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LTC3454 1A Synchronous Buck-Boost High Current LED Driver FEATURES

DESCRIPTIO
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 x 3mm) DFN Package
The LTC(R)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). 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 x 3mm) DFN package.
, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
APPLICATIO S

Cell Phone Camera Flash Cell Phone Torch Lighting Digital Cameras PDAs Misc Li-Ion LED Drivers
TYPICAL APPLICATIO
VIN 1- CELL Li-Ion 2.7V-4.2V L1 5mH 10mF VIN A SW1
High Efficiency Torch/Flash LED Driver
SW2 D
VOUT
10mF
ILED
LED LED EN2 EN1 I LED 0 0 0 (SHUTDOWN) 0 1 150mA EN1 (TORCH) 1 0 850mA 1 1 1A EN2 (FLASH) RISET1 20.5k 1%
3453 TA01a
EFFICIENCY (%)
B
C
VC 0.1mF
1MHz BUCK-BOOST LTC3454
ISET1 ISET2
LED: LUMILEDS LXL-PWF1 L1: SUMIDA CDRH6D28-5RONC
GND (EXPOSED PAD)
RISET2 3.65k 1%
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LED Power Efficiency vs VIN
100 95 90 85 80 75 70 65 TA = 25C EFFICIENCY = (VOUT - VLED)ILED/VINIIN 60 2.7 3.1 3.5 3.9 4.3 4.7 5.1 VIN (V) ILED = 1A ILED = 150mA 5.5
3454 TA01b
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LTC3454 ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW EN1 EN2 ISET1 ISET2 LED 1 2 3 4 5 11 10 SW1 9 VIN 8 VC 7 VOUT 6 SW2
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...................-65C to 125C Operating Temperature Range (Note 2) ...-40C to 85C Junction Temperature (Note 3) ............................. 125C
DD PACKAGE 10-LEAD (3mm x 3mm) PLASTIC DFN
TJMAX = 125C, JA = 40C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER PART NUMBER LTC3454EDD
DD PART MARKING LBQX
Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges.
the full The ELECTRICAL CHARACTERISTICS25C, V denotes R specifications which apply over the(Noteoperating = 3.6V, = 20.5k unless otherwise noted. 2) temperature range, otherwise specifications are at T =
A IN ISET
PARAMETER Input Supply Voltage (VIN) Input DC Supply Current Normal Operation Shutdown UVLO Undervoltage Lockout Threshold VEN1, VEN2 DC Threshold for Normal Operation (VIH) VEN1, VEN2 DC Threshold for Shutdown (VIL) VEN1, VEN2 Input Current ISET1 and ISET2 Servo Voltage LED Output Current to Programming Current Ratio LED Pin Voltage Regulated Maximum VOUT PMOS Switch RON NMOS Switch RON Forward Current Limit Reverse Current Limit PMOS Switch Leakage NMOS Switch Leakage Oscillator Frequency Soft-Start Time
CONDITIONS
MIN 2.7
TYP
MAX 5.5 1200 1 10 2.3 1.2 1 812 812 3975 3925 5.35
UNITS V A A A V V V V A mV mV mA/mA mA/mA mV V m m A mA A A MHz s
(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 VIN Rising VIN Falling
1.75

3.08k RISET1||RISET2 20.5k ILED/(IISET1 + IISET2), ILED = 500mA (Note 5) ILED = 1A LED Pin Open, Programmed ILED = 1A Switches A and D (VOUT = 3.6V) Switches B and C Switch A Switch D (VOUT = 3.6V) Switches A, D Switches B, C
0.2 -1 780 788 3725 3775 4.95
825 0 5 2.05 1.90 0.68 0.66 800 800 3850 3850 105 5.15 170 130 3.4 275
2.5 -1 -1 0.9
1.0 200
1 1 1.15
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC3454 is guaranteed to meet specifications from 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls.
Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD * JA 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. 3454f
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LTC3454 TYPICAL PERFOR A CE CHARACTERISTICS
Undervoltage Lockout Threshold vs Temperature
2.4 2.3 ENABLE THRESHOLDS (mV) 2.2 UVLO THRESHOLD (V) 2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.4 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
3454 G01
VIN RISING
900 800 700 600 500 400 300 200 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
3454 G02
ENABLE THRESHOLDS (mV)
VIN FALLING
ISET1,2 Servo Voltage vs Temperature
812 808 804 VISET1,2 (mV) 800 796 792 788 -55 -35 -15 VIN = 3.6V RISET1,2 = 15k 812 808 804 800 796 792
VISET1,2 (mV)
VISET1,2 (mV)
5 25 45 65 85 105 125 TEMPERATURE (C)
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LED Current Programming Ratio vs Temperature
4050 4000 3950 3900 RATIO 3850 3800 3750 3700 3650 -55 -35 -15 PROGRAMMED ILED = 1A PROGRAMMED ILED = 500mA PROGRAMMED ILED = 150mA 5 25 45 65 85 105 125 TEMPERATURE (C)
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VIN = 3.6V
VLED (mV)
RATIO
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Enable Thresholds vs Temperature
1200 1100 1000 VIN = 3.6V 1200 1100 1000 900 800 700 600 500 400 300 200
Enable Thresholds vs VIN
TA = 25C
VIH VIL
VIH VIL
2.7
3.1
3.5
3.9 4.3 VIN (V)
4.7
5.1
5.5
3454 G03
ISET1,2 Servo Voltage vs VIN
TA = 25C RISET1 = RISET2 = 15k 812 808 804 800 796 792 788
ISET1,2 Servo Voltage vs RISET
VIN = 3.6V TA = 25C
788 2.7
3.1
3.5
3.9 4.3 VIN (V)
4.7
5.1
5.5
3
7
11
19 15 RISET (k)
23
27
31
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LED Current Programming Ratio vs VIN
4050 4000 3950 3900 3850 3800 3750 30 3700 3650 2.7 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5 90 PROGRAMMED ILED = 500mA TA = 25C 150
VLED vs Temperature
VIN = 3.6V PROGRAMMED ILED = 1A
120
60
PROGRAMMED ILED = 500mA
PROGRAMMED ILED = 100mA 5 25 45 65 85 105 125 TEMPERATURE (C)
3454 G09
0 -55 -35 -15
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LTC3454 TYPICAL PERFOR A CE CHARACTERISTICS
VLED vs VIN
60 PROGRAMMED ILED = 500mA 58 TA = 25C 56 54 VLED (mV) VOUT (V) 52 50 48 46 44 42 40 2.7 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5 5.40
VOUT (V)
Maximum Regulated VOUT vs Programmed LED Current
5.40 300 270 240 RDS (m) 210 180 150 120 VIN = 3.6V 5.35 TA = 25C 5.30 5.25 VOUT (V) 5.20 5.15 5.10 5.05 5.00 4.95 4.90 100 200 300 400 500 600 700 800 900 1000 PROGRAMMED ILED (mA)
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RDS (m)
Oscillator Frequency vs Temperature
1100 1080 1060 FREQUENCY (kHz) 1040 1020 1000 980 960 940 920 900 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
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VOUT = 3V
EFFICIENCY (%)
4
UW
VIN = 4.2V VIN = 3.6V
Maximum Regulated VOUT vs Temperature
PROGRAMMED ILED = 1A 5.35 VIN = 3.6V 5.30 5.25 5.20 5.15 5.10 5.05 5.00 4.95 4.90 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
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Maximum Regulated VOUT vs VIN
5.40 PROGRAMMED ILED = 1A 5.35 TA = 25C 5.30 5.25 5.20 5.15 5.10 5.05 5.00 4.95 4.90 2.7 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5
3454 G10
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PMOS RDS(ON) vs Temperature
MEASURED AT 500mA 200 180
NMOS RDS(ON) vs Temperature
MEASURED AT 500mA VIN = 2.7V 160
VIN = 2.7V VIN = 3.6V VIN = 5.5V VIN = 4.2V
140 120
VIN = 3.6V VIN = 5.5V VIN = 4.2V
100 80 60
90 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
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40 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
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LED Power Efficiency vs LED Current
100 95 VIN = 5.5V 90 85 80 75 VIN = 3.6V TA = 25C 65 EFFICIENCY = (VOUT - VLED)ILED/VINIIN FRONT PAGE APPLICATION 60 100 200 300 400 500 600 700 800 900 1000 ILED (mA)
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VIN = 2.7V
70
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LTC3454 TYPICAL PERFOR A CE CHARACTERISTICS
Output Voltage Ripple Back Page Application Start-Up Transient Back Page Application
CH1, VOUT 1V/DIV CH2, ILED 500mA FINAL VALUE 0V, 0A CH3, VEN1 1V/DIV 0V VIN = 3.6V ILED = 500mA 500ns/DIV
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20mV/DIV
PI FU CTIO S
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 4.7H/5H. VOUT (Pin 7): Buck-Boost Output Rail. Bypass to GND with a ceramic capacitor. Recommended value is 10F. VC (Pin 8): Compensation Point for the Internal Error Amplifier Output. Connect a ceramic capacitor from VC to GND. Recommended value is 0.1F. VIN (Pin 9): Voltage Input Supply Pin (2.7V VIN 5.5V). Bypass to GND with a ceramic capacitor. Recommended value is 10F. SW1 (Pin 10): Switching Node. External inductor connects between SW1 and SW2. Recommended value is 4.7H/5H. Exposed Pad (Pin 11): Ground Pin. Solder to PCB ground for electrical contact and optimal thermal performance.
UW
VIN = 3.6V ILED = 500mA
5ms/DIV
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LTC3454 BLOCK DIAGRA W
OPTIONAL OPTIONAL 10 VIN 2.7V TO 5.5V 9 VIN SWITCH A SW1 SW2 6 SWITCH D 7 VOUT SWITCH B UNDERVOLTAGE LOCKOUT UV FORWARD CURRENT LIMIT GATE DRIVERS AND ANTICROSSCONDUCTION SWITCH C
+
OVERTEMP PROTECTION OT 3.4A BANDGAP REFERENCE 1.23V
- + -
AB PWM COMPARATOR UV 1MHz OSCILLATOR CD PWM COMPARATOR OT LOGIC
8
VC VOUT 1.23V 377k
- +
SAFETY ERROR AMP
AUTOZEROING ERROR AMP
-
R
+
1.23V SOFT START CLAMP
123k
LED CURRENT SETTING AMP 1 800mV
+
IISET1 CURRENT MIRROR
-
3 RISET1 800mV ISET1 LED CURRENT SETTING AMP 2 IISET2
I 3850 I R
+ -
4 RISET2 1
ISET2 EN1 EN2 SHUTDOWN 11 EXPOSED PAD (GND)
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2
6
+ -
275mA
REVERSE CURRENT LIMIT
+ -
LED 5
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LTC3454 OPERATIO
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 9 PMOS A SW1 10 NMOS B SW2 6 NMOS C 0% DUTY CYCLE VOUT 7 PMOS D DMIN BOOST DMAX BUCK FOUR SWITCH PWM BUCK/BOOST REGION V2 (1.55V) D ON, C OFF PWM AB SWITCHES BUCK REGION V1 (0.9V) CONTROL VOLTAGE, VC
Figure 1. Simplified Diagram of Internal Power Switches
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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: 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 Four-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 A ON, B OFF BOOST REGION PWM CD SWITCHES V3 (1.65V) V4 (2.1V)
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Figure 2. Switch Control vs Control Voltage, VC
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LTC3454 APPLICATIO S I FOR ATIO
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 130C for any reason, all four switches are shut off immediately. The overtemperature protection circuit has a typical hysteresis of 11C. 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 200s to linearly slew from 0.9V to 2.1V. This has the effect of
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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 GND VIN GND VIN EN2 GND GND VIN VIN ILOAD (A) 0 (SHUTDOWN) 3850 * 0.8V/RISET1 3850 * 0.8V/RISET2 3850 * (0.8V/RISET1 + 0.8V/RISET2)
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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 APPLICATIO S I FOR ATIO
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: L>
( VIN(MAX) - VOUT ) * VOUT * 100%
f * VIN(MAX ) * %Ripple * IOUT
where f = operating frequency, Hz %Ripple = allowable inductor current ripple, % 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 4.7H/5H inductor value is recommended. See Table 1 for a list of component suppliers.
Table 1. Inductor Vendor Information
SUPPLIER Coilcraft Cooper/Coiltronics Murata Sumida Toko Vishay-Dale WEB SITE www.coilcraft.com www.cooperet.com www.murata.com www.japanlink.com/sumida www.toko.com www.vishay.com
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Input Capacitor Selection Since the VIN pin is the supply voltage for the IC it is recommended to place at least a 2.2F, low ESR bypass capacitor to ground. See Table 2 for a list of component suppliers.
Table 2. Capacitor Vendor Information
SUPPLIER AVX Sanyo Taiyo Yuden TDK WEB SITE www.avxcorp.com www.sanyovideo.com www.t-yuden.com www.component.tdk.com
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Output Capacitor Selection The bulk value of the capacitor is set to reduce the ripple due to charge into the capacitor each cycle. The steady state ripple due to charge is given by:
%Ripple _ Boost = IOUT(MAX ) * VOUT - VIN(MIN) * 100% % COUT * VOUT * f
2
(
)
%Ripple _ Buck =
( 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 10F capacitor value is recommended. See Table 2 for a list of component suppliers. Optional Schottky Diodes 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 diode such as an MBRM120T3 or equivalent. Do not use
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LTC3454 APPLICATIO S I FOR ATIO
ordinary rectifier diodes, since the slow recovery times will compromise efficiency. In applications in which VIN is greater than 4V and VOUT to GND short-circuit protection is needed, a Schottky diode such as MBRM12OT3 or equivalent may be used from SW1 to GND and/or a 2/1nF 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. 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 * * L * COUT where COUT is the output filter capacitor. The output filter zero is given by: fFILTER _ ZERO = 1 2 * * 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: fRHPZ VIN = Hz 2 * * IOUT * L * VOUT
2
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 and slower transient response. To ensure proper phase margin, the loop is required to be crossed over a decade before the LC double pole. The unity-gain frequency of the error amplifier with the Type I compensation is given by: gm fUG = 2 * * CVC
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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 0.1F 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) = 3080 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. 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. 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.
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LTC3454 APPLICATIO S I FOR ATIO
VIN ENx LTC3454 ISETx RSET RMIN VOLTAGE DAC VDAC LED ILED = 3850 0.8V - VDAC RSET CURRENT DAC
(3a)
VIN ENx LTC3454 ISETx RMIN RPOT LED ILED = 3850 0.8V RMIN + RPOT RSET 100 VOUT VIN ENx
(3c)
Figure 3. Brightness control Methods: (a) Using Voltage DAC, (b) Using Current DAC, (c) Using Potentiometer, (d) Using PWM Input
PACKAGE DESCRIPTIO
DD Package 10-Lead Plastic DFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1699)
R = 0.115 TYP 6 0.675 0.05 0.38 0.10 10
3.50 0.05 1.65 0.05 2.15 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.38 0.05 (2 SIDES) 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 PIN 1 TOP MARK (SEE NOTE 6)
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.
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VOUT VIN ENx LTC3454 ISETx LED IDAC 0.8V RMIN ILED = 3850 * IDAC VOUT
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(3b)
VOUT
LTC3454 ISETx LED RSET RMIN VPWM = 3850 DVCC fPWM 10kHz ILED = 3850 0.8V - VPWM RSET 0.8V - (DC% * VDVCC) RSET
(3d)
3454 F03
3.00 0.10 (4 SIDES)
1.65 0.10 (2 SIDES)
(DD10) DFN 1103
5 0.200 REF 0.75 0.05 2.38 0.10 (2 SIDES)
1
0.25 0.05 0.50 BSC
0.00 - 0.05
BOTTOM VIEW--EXPOSED PAD 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
3454f
11
LTC3454 TYPICAL APPLICATIO
500mA LED Flashlight Driver
L1 4.7mH 2.2mF 3- CELL ALKALINE 4.5V EN1 VIN SWA SW1 SW2 SWD VOUT 4.7mF LED EFFICIENCY (%) LED EN2 ILED = 500mA 100 95 90 85 80 75 70 ISET1 ISET2 LTC3454 GND (EXPOSED PAD) LED: LUMILEDS, LXCL LW3C L1: TOKO A997AS-4R7M RISET1 6.19k 1%
3453 TA02
SWB
VC 0.1mF
RELATED PARTS
PART NUMBER LT1618 DESCRIPTION COMMENTS VIN: 1.6V to 18V, VOUT(MAX) = 34V, IQ = 1.8mA, ISD = <1A, MS10 Package/EDD Package VIN: 2.6V to 16V, VOUT(MAX) = 34V, IQ = 4.2mA/5.5mA, ISD = <1A, ThinSOT Package VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD = <1A, ThinSOT Package VIN: 2.5V to 10V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD = <1A, ThinSOT Package/SC70 Package VIN: 2.8V to 4.5V, VOUT(MAX) = 6V, IQ = 50A, ISD = <1A, QFN-24 Package VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300A, ISD = <2.5A, DFN Package VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300A, ISD = <2.5A, DFN Package VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 25A/50A, ISD = <1A, MS-10 Package/DFN Package VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 28A, ISD = <1A, DFN Package VIN: 1V to 3.2V, VOUT(MAX) = 4V, IQ = 20A, ISD = 20A, DFN Package VIN: 2.7V to 5.5V, Up to 500mA Continuous Output Current, QFN-16 Package VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD = <1A, ThinSOT Package VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD = <16A, DFN Package VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 6.5mA, ISD = <1A, DFN Package/TSOPP Package
3454f
Constant Current, Constant Voltage 1.4MHz, High Efficiency Boost Regulator LT1930/LT1930A 1A (ISW), 1.2MHz/2.2MHz, High Efficiency Step-Up DC/DC Converter LT1932 Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator LT1937 Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator LTC3205 High Efficiency, Multi-Display LED Controller LTC3215 LTC3216 LTC3440/ LTC3441 LTC3443 LTC3490 LTC3453 700mA Low Noise Charge Pump LED Driver 1A Low Noise High Current Charge Pump LED Driver with Independent Flash/Torch Current 600mA/1.2A IOUT, 2MHz/1MHz, Synchronous Buck-Boost DC/DC Converter 600mA/1.2A IOUT, 600kHz, Synchronous Buck-Boost DC/DC Converter Single Cell 350mA LED Driver Synchronous Buck-Boost High Power White LED Driver
LT3465/LT3465A Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode LT3466 Dual Constant Current, 2MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode LT3479 3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection
12 Linear Technology Corporation
(408) 432-1900
1630 McCarthy Blvd., Milpitas, CA 95035-7417
FAX: (408) 434-0507 www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2005
U
LED Power Efficiency vs VIN
SWC 1MHz BUCK-BOOST 65 ILED = 500mA TA = 25C EFFICIENCY = (VOUT - VLED)ILED/VINIIN 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5 60 2.7
3454 TA02b
LT 1105 * PRINTED IN USA

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