View LT3477EUF_4342337.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

LT3477 3A, DC/DC Converter with Dual Rail-to-Rail Current Sense
FEATURES

DESCRIPTIO
Dual 100mV Rail-to-Rail Current Sense Amplifiers Wide Input Voltage Range: 2.5V to 25V 3A, 42V Internal Switch High Efficiency Power Conversion: Up to 93% Drives LEDs in Boost, Buck-Boost or Buck Mode Frequency Set by External Resistor: 200kHz to 3.5MHz Programmable Soft-Start Low VCESAT Switch: 0.3V at 2.5A Capable of Positive and Negative Output Voltages (Boost, Inverting, SEPIC, Flyback) Available in Thermally Enhanced 20-Lead (4mm x 4mm) QFN and 20-Lead TSSOP Packages
APPLICATIO S

The LT(R)3477 is a current mode, 3A DC/DC step-up converter with dual rail-to-rail current sense amplifiers and an internal 3A, 42V switch. It combines a traditional voltage feedback loop and two unique current feedback loops to operate as a constant-current, constant-voltage source. Both current sense voltages are set at 100mV and can be adjusted independently using the IADJ1 and IADJ2 pins. Efficiency of up to 91% can be achieved in typical applications. The LT3477 features a programmable soft-start function to limit inductor current during start-up. Both inputs of the error amplifier are available externally allowing positive and negative output voltages (Boost, Inverting, SEPIC, Flyback). The switching frequency is programmable from 200kHz to 3.5MHz through an external resistor. Available in thermally enhanced 20-pin (4mm x 4mm) QFN and 20-pin TSSOP packages, the LT3477 provides a complete solution for both constant-voltage and constant-current applications.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
High Power LED Driver DSL Modems Distributed Power Input/Output Current Limited Boost, SEPIC, Inverting, Flyback Converters Constant-Voltage, Constant-Current Source
TYPICAL APPLICATIO
VIN 5V 3.3F ISP1 VIN IADJ1 IADJ2 SHDN SHDN LT3477 ISN1 10H
330mA LED Driver with Open LED Protection Efficiency
90
SW FBN 200k 3.3F
85 80
EFFICIENCY (%)
10k
75 70 65 60 55
ISP2 0.3 ISN2 RT
VC VREF 1k FBP GND SS
330mA 33nF 22k
50
0
4.7nF
3477 TA01a
U
0.1 0.2 IOUT (A)
3477 TA01b
U
U
0.3
0.4
3477fb
1
LT3477
ABSOLUTE MAXIMUM RATINGS
SW Pin Voltage ....................................................... 42V VIN, SHDN Pin Voltage ............................................ 25V FBP, FBN Pin Voltage ................................................ 6V VREF Pin Voltage ....................................................... 6V RT, VC , SS Pin Voltage .............................................. 6V IADJ1, IADJ2 Pin Voltage ........................................... 25V ISP1, ISP2, ISN1, ISN2 Pin Voltage .............................. 42V
PACKAGE/ORDER INFORMATION
TOP VIEW
GND ISN1 SW SW NC
20 19 18 17 16 NC 1 NC 2 VIN 3 RT 4 SHDN 5 6
SS
15 ISP1 14 ISN2 21 13 ISP2 12 IADJ1 11 IADJ2 7
VC
8
FBN
9 10
FBP VREF
UF PACKAGE 20-LEAD (4mm x 4mm) PLASTIC QFN
TJMAX = 125C, JA = 37C/W EXPOSED PAD (PIN 21) IS PGND (MUST BE SOLDERED TO PCB)
ORDER PART NUMBER LT3477EUF LT3477IUF
Order Options Tape and Reel: Add #TR
UF PART MARKING 3477 3477
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 indicates specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 2.5V, VSHDN = 2.5V.
PARAMETER Minimum Input Voltage Quiescent Current Reference Voltage Reference Voltage Line Regulation Maximum VREF Pin Current VSHDN = 0V VSHDN = 2.5V, VC = 0.3V (Not Switching) E Grade I Grade 2.5V < VIN < 25V, VC = 0.3V Out of Pin

ELECTRICAL CHARACTERISTICS
CONDITIONS
2
U
U
W
WW
U
W
(Note 1)
Junction Temperature .......................................... 125C Operating Temperature Range (Note 2) LT3477E ............................................. - 40C to 85C LT3477I ............................................ - 40C to 125C Storage Temperature Range ................. - 65C to 125C Lead Temperature (Soldering, 10 sec) TSSOP .............................................................. 300C
TOP VIEW VIN RT SHDN SS VC FBN FBP VREF IADJ2 1 2 3 4 5 6 7 8 9 21 20 NC 19 NC 18 NC 17 SW 16 SW 15 GND 14 ISN1 13 ISP1 12 ISN2 11 ISP2
IADJ1 10
FE PACKAGE 20-LEAD PLASTIC TSSOP
TJMAX = 150C, JA = 40C/W EXPOSED PAD (PIN 21) IS PGND (MUST BE SOLDERED TO PCB)
ORDER PART NUMBER LT3477EFE LT3477IFE
SW PART MARKING
MIN
TYP 2.3 0.1 5.0
MAX 2.5 1.0 7.5 1.250 1.260 0.03 100
UNITS V A mA V V %/V A
3477fb
1.216 1.210
1.235 1.235 0.01
LT3477
The indicates specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 2.5V, VSHDN = 2.5V.
PARAMETER Soft-Start Pin Current FBP Pin Bias Current FBN Pin Bias Current Feedback Amplifier Offset Voltage Feedback Amplifier Voltage Gain Voltage Feedback Amplifier Transconductance Feedback Amplifier Sink Current Feedback Amplifier Source Current Current Sense Amplifier Sense Voltage VFBP = 1.25V, VFBN = 1.5V, VC = 1V VFBP = 1.25V, VFBN = 1V, VC = 0.5V Positive Rail, VCM = 25V, E Grade Positive Rail, VCM = 25V, I Grade Ground RT = 17.2k RT = 107.4k RT = 2.44k RT = 17.2k (Note 3) ISW = 1A (Note 3) SW = 40V VSHDN = 5V VSHDN = 0V 0.3

ELECTRICAL CHARACTERISTICS
CONDITIONS SS = 0.5V, Out of Pin
MIN
TYP 9 25 25
MAX 100 100 6
UNITS A nA nA mV V/V S A A
FBP - FBN, VC = 1V
-2
2 500 500 10 10
97.5 97.5 88 0.9 160 2.7 87 3
100 100 100 1 200 3.5 93 4 150 0.2 30 0.1 1.5
102.5 103 112 1.15 240 4.3 5 200 5 60 1 2
mV mV mV MHz kHz MHz % A mV A A A V
Switching Frequency
Maximum Switch Duty Cycle Switch Current Limit Switch VCESAT Switch Leakage Current SHDN Pin Current SHDN Pin Threshold
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 LT3477E is guaranteed to meet performance specifications from 0C to 70C. Specifications over the - 40C to 85C operating
junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3477I is guaranteed over the full -40C to 125C operating junction temperature range. Note 3: Switch current limit and switch VCESAT for UF package guaranteed by design and/or correlation to static test.
TYPICAL PERFOR A CE CHARACTERISTICS
Switch VCE(SAT)
0.50 0.45 0.40 0.35 VCE(SAT) (V) 0.30 0.25 0.20 0.15 0.10 0.05 0 0 0.5 1.5 2 1 SWITCH CURRENT (A) 2.5 3
3477 G01
CURRENT (A)
125C 25C
VREF (V)
-50C
UW
Switch Current Limit
5 1.27 1.26 1.25 3 1.24 1.23 1 1.22
VREF
4
VIN = 25V VIN = 2.5V
2
0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3477 G02
1.21 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C) 3477 G03
3477fb
3
LT3477 TYPICAL PERFOR A CE CHARACTERISTICS
SHDN Pin Turn-On Threshold
1.6
50 -50C
QUIESCENT CURRENT (mA)
SHDN PIN CURRENT (A)
SHDN THRESHOLD (V)
1.4
1.2
1.0 -50
-25
50 25 75 0 TEMPERATURE (C)
Soft-Start Pin Current
20
OFFSET VOLTAGE (mV)
15
FREQUENCY (MHz)
ISS (A)
10
5
0 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3477 G07
FBP Pin Bias Current
50 40 30 20 10 0 -10 -50 -25 "+" INDICATES THE CURRENT FLOWS OUT OF PIN 50 40 30 20 10 0
FBN PIN BIAS CURRENT (nA)
FBP PIN BIAS CURRENT (nA)
50 25 75 0 TEMPERATURE (C)
4
UW
100
3477 G04
SHDN Pin Current
6
Quiescent Current
VC = 0.3V
40
5
30 25C 20 125C 10
4
3
0
125
0
5
15 10 VSHDN (V)
20
25
3477 G05
2 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3477 G06
Oscillator Frequency
2.0 RT = 10k
Feedback Amplifier Offset Voltage
4 3 2 VC = 1V 1 0 -1 -2 -3 VC = 0.5V
1.6 1.2
RT = 15k RT = 20k
0.8
0.4
0 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3477 G08
-4 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3477 G09
FBN Pin Bias Current
"+" INDICATES THE CURRENT FLOWS OUT OF PIN
100
125
-10 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
3477 G10
3477 G11
3477fb
LT3477 TYPICAL PERFOR A CE CHARACTERISTICS
Current Sense Voltage vs Temperature
104 120 100
VOLTAGE SENSE (mV)
CURRENT SENSE VOLTAGE (mV)
103 102 101 100 99 98 97 96 -50 -25 0 50 75 25 TEMPERATURE (C) 100 125 VCM = 42V VCM = 10V
PI FU CTIO S
(QFN/TSSOP)
NC (Pins 1, 2, 20/Pins 18, 19, 20): No Connect Pin. Okay to connect to ground or VIN, or to float. VIN (Pin 3/Pin 1): Input Supply. Must be locally bypassed. Powers the internal control circuitry. RT (Pin 4/Pin 2): Timing Resistor Pin. Adjusts the switching frequency. Connect a 17.2k resistor between RT and GND for a 1MHz switching frequency. Do not leave this pin open. See Table 4 for additional RT values and switching frequencies. SHDN (Pin 5/Pin 3): Shutdown. Tie to 2V or greater to enable the device. Tie below 0.3V to turn off the device. SS (Pin 6/Pin 4): Soft-Start. Place a soft-start capacitor here. Leave floating if not in use. VC (Pin 7/Pin 5): Compensation Pin for Error Amplifier. Connect a series RC from this pin to GND. Typical values are 1k and 4.7nF. FBN (Pin 8/Pin 6): The Inverting Input to the Error Amplifier. Connect resistive divider tap here for positive output voltage.
UW
Current Sense Voltage vs IADJ
VCM = 10V
80 60 40 20 0 0
100 200 300 400 500 600 700 800 IADJ VOLTAGE (mV)
2477 G13
3477 G14
U
U
U
FBP (Pin 9/Pin 7): The Noninverting Input to the Error Amplifier. Connect resistive divider tap here for negative output voltage. VREF (Pin 10/Pin 8): Bandgap Voltage Reference. Internally set to 1.235V. Connect this pin to FBP if generating a positive output or to an external resistor divider if generating a negative voltage. This pin can provide up to 100A of current and can be locally bypassed with a 100pF capacitor. IADJ2 (Pin 11/Pin 9): Second Current Sense Adjustment. Setting IADJ2 to be less than 625mV leads to adjustment of the sensed voltage of the second current sense amplifier linearly. If IADJ2 is tied to higher than 650mV, the default current sense voltage is 100mV. If current sense amplifier 2 is not used, always tie IADJ2 to higher than 650mV. IADJ1 (Pin 12/Pin 10): First Current Sense Adjustment. Setting IADJ1 to be less than 625mV leads to adjustment of the sensed voltage of the first current sense amplifier linearly. If IADJ1 is tied to higher than 650mV, the default current sense voltage is 100mV. If current sense amplifier 1 is not used, always tie IADJ1 to higher than 650mV.
3477fb
5
LT3477
PI FU CTIO S
ISP2 (Pin 13/ Pin 11): Second Current Sense (+) Pin. The noninverting input to the second current sense amplifier. Connect to ISN2 if not used. ISN2 (Pin 14/ Pin 12): Second Current Sense (-) Pin. The inverting input to the second current sense amplifier. Connect to ISP2 if not used. ISP1 (Pin 15/Pin 13): First Current Sense (+) Pin. The noninverting input to the first current sense amplifier. Connect to ISN1 if not used. ISN1 (Pin 16/Pin 14): First Current Sense (-) Pin. The inverting input to the first current sense amplifier. Connect to ISP1 if not used.
BLOCK DIAGRA
ISP1
+
IA1
ISN1 IADJ1
-
VADJ
ISP2
+
IA2
ISN2 IADJ2 FBP
-
VADJ
FBN
-
VREF 1.25V SHDN VIN
-
VREF
OSCILLATOR RT
3477 F01
Figure 1. LT3477 Block Diagram
6
+
VA
+
+
SLOPE
-
W
U
U
U
(QFN/TSSOP)
GND (Pins 17/Pin 15): Ground. Tie directly to local ground plane. SW (Pins 18, 19/Pins 16, 17): Switch Pins. Collector of the internal NPN power switch. Connect the inductor and diode here and minimize the metal trace area connected to this pin to minimize electromagnetic interference. Exposed Pad (Pin 21/Pin 21): Power Ground. Must be soldered to PCB ground for electrical contact and rated thermal performance.
SS
VC
SW
- + +
A1
- + +
A2
A3
A4
R S
Q
Q1
3477fb
LT3477
OPERATIO
The LT3477 uses a fixed frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the Block Diagram in Figure 1. The start of each oscillator cycle sets the SR latch and turns on power switch Q1. The signal at the noninverting input of the PWM comparator (A4 SLOPE) is proportional to the sum of the switch current and oscillator ramp. When SLOPE exceeds VC (the output of the feedback amplifier), the PWM comparator resets the latch and turns off the power switch. In this manner, the feedback amplifier and PWM comparators set the correct peak current level to keep the output in regulation. Amplifier A3 drives A4 inverting input. A3 has three inputs, one from the voltage feedback loop and the other two from the current feedback loop. Whichever feedback input is higher takes precedence, forcing the converter into either a constant-current or a constant-voltage mode. The LT3477 is designed to transition cleanly between the two modes of operation. Current sense amplifier IA1 senses the voltage between the ISP1 and ISN1 pins and provides a pre-gain to amplifier A1. When the voltage between ISP1 and ISN1 reaches 100mV, the output of IA1 provides VADJ to the inverting input of A1 and the converter is in constantcurrent mode. If the current sense voltage exceeds 100mV, the output of IA1 will increase causing the output of A3 to decrease, thus reducing the amount of current delivered to
U
the output. In this manner the current sense voltage is regulated to 100mV. The current sense level is also pin adjustable by IADJ1. Forcing IADJ1 to less than 625mV will overwrite VADJ voltage that's set internally, thus providing current level control. The second current sense amplifier, IA2, works the same as the first current sense amplifier IA1. Both current sense amplifiers provide rail-to-rail current sense operation. Similarly, for positive output voltage operation where FBP is tied to VREF, if the FBN pin increases above VREF, the output of A3 will decrease to reduce the peak current level and regulate the output (constant-voltage mode). For negative output voltage operation where FBN is tied to GND, if the FBP pin decreases below GND level, the output of A3 will decrease to reduce the peak current level and regulate the output (constant-voltage mode). The LT3477 also features a soft-start function. During start-up, 9A of current charges the external soft-start capacitor. The SS pin directly limits the rate of voltage rise on the VC pin, which in turn limits the peak switch current. The switch current is constantly monitored and not allowed to exceed the nominal value of 3A. If the switch current reaches 3A, the SR latch is reset regardless of the output of the PWM comparator. Current limit protects the power switch and external components.
3477fb
7
LT3477
APPLICATIO S I FOR ATIO
Capacitor Selection
Low ESR (equivalent series resistance) ceramic capacitors should be used at the output to minimize the output ripple voltage. Use only X5R or X7R dielectrics, as these materials retain their capacitance over wider voltage and temperature ranges better than other dielectrics. A 4.7F to 10F output capacitor is sufficient for most high output current designs. Converters with lower output currents may need only a 1F or 2.2F output capacitor.
Table 1. Ceramic Capacitor Manufacturers
MANUFACTURER Taiyo Yuden AVX Murata TDK PHONE (408) 573-4150 (803) 448-9411 (714) 852-2001 (847) 803-6100 WEB www.t-yuden.com www.avxcorp.com www.murata.com www.component.tdk.com
Inductor Selection Several inductors that work well with the LT3477 are listed in Table 2. However, there are many other manufacturers and devices that can be used. Consult each manufacturer for more detailed information and their entire range of parts. Ferrite core inductors should be used to obtain the best efficiency. Choose an inductor that can handle the necessary peak current without saturating, and ensure that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. A 4.7H or 10H inductor will suffice for most LT3477 applications. Inductor manufacturers specify the maximum current rating as the current where the inductance falls to some
Table 2. Suggested Inductors
MANUFACTURER PART NUMBER CDRH6D283R0 CDRH6D28100 CDRH4D284R7 LM N 05D B4R7M LM N 05D B100K LQH55DN4R7M01L LQH55DN100M01K FDV0630-4R7M IDC (A) 3 1.7 1.32 2.2 1.6 2.7 1.7 4.2 INDUCTANCE (H) 3 10 4.7 4.7 10 4.7 10 4.7
8
U
percentage of its nominal value--typically 65%. An inductor can pass a current larger than its rated value without damaging it. Aggressive designs where board space is precious will exceed the maximum current rating of the inductor to save board space. Consult each manufacturer to determine how the maximum inductor current is measured and how much more current the inductor can reliably conduct. Diode Selection Schottky diodes, with their low forward voltage drop and fast switching speed, are ideal for LT3477 applications. Table 3 lists several Schottky diodes that work well with the LT3477. The diode's average current rating must exceed the average output current. The diode's maximum reverse voltage must exceed the output voltage. The diode conducts current only when the power switch is turned off (typically less than 50% duty cycle), so a 3A diode is sufficient for most designs. The companies below also offer Schottky diodes with higher voltage and current ratings.
Table 3. Suggested Diodes
MANUFACTURER MAX MAX REVERSE PART NUMBER CURRENT (A) VOLTAGE (V) MANUFACTURER UPS340 UPS315 B220 B230 B240 B320 B330 B340 SBM340 3 3 2 2 2 3 3 3 3 40 15 20 30 40 20 30 40 40 Microsemi www.microsemi.com Diodes, Inc www.diodes.com MAX DCR (m) 24 65 72 49 10 57 130 49 LxWxH (mm) 6.7 x 6.7 x 3.0 6.7 x 6.7 x 3.0 5.0 x 5.0 x 3.0 5.9 x 6.1 x 2.8 5.9 x 6.1 x 2.8 5.7 x 5.0 x 4.7 5.7 x 5.0 x 4.7 7.0 x 7.7 x 3.0 MANUFACTURER Sumida www.sumida.com Taiyo Yuden www.t-yuden.com Murata www.murata.com Toko www.toko.com
3477fb
W
UU
LT3477
APPLICATIO S I FOR ATIO
Setting Positive Output Voltages
To set a positive output voltage, select the values of R1 and R2 (see Figure 2) according to the following equation: R1 VOUT = 1.235V 1 + R2
FBP LT3477 VREF R1 FBN R2
3477 F02
VOUT
Figure 2. Positive Output Voltage Feedback Connections
Setting Negative Output Voltages To set a negative output voltage, select the values of R3 and R4 (see Figure 3) according to the following equation: R3 VOUT = -1.235V R4
-VOUT
R3
FBP LT3477 VREF FBN
3477 F03
R4
Figure 3. Negative Output Voltage Feedback Connections
Selecting RSENSE/Current Sense Adjustment Using the following formula to choose the correct current sense resistor value (for constant current or fail-safe operation).
RSENSE =
100mV ISENSE
U
For designs needing an adjustable current level, the IADJ1 and IADJ2 pins are provided for the first and the second current sense amplifiers, respectively. With the IADJ1 and IADJ2 pins tied higher than 650mV, the nominal current sense voltage is 100mV (appearing between the ISP1 and ISN2 or ISP2 and ISN2 pins). Applying a positive DC voltage less than 600mV to the IADJ1 and IADJ2 pins will decrease the current sense voltage according to the following formula:
ISENSE = 100mV VIADJ * R SENSE 618mV
W
UU
For example, if 309mV is applied to the IADJ1 pin and RSENSE is 0.5, the current sense will be reduced from 200mA to 100mA. The adjustability allows the regulated current to be reduced without changing the current sense resistor (e.g., to adjust brightness in an LED driver or to reduce the charge current in a battery charger). Considerations When Sensing Input Current In addition to regulating the DC output current for currentsource applications, the constant-current loop of the LT3477 can also be used to provide an accurate input current limit. Boost converters cannot provide output short-circuit protection, but the surge turn-on current can be drastically reduced using the LT3477 current sense at the input. SEPICs, however, have an output that is DCisolated from the input, so an input current limit not only helps soft-start the output but also provides excellent short-circuit protection. When sensing input current, the sense resistor should be placed in front of the inductor (between the decoupling capacitor and the inductor). This will regulate the average inductor current and maintain a consistent inductor ripple current, which will, in turn, maintain a well regulated input current. Do not place the sense resistor between the input source and the input decoupling capacitor, as this may allow the inductor ripple current to vary widely (even though the average input current and the average inductor current will still be regulated). Since the inductor current
3477fb
9
LT3477
APPLICATIO S I FOR ATIO
is a triangular waveform (not a DC waveform like the output current) some tweaking of the compensation values (RC and CC on the VC pin) may be required to ensure a clean inductor ripple current while the constant-current loop is in effect. For these applications, the constantcurrent loop response can usually be improved by reducing the RC value or by adding a capacitor (with a value of approximately CC/10) in parallel with the RC and CC compensation network. Frequency Compensation The LT3477 has an external compensation pin (VC), which allows the loop response to be optimized for each application. An external resistor and capacitor (or sometimes just a capacitor) are placed at the VC pin to provide a pole and a zero (or just a pole) to ensure proper loop compensation. Several other poles and zeroes are present in the closedloop transfer function of a switching regulator, so the VC pin pole and zero are positioned to provide the best loop response. A thorough analysis of the switching regulator control loop is not within the scope of this data sheet, and will not be presented here, but values of 1k and 4.7nF will be a good choice for many designs. For those wishing to optimize the compensation, use the 1k and 4.7nF as a starting point. Board Layout As with all switching regulators, careful attention must be paid to the PCB board layout and component placement. To maximize efficiency, switch rise and fall times are made as short as possible. To prevent radiation and high frequency resonance problems, proper layout of the high frequency switching path is essential. Minimize the length and area of all traces connected to the SW pin and always use a ground plane under the switching regulator to minimize interplane coupling. The signal path including the switch, output diode D1 and output capacitor COUT, contains nanosecond rise and fall times and should be kept as short as possible.
SWITCH FREQUENCY (MHz)
10
U
Soft-Start For many applications, it is necessary to minimize the inrush current at start-up. The built-in soft-start circuit significantly reduces the start-up current spike and output voltage overshoot. A typical value for the soft-start capacitor is 10nF. Switching Frequency The switching frequency of the LT3477 is set by an external resistor attached to the RT pin. Do not leave this pin open. A resistor must always be connected for proper operation. See Table 4 and Figure 4 for resistor values and corresponding frequencies. Increasing switching frequency reduces output voltage ripple but also reduces efficiency. The user should set the frequency for the maximum tolerable output voltage ripple.
Table 4. Switching Frequency
SWITCHING FREQUENCY (MHz) 3.5 3 2.5 2 1.5 1 0.5 0.2 RT (k) 2.43 3.65 4.87 6.81 10.2 17.4 43.2 107
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0.1 10 RT (k) 100
3477 F04
W
UU
Figure 4. Switch Frequency
3477fb
LT3477
APPLICATIO S I FOR ATIO
PWM Dimming For LED applications where a wide dimming range is required, two competing methods are available: analog dimming and PWM dimming. The easiest method is to simply vary the DC current through the LED--analog dimming--but changing LED current also changes its chromaticity, undesirable in many applications. The better method is PWM dimming, which switches the LED on and off, using the duty cycle to control the average current. PWM dimming offers several advantages over analog dimming and is the method preferred by LED manufacturers. By modulating the duty cycle of the PWM signal, the average LED current changes proportionally as illustrated in Figure 5. The chromaticity of the LEDs remains unchanged in this scheme since the LED current is either zero or at programmed current. Another advantage of PWM dimming over analog dimming is that a wider dimming range is possible. The LT3477 is a DC/DC converter that is ideally suited for LED applications. For the LT3477, analog dimming offers a dimming ratio of about 10:1; whereas, PWM dimming with the addition of a few external components results in a wider dimming range of 500:1. The technique requires a PWM logic signal applied to the gate of both NMOS (refer to Figure 7). When the PWM signal is taken high the part runs in normal operation and ILED = 100mV/RSENSE runs
100
RT = 6.81k
LED CURRENT (mA)
DIMMING RANGE: 1
10
1
0.1
0.01 0.1
VIN = 5V BOOST 4 LEDs PWM FREQUENCY = 100Hz 1 10 PWM DUTY CYCLE (%) 100
3477 F05
Figure 5. LED Current vs PWM Duty Cycle Wide Dimming Range (500:1)
U
through the LEDs. When the PWM input is taken low, the LEDs are disconnected and turn off. This unique external circuitry produces a fast rise time for the LED current, resulting in a wide dimming range of 500:1 at a PWM frequency of 100Hz. The LED current can be controlled by feeding a PWM signal with a broad range of frequencies. Dimming below 80Hz is possible, but not desirable, due to perceptible flashing of LEDs at lower PWM frequencies. The LED current can be controlled at higher frequencies, but the dimming range decreases with increasing PWM frequency, as seen in Figure 6. PWM dimming can be used in Boost (shown in Figure 7), Buck mode (shown in Figure 8) and Buck-Boost mode (shown in Figure 9). For the typical boost topology, efficiency exceeds 80%. Buck mode can be used to increase the power handling capability for higher current LED applications. A Buck-Boost LED driver works best in applications where the input voltage fluctuates to higher or lower than the total LED voltage drop. In high temperature applications, the leakage of the Schottky diode D1 increases, which in turn, discharges the output capacitor during the PWM "off" time. This results in a smaller effective LED dimming ratio. Consequently, the dimming range decreases to about 200:1 at 85C.
1000 RT = 6.81k 100 10 1 0.1 1 10 100
3477 F06
W
UU
PWM FREQUENCY (kHz)
Figure 6. Dimming Range vs PWM Frequency
3477fb
11
LT3477
APPLICATIO S I FOR ATIO
VIN 5V
C1 3.3F
CSS 33nF VC GND D2 5V 0 100Hz 100k PWM NMOS1 RC 2.4k CC 10nF
C1: TAIYO YUDEN EMK316BJ335ML C2: TAIYO YUDEN UDK325BJ106MM L1: TOKO D53LC (PN# A915AY-2ROM) D1: ZETEX ZLLS1000 D2: DIODES INC 1N4148 NMOS1: ZETEX 2N7002 NMOS2: FAIRCHILD FDG327N LED1 TO LED4: LUMILEDS LXHL-BW02
Figure 7a. 5V to 4 White LEDs: Boost with PWM Dimming
PWM 5V/DIV
EFFICIENCY (%)
IL 1A/DIV
ILED 200mA/DIV
3477 F07b
VIN = 5V 4 LEDs 300mA
10s/DIV PWM FREQ = 100Hz BOOST
Figure 7b. PWM Dimming Waveforms
12
U
L1 2.0H D1 OUT 1M FBN FBP LT3477 VREF ISP2 ISN2 RT 6.81k LED1 LED2 LED3 LED4 NMOS2 300mA 75k C2 10F ISP1 VIN IADJ1 IADJ2 SHDN SS ISN1 SW RSENSE 0.33
3477 F07a
W
UU
85 EFFICIENCY 80 75 70 65 60 55 50 0 20 VIN = 5V BOOST 4 LEDs, 300mA PWM FREQUENCY = 100Hz 40 60 80 PWM DUTY CYCLE (%) LED CURRENT
350 300 250 200 150 100 50
0 100
3477 F07c
Figure 7c. Efficiency and LED Current vs PWM Duty Cycle
3477fb
LT3477
APPLICATIO S I FOR ATIO
PVIN 32V
C1 2.2F
C1: NIPPON NTS40X5R1H225M C2: TAIYO YUDEN GMK316BJ105ML C3: TAIYO YUDEN LMK316BJ335KL L1: TOKO D53LC (PN# A915AY-100M) D1: ZETEX ZLLS400 D2: DIODES INC 1N4148 NMOS1, NM0S2: ZETEX 2N7002 PMOS: SILICONIX Si2303BDS LED1 TO LED6: LUMILEDS LXHL-BW02
VIN 3.3V
C3 3.3F
CSS 33nF VC GND D2 5V 0 100Hz 100k CC 0.1F PWM NMOS1
Figure 8a. 32V to 6 White LEDs: Buck Mode with PWM Dimming
PWM 5V/DIV
IL 500mA/DIV ILED 500mA/DIV
3477 F08b
PVIN = 32V 6 LEDs 300mA
Figure 8b. PWM Dimming Waveforms
3477fb
U
RSENSE 0.33 LED1 300mA
W
UU
* * *
LED6 1k 1k NMOS2 C2 1F PWM
PMOS
L1 10H
D1 280k
ISP1 VIN IADJ1 IADJ2 SHDN SS
ISN1
SW FBN FBP
10k
LT3477
VREF ISP2 ISN2 RT 6.81k
3477 F08a
2ms/DIV PWM FREQUENCY = 100Hz BUCK MODE
13
LT3477
APPLICATIO S I FOR ATIO
C1: TAIYO YUDEN LMK316BJ335ML C2: TAIYO YUDEN UDK325BJ106MM L1: TOKO D53LC (PN# A915AY-4R7M) D1: ZETEX ZLLS1000 D2: DIODES INC 1N4148 NMOS1, NMOS2: ZETEX 2N7002 PMOS: SILICONIX Si2303BDS LED1, LED2: LUMILEDS LXHL-BW02
VIN 10V
D2 5V 0 100Hz 100k PWM NMOS1 RC 1.5k CC 10nF
Figure 9a. 10V to 2 White LEDs: Buck-Boost Mode with PWM Dimming
PWM 10V/DIV
IL 1A/DIV ILED 500mA/DIV
3477 F09b
VIN = 10V 2 LEDs 300mA
Figure 9b. PWM Dimming Waveforms
14
U
1k NMOS2 1k LED2 PMOS PWM 300mA LED1 L1 4.7H D1 RSENSE 0.33 1M FBN FBP VREF LT3477 SS CSS 33nF VC GND
3477 F09a
W
UU
C1 3.3F
ISP1 VIN IADJ1 IADJ2 SHDN
ISN1
SW
49.9k
ISP2 ISN2 RT 6.81k C2 10F
2ms/DIV PWM FREQUENCY = 100Hz BUCK-BOOST MODE
3477fb
LT3477
TYPICAL APPLICATIO S
5.5V SEPIC Converter with Short-Circuit Protection
R1 0.04 C1 3.3F ISP1 VIN IADJ1 IADJ2 SHDN SHDN LT3477 ISN1 SW L1 4.7H C2 10F
90
VIN 3V TO 16V
EFFICIENCY (%)
VC VREF RC 1k CC 4.7nF FBP GND SS
C1: TAIYO YUDEN LMK316BJ335ML C2: TAIYO YUDEN LMK325BJ106MN C3: TAIYO YUDEN LMK316BJ106ZL D1: DIODES INC. DFLS130L L1, L2: TOKO FDV0630-4R7M
800mA, 5V to 12V Boost Converter with Accurate Input Current Limit
VIN 5V R1 0.033 C1 2.2F ISP1 VIN IADJ1 IADJ2 SHDN SHDN VC VREF RC 1k CC 4.7nF C1: TAIYO YUDEN LMK316BJ225MD C2: AVX 1206YD106MAT D1: DIODES INC. B320A L1: TOKO FDV0630-4R7M FBP GND SS C3 10nF R2 17.8k
3477 TA04a
L1 4.7H
FBN ISP2 LT3477 ISN2 RT R4 23.2k
C2 10F
EFFICIENCY (%)
ISN1
U
Efficiency
VIN = 3V
D1
R3 0.15
5.5V 670mA R4 34.8k
85 80 75 70 65 60 55
L2 4.7H
FBN ISP2 ISN2 RT
50 0 0.1 0.2 0.3 0.4 IOUT (A) 0.5 0.6 0.7
C4 33nF
R2 18.2k
C3 10F
3477 TA02a
R5 10k
3477 TA02b
Efficiency
90
D1 12V 0.8A SW R3 200k
85 80 75 70 65 60 55 50 0 0.1 0.2 0.3 0.4 0.5 IOUT (A) 0.6 0.7 0.8
3477 TA04b
3477fb
15
LT3477
TYPICAL APPLICATIO S
87% Efficient, 4W LED Driver
VIN 5V R4 0.05 C1 3.3F L2 10H D1 R2 200k FBN R1 10k LT3477 ISP2 ISN2 RT GND SS C3 33nF R3 22k LED1 LED2 LED3 LED4
3477 TA03a
EFFICIENCY (%)
ISP1 VIN IADJ1 IADJ2
ISN1
SW
SHDN
SHDN
VC VREF RC 1k CC 4.7nF FBP
C1: TAIYO YUDEN LMK316BJ335ML C2: TAIYO YUDEN TMK325BJ335MN D1: DIODES INC. DFLS120L L1: TOKO A915AY-100M
1A Buck Mode High Current LED Driver
PVIN 32V R1 0.1 LED1 1A LED4 C1 2.2F
* * *
EFFICIENCY (%)
ISP1 VIN 3.3V C3 3.3F SHDN VIN IADJ1 IADJ2 SHDN
ISN1
SW FBN
LT3477
VC RC 1k CC 4.7nF VREF FBP GND SS
C1: NIPPON UNITED CHEMICON NTS40X5R1H225M C2: TAIYO YUDEN GMK316BJ105ML C3: TAIYO YUDEN LMK316BJ475 L1: TOKO A814AY-330M D1: DIODES INC DFLS140
3477fb
16
U
Efficiency
90 85
C2 3.3F
80 75 70 65 60 55 50 0 0.1 0.2 IOUT (A)
3477 TA01b
330mA
R6 0.3
0.3
0.4
Efficiency
LED STRING C2 1F
100 90
L1 33H D1 R3 280k
80
70 60 50 40 30
R4 10k ISP2 ISN2 RT
20
0
0.2
0.4 0.6 LED CURRENT (A)
0.8
1
3477 TA05b
C4 33nF
R2 22k
3477 TA05a
LT3477
TYPICAL APPLICATIO S
Buck-Boost Mode LED Driver
LED2 LED1
VIN 2.7V TO 10V
LED BRIGHTNESS CONTROL 0mV TO 650mV
SHDN
90 85 VIN = 8V 80
EFFICIENCY (%)
75 VIN = 4.2V 70 65 60 55 50 0 0.2 0.4 0.6 IOUT (A) 0.8 1.0
3477 TA06b
U
L1 4.7H
D1
R1 0.1 R3 200k
C1 3.3F
ISP1 VIN IADJ1 IADJ2
SHDN
ISN1
SW1
FBN
LT3477
ISP2
ISN2 RT
VC VREF FBP GND SS
CC 10nF C1: TAIYO YUDEN LMK316BJ335ML C2: MURATA GRM31CR71E475KA88L D1: DIODES, INC. B320A L1: TOKO FDV0630-4R7M
C3 33nF
R2 18k
R4 10k
C2 4.7F
3477 TA06a
Efficiency
VIN (V) 2.7 3.6 4.2 5 8
IOUT (A) 0.57 0.74 0.83 0.93 1.0
3477fb
17
LT3477
PACKAGE DESCRIPTIO
4.50 0.05 3.10 0.05 2.45 0.05 (4 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS BOTTOM VIEW--EXPOSED PAD 4.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6) 2.45 0.10 (4-SIDES) 0.75 0.05 R = 0.115 TYP 19 20 0.38 0.10 1 2 PIN 1 NOTCH R = 0.30 TYP
NOTE: 1. DRAWING IS PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-1)--TO BE APPROVED 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
18
U
UF Package 20-Lead Plastic QFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1710)
0.70 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC
(UF20) QFN 10-04
0.200 REF 0.00 - 0.05
0.25 0.05 0.50 BSC
3477fb
LT3477
PACKAGE DESCRIPTIO
3.86 (.152)
6.60 0.10 4.50 0.10
SEE NOTE 4
RECOMMENDED SOLDER PAD LAYOUT
4.30 - 4.50* (.169 - .177)
0.09 - 0.20 (.0035 - .0079)
0.50 - 0.75 (.020 - .030)
NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE
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.
U
FE Package 20-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation CB
6.40 - 6.60* (.252 - .260) 3.86 (.152) 20 1918 17 16 15 14 13 12 11
2.74 (.108) 0.45 0.05 1.05 0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10
6.40 2.74 (.252) (.108) BSC
0.25 REF
1.20 (.047) MAX
0 - 8
0.65 (.0256) BSC
0.195 - 0.30 (.0077 - .0118) TYP
0.05 - 0.15 (.002 - .006)
FE20 (CB) TSSOP 0204
4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE
3477fb
19
LT3477
TYPICAL APPLICATIO
PVIN 32V
Buck Mode High Current LED Driver
C1 2.2F R1 0.1
LED1 1A LED4
* * *
EFFICIENCY (%)
ISP1 VIN 3.3V C3 3.3F SHDN VIN IADJ1 IADJ2 SHDN
ISN1
SW FBN
LT3477
VC RC 1k CC 4.7nF VREF FBP GND SS
C1: NIPPON UNITED CHEMICON NTS40X5R1H225M C2: TAIYO YUDEN GMK316BJ105ML C3: TAIYO YUDEN LMK316BJ475 L1: TOKO A814AY-330M D1: DIODES INC DFLS140
RELATED PARTS
PART NUMBER LT1618 LT3436 LTC 3453 LT3466
(R)
DESCRIPTION Constant Current, Constant Voltage 1.4MHz, High Efficiency Boost Regulator 3A (ISW), 800kHz, 34V Step-Up DC/DC Converter Synchronous Buck-Boost High Power White LED Driver Dual Constant Current, 2MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode 3A, 42V Full Featured Boost/Inverter Converter with Soft-Start Single Cell 350mA, 1.3MHz LED Driver
LT3479 LTC3490
20
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
U
Efficiency
LED STRING C2 1F
100 90 80
L1 33H D1 R3 280k
70 60 50 40
R4 10k ISP2 ISN2 RT
30 20 0
0.2 0.4 0.6 LED CURRENT (A) 0.8
1
3477 TA05b
C4 33nF
R2 22k
3477 TA07
COMMENTS VIN: 1.6V to 18V, VOUT(MAX) = 5.5V, IQ = 2.5mA, ISD < 1A, QFN16 Package VIN: 3V to 25V, VOUT(MAX) = 34V, IQ = 0.9mA, ISD < 6A, TSSOP16E Package VIN: 2.7V to 5.5V, VOUT(MAX) = 5.5V, IQ = 2.5mA, ISD < 1A, QFN16 Package VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD < 16A, DFN Package
VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD < 1A, DFN/TSSOP Packages VIN: 1V to 3.2V, VOUT(MAX) = 4.7V, ISD < 1A, DFN/SO8 Packages
3477fb LT 0207 REV B * PRINTED IN THE USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2005

▲Up To Search▲

Price & Availability of LT3477EUF

	To Download LT3477EUF Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .