directly to one or both of the ISET
pins. Figure 3 shows four such methods employing a
voltage DAC, a current DAC, a simple potentiometer or a
PWM input. It is not recommended to control brightness
by PWMing the enable pins directly as this will toggle
the LTC3454 in and out of shutdown and result in erratic
operation.
3454fa
10
LTC3454
APPLICATIONS INFORMATION
LED Failure Modes
If the LED fails as an open circuit, the safety amplifier takes
control of the regulation loop to prevent VOUT runaway.
The VOUT threshold at which this occurs is about 5.15V.
The safety amplifier has no effect on loop regulation at
VOUT less than 5.15V.
VIN
If the LED fails as a short-circuit, the current limiting
circuitry detects this condition and limits the peak input
current to a safe level.
VOUT
VIN
VOUT
ENx
ENx
LTC3454
ISETx
LTC3454
ISETx
LED
LED
ILED = 3850
RSET ≥ RMIN
VOLTAGE
DAC
0.8V – VDAC
RSET
IDAC ≤
CURRENT
DAC
VDAC
ILED = 3850 • IDAC
0.8V
RMIN
(3a)
(3b)
VIN
VOUT
VIN
VOUT
ENx
ENx
LTC3454
ISETx
LTC3454
ISETx
LED
LED
RMIN
ILED = 3850
0.8V
RMIN + RPOT
RSET
100
ILED = 3850
RSET ≥ RMIN
VPWM
RPOT
= 3850
0.8V – VPWM
RSET
0.8V – (DC% • VDVCC)
RSET
DVCC
fPWM ≥ 10kHz
(3c)
(3d)
3454 F03
Figure 3. Brightness Control Methods: (a) Using Voltage DAC, (b) Using Current DAC, (c) Using Potentiometer, (d) Using PWM Input
PACKAGE DESCRIPTION
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699)
R = 0.115
TYP
6
0.38 p 0.10
10
0.675 p0.05
3.00 p0.10
(4 SIDES)
3.50 p0.05
1.65 p0.05
2.15 p0.05 (2 SIDES)
PACKAGE
OUTLINE
1.65 p 0.10
(2 SIDES)
PIN 1
TOP MARK
(SEE NOTE 6)
(DD) DFN 1103
5
0.200 REF
0.25 p 0.05
0.50
BSC
2.38 p0.05
(2 SIDES)
1
0.25 p 0.05
0.50 BSC
0.75 p0.05
2.38 p0.10
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
3454fa
Information furnished 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.
11
LTC3454
TYPICAL APPLICATION
500mA LED Flashlight Driver
L1
4.7μH
3-CELL
ALKALINE
4.5V
VIN
2.2μF
LED Power Efficiency vs VIN
SW2
SW1
VOUT
4.7μF
100
ILED = 500mA
95
EN1
SWD
SWB
LED
90
EFFICIENCY (%)
SWA
LED
SWC
EN2
VC
80
75
70
1MHz
BUCK-BOOST
ISET1
ISET2
0.1μF
LTC3454
GND (EXPOSED PAD)
LED: LUMILEDS, LXCL LW3C
L1: TOKO A997AS-4R7M
85
RISET1
6.19k
1%
3453 TA02
65
ILED = 500mA
TA = 25°C
EFFICIENCY = (VOUT – VLED)ILED/VINIIN
60
2.7
3.1
3.5
3.9 4.3
VIN (V)
4.7
5.5
5.1
3454 TA02b
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DESCRIPTION
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3A, Full Featured DC/DC Converter with Soft-Start and
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VIN: 1.6V to 18V, VOUT(MAX) = 34V, IQ = 1.8mA, ISD < 1μA,
MS10 Package/EDD Package
VIN: 2.6V to 16V, VOUT(MAX) = 34V, IQ = 4.2mA/5.5mA, ISD < 1μA,
ThinSOT Package
VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD < 1μA,
ThinSOT Package
VIN: 2.5V to 10V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1μA,
ThinSOT Package/SC70 Package
VIN: 2.8V to 4.5V, VOUT(MAX) = 6V, IQ = 50μA, ISD < 1μA,
QFN-24 Package
VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300μA, ISD < 2.5μA,
DFN Package
VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300μA, ISD < 2.5μA,
DFN Package
VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 25μA/50μA, ISD
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