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LT3970/LT3970-3.3/LT3970-5 40V, 350mA Step-Down Regulator with 2.5A Quiescent Current and Integrated Diodes FEATURES
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DESCRIPTION
The LT(R)3970 is an adjustable frequency monolithic buck switching regulator that accepts a wide input voltage range up to 40V, and consumes only 2.5A of quiescent current. A high efficiency switch is included on the die along with the catch diode, boost diode, and the necessary oscillator, control and logic circuitry. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping the output ripple below 5mV in a typical application. Current mode topology is used for fast transient response and good loop stability. A catch diode current limit provides protection against shorted outputs and overvoltage conditions. An enable pin with accurate threshold is available, producing a low shutdown current of 0.7A. A power good flag signals when VOUT reaches 90% of the programmed output voltage. The LT3970 is available in small 10-pin MSOP and 3mm x 2mm DFN packages.
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
n n n n n n n n n n n n
Low Ripple Burst Mode(R) Operation 2.5A IQ at 12VIN to 3.3VOUT Output Ripple < 5mVP-P Wide Input Voltage Range: 4.2V to 40V Operating Adjustable Switching Frequency: 200kHz to 2.2MHz Integrated Boost and Catch Diodes 350mA Output Current Fixed Output Voltages: 3.3V, 5V 1.8A IQ at 12VIN Accurate 1V Enable Pin Threshold Low Shutdown Current: IQ = 0.7A Internal Sense Limits Catch Diode Current Power Good Flag Output Voltage: 1.21V to 25V Internal Compensation Small 10-Pin MSOP and (3mm x 2mm) DFN Packages
APPLICATIONS
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Automotive Battery Regulation Power for Portable Products Industrial Supplies
TYPICAL APPLICATION
5V Step-Down Converter
90 VIN 6V TO 40V VIN OFF ON EN PG RT 226k f = 600kHz GND BOOST LT3970 SW BD 22pF 2.2F FB 316k 50
3490 TA01a
Efficiency
VIN = 12V 1000
0.22F 80 EFFICIENCY (%) 22H VOUT 5V 350mA 1M 22F
100 POWER LOSS (mW)
70
10
60
1
0.1
40 0.01
0.01 0.1 1 10 LOAD CURRENT (mA) 100
3970 TA01b
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LT3970/LT3970-3.3/LT3970-5 ABSOLUTE MAXIMUM RATINGS (Note 1)
VIN, EN Voltage .........................................................40V BOOST Pin Voltage ...................................................55V BOOST Pin Above SW Pin.........................................30V FB/VOUT, RT Voltage ....................................................6V PG, BD Voltage .........................................................30V Operating Junction Temperature Range (Note 2) LT3970E/LT3970E-X ........................... -40C to 125C LT3970I/LT3970I-X ............................ -40C to 125C LT3970H-X ......................................... -40C to 150C Storage Temperature Range................... -65C to 150C Lead Temperature (Soldering, 10 sec) MS Only ............................................................ 300C
PIN CONFIGURATION
TOP VIEW *FB/VOUT 1 EN 2 VIN 3 GND 4 GND 5 11 GND 10 RT 9 PG 8 BD 7 BOOST 6 SW *FB/VOUT EN VIN GND GND 1 2 3 4 5 TOP VIEW 10 9 8 7 6 RT PG BD BOOST SW
DDB PACKAGE 10-LEAD (3mm x 2mm) PLASTIC DFN JA = 76C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
MS PACKAGE 10-LEAD PLASTIC MSOP JA = 100C/W
*FB for LT3970, VOUT for LT3970-3.3, LT3970-5.
ORDER INFORMATION
LEAD FREE FINISH LT3970EDDB#PBF LT3970IDDB#PBF LT3970EMS#PBF LT3970IMS#PBF LT3970EDDB-3.3#PBF LT3970IDDB-3.3#PBF LT3970EMS-3.3#PBF LT3970IMS-3.3#PBF LT3970HMS-3.3#PBF LT3970EDDB-5#PBF LT3970IDDB-5#PBF LT3970EMS-5#PBF LT3970IMS-5#PBF LT3970HMS-5#PBF TAPE AND REEL LT3970EDDB#TRPBF LT3970IDDB#TRPBF LT3970EMS#TRPBF LT3970IMS#TRPBF LT3970EDDB-3.3#TRPBF LT3970IDDB-3.3#TRPBF LT3970EMS-3.3#TRPBF LT3970IMS-3.3#TRPBF LT3970HMS-3.3#TRPBF LT3970EDDB-5#TRPBF LT3970IDDB-5#TRPBF LT3970EMS-5#TRPBF LT3970IMS-5#TRPBF LT3970HMS-5#TRPBF PART MARKING* LFCZ LFCZ LTFDB LTFDB LFQH LFQH LTFQG LTFQG LTFQG LFQF LFQF LTFQD LTFQD LTFQD PACKAGE DESCRIPTION 10-Lead (3mm x 2mm) Plastic DFN 10-Lead (3mm x 2mm) Plastic DFN 10-Lead Plastic MSOP 10-Lead Plastic MSOP 10-Lead (3mm x 2mm) Plastic DFN 10-Lead (3mm x 2mm) Plastic DFN 10-Lead Plastic MSOP 10-Lead Plastic MSOP 10-Lead Plastic MSOP 10-Lead (3mm x 2mm) Plastic DFN 10-Lead (3mm x 2mm) Plastic DFN 10-Lead Plastic MSOP 10-Lead Plastic MSOP 10-Lead Plastic MSOP TEMPERATURE RANGE -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 150C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 150C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. 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/
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LT3970/LT3970-3.3/LT3970-5 ELECTRICAL CHARACTERISTICS
PARAMETER Minimum Input Voltage Quiescent Current from VIN VEN Low VEN High VEN High, -40C to 125C VEN High, -40C to 150C
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 12V, VBD = 3.3V unless otherwise noted. (Note 2)
CONDITIONS
l
MIN
TYP 4 0.7 1.7
MAX 4.2 1.2 2.7 3.5 4 1.225 1.235 3.34 3.366 3.366 5.06 5.1 5.1 20 0.01 2.64 960 240 865 500 2 2 2 1.8 10 30 1.06 1.07 160 13.5 1
UNITS V A A A A V V V V V V V V nA %/V MHz kHz kHz mA mA mV A mV A mV A V mA nA V V mV mV mV % % A A ns
l l l
LT3970 Feedback Voltage LT3970-3.3 Output Voltage -40C to 125C -40C to 150C LT3970-5 Output Voltage -40C to 125C -40C to 150C LT3970 FB Pin Bias Current (Note 3) FB/Output Voltage Line Regulation Switching Frequency VFB = 1.21V 4.2V < VIN < 40V RT = 41.2k, VIN = 6V RT = 158k, VIN = 6V RT = 768k, VIN = 6V VIN = 5V, VFB = 0V VIN = 5V ISW = 200mA ISCH = 100mA, VIN = VBD = NC VSW = 12V ISCH = 50mA, VIN = NC, VBOOST = 0V VREVERSE = 12V VIN = 5V ISW = 200mA, VBOOST = 15V VEN = 12V EN Rising, VIN 4.2V EN Rising, VIN 4.2V VFB Rising VFB Rising VPG = 3V VPG = 0.4V VIN = 10V
1.195 1.185 3.26 3.234 3.217 4.94 4.9 4.875
1.21 1.21 3.3 3.3 3.3 5 5 5 0.1 0.0002
l l l l l
1.76 640 160 535 350
2.25 800 200 700 400 175 0.05 650 0.05 875 0.02
Switch Current Limit Catch Schottky Current Limit Switch VCESAT Switch Leakage Current Catch Schottky Forward Voltage Catch Schottky Reverse Leakage Boost Schottky Forward Voltage Boost Schottky Reverse Leakage Minimum Boost Voltage (Note 4) BOOST Pin Current EN Pin Current LT3970 EN Voltage Threshold LT3970-X EN Voltage Threshold EN Voltage Hysteresis LT3970 PG Threshold Offset from Feedback Voltage LT3970 PG Hysteresis LT3970-X PG Threshold Offset from Output Voltage LT3970-X PG Hysteresis as % of Output Voltage PG Leakage PG Sink Current Minimum Switch On-Time Minimum Switch Off-Time
l
1.4 7 1
l l
0.94 0.93 80 6.5
1 1 30 120 12 10 1.0 0.01
l l
30
80 90 100 160
ns
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LT3970/LT3970-3.3/LT3970-5 ELECTRICAL CHARACTERISTICS
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 LT3970E is guaranteed to meet performance specifications from 0C to 125C junction temperature. Specifications over the -40C to 125C operating junction temperature range are assured by design, characterization, and correlation with statistical process controls. The LT3970I is guaranteed over the full -40C to 125C operating junction temperature range. The LT3970H is guaranteed over the full -40C to 150C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125C. Note 3: Bias current flows into the FB pin. Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch.
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25C, unless otherwise noted.
Efficiency, VOUT = 3.3V
90 80 70 EFFICIENCY (%) 60 50 40 30 20 0.01 FRONT PAGE APPLICATION VOUT = 3.3V R1 = 1M R2 = 576k 0.1 1 10 LOAD CURRENT (mA) 100
3970 G01
Efficiency, VOUT = 5V
90 FRONT PAGE APPLICATION 80 VIN = 24V EFFICIENCY (%) 70 60 50 40 30 0.01 VIN = 36V 1.220
LT3970 Feedback Voltage
FEEDBACK VOLTAGE (V) 100
3970 G02
VIN = 24V
VIN = 12V
VIN = 12V
1.215 1.210
VIN = 36V
1.205
1.200
0.1
1 10 LOAD CURRENT (mA)
1.195 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3970 G03
LT3970-3.3 Output Voltage
3.32 5.04
LT3970-5 Output Voltage
4.0
No-Load Supply Current
FRONT PAGE APPLICATION VOUT = 3.3V R1 = 1M R2 = 576k LT3970-5 LT3970-3.3
3.31 OUTPUT VOLTAGE (V) 3.30 OUTPUT VOLTAGE (V)
5.02 5.00 SUPPLY CURRENT (A)
3.5
3.0
3.29
4.98
2.5
3.28
4.96
2.0
3.27 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3970 G03a
4.94 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3970 G03b
1.5
5
10
25 30 15 20 INPUT VOLTAGE (V)
35
40
3970 G04
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LT3970/LT3970-3.3/LT3970-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25C, unless otherwise noted.
No-Load Supply Current
15 FRONT PAGE APPLICATION VIN = 12V VOUT = 3.3V R1 = 1M R2 = 576k 550
Maximum Load Current
FRONT PAGE APPLICATION VOUT = 3.3V TYPICAL LOAD CURRENT (mA) 600
Maximum Load Current
FRONT PAGE APPLICATION VOUT = 5V TYPICAL
12 SUPPLY CURRENT (A) 9
LOAD CURRENT (mA)
500
550
500 MINIMUM
450
MINIMUM
6
450
400
3
400
0 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3970 G05
350 5 10 30 15 20 25 INPUT VOLTAGE (V) 35 40
350 5 10 30 15 20 25 INPUT VOLTAGE (V) 35 40
3870 G06
3870 G07
Maximum Load Current
600 LIMITED BY CURRENT LIMIT 500 400 300 200 100 FRONT PAGE APPLICATION VIN = 12V VOUT = 5V 0 25 50 75 100 125 150 TEMPERATURE (C)
3970 G08
Load Regulation
0.20 H GRADE LOAD REGULATION (%) 0.15 0.10 0.05 0 -0.05 -0.10 -0.15 FRONT PAGE APPLICATION REFERENCED FROM VOUT AT 100mA LOAD -0.20 50 100 150 200 250 300 350 0 LOAD CURRENT (mA)
3970 G09
Switch Current Limit
800 700 600 500 CATCH DIODE VALLEY CURRENT LIMIT 400 300 200 0 20 40 60 DUTY CYCLE (%) 80 100
3970 G10
LIMITED BY MAXIMUM JUNCTION TEMPERATURE; JA = 76C/W
0 -50 -25
SWITCH CURRENT LIMIT (mA)
LOAD CURRENT (mA)
SWITCH PEAK CURRENT LIMIT
Switch Current Limit
800 SWITCH PEAK CURRENT LIMIT SWITCH CURRENT LIMIT (mA) 700 600 500 CATCH DIODE VALLEY CURRENT LIMIT 400 300 200 -50 -25 FREQUENCY (MHz) 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 25 50 75 100 125 150 TEMPERATURE ( C)
3970 G11
Switching Frequency
200 SWITCH ON-TIME/SWITCH OFF-TIME (ns) 180 160 140 120 100 80 60 40 20
Minimum Switch On-Time/Switch Off-Time
LOAD CURRENT = 175mA
MINIMUM OFF-TIME
MINIMUM ON-TIME
0 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3970 G12
0 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3970 G13
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LT3970/LT3970-3.3/LT3970-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25C, unless otherwise noted.
Switch VCESAT (ISW = 200mA) vs Temperature
250 400
Switch VCESAT
16 14
BOOST Pin Current
SWITCH VCESAT (mV)
200
SWITCH VCESAT (mV)
300
BOOST PIN CURRENT (mA)
12 10 8 6 4
200
150
100
100 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3970 G14
0
2 0 100 300 400 200 SWITCH CURRENT (mA) 500
3970 G15
0
100
200 300 400 SWITCH CURRENT (mA)
500
3970 G16
Minimum Input Voltage, VOUT = 3.3V
5.0 FRONT PAGE APPLICATION VOUT = 3.3V 6.5
Minimum Input Voltage, VOUT = 5V
FRONT PAGE APPLICATION VOUT = 5V BOOST DIODE VF (V) TO START 5.5 TO RUN 5.0 1.2 1.0 0.8 0.6 0.4 0.2 0 0 50 100 150 200 250 LOAD CURRENT (mA) 300 350
Boost Diode Forward Voltage
4.5 INPUT VOLTAGE (V) INPUT VOLTAGE (V)
6.0
4.0 TO START/RUN 3.5
3.0
4.5
-50C 25C 125C 150C 0 50 100 150 BOOST DIODE CURRENT (mA) 200
3970 G19
2.5 0 50
100 150 200 250 LOAD CURRENT (mA)
300
350
4.0
3970 G17
3970 G18
Catch Diode Forward Voltage
1.0 20
Catch Diode Leakage
VR = 12V 92
Power Good Threshold
CATCH DIODE LEAKAGE (A)
0.8 CATCH DIODE VF (V)
16 91 12 THRESHOLD (%) 0 25 50 75 100 125 150 TEMPERATURE (C)
3970 G21
0.6
90
0.4 -50C 25C 125C 150C 0 100 300 200 CATCH DIODE CURRENT (mA) 400
3970 G20
8
89 4
0.2
0
0 -50 -25
88 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3970 G22
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LT3970/LT3970-3.3/LT3970-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25C, unless otherwise noted.
EN Threshold
1.050 VOUT 100mV/DIV VOUT 100mV/DIV
Transient Load Response; Load Current is Stepped from 10mA (Burst Mode Operation) to 110mA
Transient Load Response; Load Current is Stepped from 100mA to 200mA
THRESHOLD VOLTAGE (V)
1.025
1.000
IL 100mA/DIV
IL 100mA/DIV
0.975
100s/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V 0 25 50 75 100 125 150 TEMPERATURE (C)
3970 G23
3970 G24
100s/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V
3970 G25
0.950 -50 -25
Switching Waveforms, Burst Mode Operation
VSW 5V/DIV IL 100mA/DIV VOUT 5mV/DIV 2s/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V ILOAD = 10mA
3970 G26
Switching Waveforms, Full Frequency Continuous Operation
VSW 5V/DIV IL 200mA/DIV VOUT 5mV/DIV 1s/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V ILOAD = 350mA
3970 G27
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LT3970/LT3970-3.3/LT3970-5 PIN FUNCTIONS
FB (Pin 1, LT3970 Only): The LT3970 Regulates the FB Pin to 1.21V. Connect the feedback resistor divider tap to this pin. VOUT (Pin 1, LT3970-3.3 and LT3970-5 Only): The LT39703.3 and LT3970-5 regulate the VOUT Pin to 3.3V and 5V respectively. This pin connects to the internal feedback divider that programs the fixed output voltage. EN (Pin 2): The part is in shutdown when this pin is low and active when this pin is high. The hysteretic threshold voltage is 1V going up and 0.97V going down. Tie to VIN if shutdown feature is not used. The EN threshold is accurate only when VIN is above 4.2V. If VIN is lower than 4.2V, ground EN to place the part in shutdown. VIN (Pin 3): The VIN pin supplies current to the LT3970's internal circuitry and to the internal power switch. This pin must be locally bypassed. GND (Pins 4, 5, Exposed Pad (Pin 11, DFN Only)): Ground. Must be soldered to PCB. SW (Pin 6): The SW pin is the output of an internal power switch. Connect this pin to the inductor. BOOST (Pin 7): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. BD (Pin 8): This pin connects to the anode of the boost diode. This pin also supplies current to the LT3970's internal regulator when BD is above 3.2V. PG (Pin 9): The PG pin is the open-drain output of an internal comparator. PG remains low until the FB pin is within 10% of the final regulation voltage. PG is valid when VIN is above 4.2V and EN is high. RT (Pin 10): A resistor is tied between RT and ground to set the switching frequency.
BLOCK DIAGRAM
VIN C1 3 VIN
INTERNAL 1.21V REF 1V EN
+ -
SHDN
- +
DBOOST SLOPE COMP SWITCH LATCH R
BD
8
2
BOOST
7
10 RT 9
RT PG
OSCILLATOR 200kHz TO 2.2MHz
Q S SW
C3 L1 6 C2 VOUT
+ -
1.09V
+
ERROR AMP
VC
Burst Mode DETECT
-
R2 LT3970 ONLY FB R2 1 R1 LT3970-3.3 LT3970-5 ONLY* 1 R1
DCATCH
GND (4, 5)
VOUT
3990 BD
*LT3970-3.3: R1 = 12.65M, R2 = 7.35M LT3970-5: R1 = 15.15M, R2 = 4.85M
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LT3970/LT3970-3.3/LT3970-5 OPERATION
The LT3970 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, sets an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC (see Block Diagram). An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC node. If the error amplifier's output increases, more current is delivered to the output; if it decreases, less current is delivered. Another comparator monitors the current flowing through the catch diode and reduces the operating frequency when the current exceeds the 400mA bottom current limit. This foldback in frequency helps to control the output current in fault conditions such as a shorted output with high input voltage. Maximum deliverable current to the output is therefore limited by both switch current limit and catch diode current limit. An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the VIN pin, but if the BD pin is connected to an external voltage higher than 3.2V, bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. If the EN pin is low, the LT3970 is shut down and draws 0.7A from the input. When the EN pin exceeds 1V, the switching regulator will become active. The switch driver operates from either VIN or from the BOOST pin. An external capacitor is used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for efficient operation. To further optimize efficiency, the LT3970 automatically switches to Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down reducing the input supply current to 1.7A. The LT3970 contains a power good comparator which trips when the FB pin is at 90% of its regulated value. The PG output is an open-drain transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT3970 is enabled and VIN is above 4.2V.
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LT3970/LT3970-3.3/LT3970-5 APPLICATIONS INFORMATION
FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to: R1=R2 VOUT -1 1.21 where VIN is the typical input voltage, VOUT is the output voltage, VD is the integrated catch diode drop (~0.7V), and VSW is the internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to accommodate high VIN/VOUT ratio. Lower frequency also allows a lower dropout voltage. The input voltage range depends on the switching frequency because the LT3970 switch has finite minimum on and off times. The switch can turn on for a minimum of ~150ns and turn off for a minimum of ~160ns (note that the minimum on-time is a strong function of temperature). This means that the minimum and maximum duty cycles are: DCMIN = fSW * tON(MIN) DCMAX = 1 - fSW * tOFF(MIN) where fSW is the switching frequency, the tON(MIN) is the minimum switch on-time (~150ns), and the tOFF(MIN) is the minimum switch off-time (~160ns). These equations show that duty cycle range increases when switching frequency is decreased. A good choice of switching frequency should allow adequate input voltage range (see next section) and keep the inductor and capacitor values small. Input Voltage Range The minimum input voltage is determined by either the LT3970's minimum operating voltage of 4.2V or by its maximum duty cycle (as explained in previous section). The minimum input voltage due to duty cycle is: VIN(MIN) = VOUT + VD -V +V 1- fSW * tOFF(MIN) D SW
Reference designators refer to the Block Diagram. Note that choosing larger resistors will decrease the quiescent current of the application circuit. Setting the Switching Frequency The LT3970 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.2MHz by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching frequency is in Table 1.
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz) 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 RT VALUE (k) 768 499 357 280 226 158 124 100 80.6 68.1 57.6 49.9 42.2
Operating Frequency Trade-Offs Selection of the operating frequency is a trade-off between efficiency, component size, minimum dropout voltage and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efficiency, lower maximum input voltage, and higher dropout voltage. The highest acceptable switching frequency (fSW(MAX)) for a given application can be calculated as follows: fSW(MAX ) = VOUT + VD tON(MIN) ( VIN - VSW + VD )
where VIN(MIN) is the minimum input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.7V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT), and tOFF(MIN) is the minimum switch off-time (160ns). Note that higher switching frequency will increase the minimum input voltage. If a lower dropout voltage is desired, a lower switching frequency should be used.
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LT3970/LT3970-3.3/LT3970-5 APPLICATIONS INFORMATION
The highest allowed V IN during normal operation (VIN(OP-MAX)) is limited by minimum duty cycle and can be calculated by the following equation: VIN(OP-MAX ) = VOUT + VD -V +V fSW * tON(MIN) D SW where VD is the voltage drop of the catch diode (~0.7V), L is in H and fSW is in MHz. The inductor's RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions (start-up or short circuit) and high input voltage (>30V), the saturation current should be above 500mA. To keep the efficiency high, the series resistance (DCR) should be less than 0.1, and the core material should be intended for high frequency applications. Table 2 lists several vendors and suitable types. This simple design guide will not always result in the optimum inductor selection for a given application. As a general rule, lower output voltages and higher switching frequency will require smaller inductor values. If the application requires less than 350mA load current, then a lesser inductor value may be acceptable. This allows use of a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. There are several graphs in the Typical Performance Characteristics section of this data sheet that show the maximum load current as a function of input voltage for several popular output voltages. Low inductance may result in discontinuous mode operation, which is acceptable but reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Linear Technology Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. See Application Note 19. Input Capacitor Bypass the input of the LT3970 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 1F to 4.7F ceramic capacitor is adequate to bypass the LT3970 and will easily handle
where tON(MIN) is the minimum switch on-time (~150ns). However, the circuit will tolerate inputs up to the absolute maximum ratings of the VIN and BOOST pins, regardless of chosen switching frequency. During such transients where VIN is higher than VIN(OP-MAX), the switching frequency will be reduced below the programmed frequency to prevent damage to the part. The output voltage ripple and inductor current ripple may also be higher than in typical operation, however the output will still be in regulation. Inductor Selection For a given input and output voltage, the inductor value and switching frequency will determine the ripple current. The ripple current increases with higher VIN or VOUT and decreases with higher inductance and faster switching frequency. A good starting point for selecting the inductor value is: L=3 VOUT + VD fSW
Table 2. Inductor Vendors
VENDOR Coilcraft Sumida Toko Wurth Elektronik Coiltronics Murata URL www.coilcraft.com www.sumida.com www.tokoam.com www.we-online.com www.cooperet.com www.murata.com
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LT3970/LT3970-3.3/LT3970-5 APPLICATIONS INFORMATION
the ripple current. Note that larger input capacitance is required when a lower switching frequency is used (due to longer on-times). If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic capacitor. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3970 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 1F capacitor is capable of this task, but only if it is placed close to the LT3970 (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3970. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3970 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3970's voltage rating. This situation is easily avoided (see the Hot Plugging Safely section). Output Capacitor and Output Ripple The output capacitor has two essential functions. It stores energy in order to satisfy transient loads and stabilize the LT3970's control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: 50 COUT = VOUT * fSW where fSW is in MHz and COUT is the recommended output capacitance in F Use X5R or X7R types. This choice will . provide low output ripple and good transient response. Transient performance can be improved with a higher value capacitor if combined with a phase lead capacitor (typically 22pF) between the output and the feedback pin. A lower value of output capacitor can be used to save space and cost but transient performance will suffer. The second function is that the output capacitor, along with the inductor, filters the square wave generated by the LT3970 to produce the DC output. In this role it determines the output ripple, so low impedance (at the switching frequency) is important. The output ripple decreases with increasing output capacitance, down to approximately 1mV. See Figure 1. Note that a larger phase lead capacitor should be used with a large output capacitor.
18 WORST-CASE OUTPUT RIPPLE (mV) 16 14 12 10 8 6 4 2 0 0 20 60 40 COUT (F) 80 100
3970 F01
FRONT PAGE APPLICATION CLEAD = 47pF FOR COUT 47F
VIN = 24V VIN = 12V
Figure 1. Worst-Case Output Ripple Across Full Load Range
When choosing a capacitor, look carefully through the data sheet to find out what the actual capacitance is under operating conditions (applied voltage and temperature). A physically larger capacitor or one with a higher voltage rating may be required. Table 3 lists several capacitor vendors.
Table 3. Recommended Ceramic Capacitor Vendors
MANUFACTURER AVX Murata Taiyo Yuden Vishay Siliconix TDK WEBSITE www.avxcorp.com www.murata.com www.t-yuden.com www.vishay.com www.tdk.com
Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT3970 due to their piezoelectric nature. When in Burst Mode operation, the LT3970's switching frequency depends on the load current, and at very light loads the LT3970 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT3970
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12
LT3970/LT3970-3.3/LT3970-5 APPLICATIONS INFORMATION
SWITCHING FREQUENCY (kHz)
operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT3970. As previously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3970 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3970's rating. This situation is easily avoided (see the Hot Plugging Safely section). Low Ripple Burst Mode Operation To enhance efficiency at light loads, the LT3970 operates in low ripple Burst Mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode operation, the LT3970 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor. Because the LT3970 delivers power to the output with single, low current pulses, the output ripple is kept below 5mV for a typical application. See Figure 2. As the load current decreases towards a no load condition, the percentage of time that the LT3970 operates in sleep mode increases and the average input current is greatly reduced resulting in high efficiency even at very low loads. Note that during Burst Mode operation, the switching frequency will be lower than the programmed switching frequency. See Figure 3.
VSW 5V/DIV IL 100mA/DIV VOUT 5mV/DIV 2s/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V ILOAD = 10mA
3970 F02
700 600 500 400 300 200 100 0 0
FRONT PAGE APPLICATION
50
100 150 200 250 LOAD CURRENT (mA)
300
350
3070 F03
Figure 3. Switching Frequency in Burst Mode Operation
At higher output loads (above ~45mA for the front page application) the LT3970 will be running at the frequency programmed by the RT resistor, and will be operating in standard PWM mode. The transition between PWM and low ripple Burst Mode is seamless, and will not disturb the output voltage. BOOST and BD Pin Considerations Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22F capacitor will work well. Figure 4 shows two ways to arrange the boost circuit. The BOOST pin must be more than 1.9V above the SW pin for best efficiency. For outputs of 2.2V and above, the standard circuit (Figure 4a) is best. For outputs between 2.2V and 2.5V, use a 0.47F boost capacitor. For output voltages below 2.2V, the boost diode can be tied to the input (Figure 4b), or to another external supply greater than 2.2V. However, the circuit in Figure 4a is more efficient because the BOOST pin current and BD pin quiescent current come from a lower voltage source. Also, be sure that the maximum voltage ratings of the BOOST and BD pins are not exceeded. The minimum operating voltage of an LT3970 application is limited by the minimum input voltage (4.2V) and by the maximum duty cycle as outlined in a previous section. For output voltages greater than 3.4V, the minimum input voltage is also limited by the boost circuit for proper start-up. If the input voltage is ramped slowly, the boost capacitor
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Figure 2. Burst Mode Operation
13
LT3970/LT3970-3.3/LT3970-5 APPLICATIONS INFORMATION
VOUT BD VIN VIN BOOST SW GND INPUT VOLTAGE (V) LT3970 C3 4.5 5.0 FRONT PAGE APPLICATION VOUT = 3.3V
4.0 TO START/RUN 3.5
(4a) For VOUT 2.2V
3.0
BD VIN VIN BOOST LT3970 SW GND C3 VOUT
2.5 0 6.5 50
100 150 200 250 LOAD CURRENT (mA)
300
350
FRONT PAGE APPLICATION VOUT = 5V TO START
6.0
3970 F04
INPUT VOLTAGE (V)
(4b) For VOUT < 2.2V; VIN < 27V
5.5 TO RUN 5.0
Figure 4. Two Circuits for Generating the Boost Voltage
may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. Figure 5 shows a plot of minimum load to start and to run as a function of input voltage. In many cases the discharged output capacitor will present a load to the switcher, which will allow it to start. The plots show the worst-case situation where VIN is ramping very slowly. For lower start-up voltage, the boost diode can be tied to VIN; however, this restricts the input range to one-half of the absolute maximum rating of the BOOST pin. Enable Pin The LT3970 is in shutdown when the EN pin is low and active when the pin is high. The rising threshold of the EN comparator is 1V, with a 30mV hysteresis. This threshold is accurate when VIN is above 4.2V. If VIN is lower than 4.2V, tie EN pin to GND to place the part in shutdown.
4.5
4.0 0 50
100 150 200 250 LOAD CURRENT (mA)
300
350
3970 F05
Figure 5. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
Adding a resistor divider from VIN to EN programs the LT3970 to regulate the output only when VIN is above a desired voltage (see Figure 6). This threshold voltage, VIN(EN), can be adjusted by setting the values R3 and R4 such that they satisfy the following equation: VIN(EN) = R3 + R4 * 1V R4
where output regulation should not start until VIN is above VIN(EN). Note that due to the comparator's hysteresis, regulation will not stop until the input falls slightly below VIN(EN).
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LT3970/LT3970-3.3/LT3970-5 APPLICATIONS INFORMATION
160 VIN R3 EN R4
3970 F06
VIN 1V
INPUT CURRENT (A)
LT3970
120 80 40 0 4
VIN(EN) = 6V R3 = 5M R4 = 1M
+ -
SHDN
OUTPUT VOLTAGE (V)
Figure 6. Enable
3 2 1 0 0 1 2 3 4 5 6 7 8 INPUT VOLTAGE (V) 160
Be aware that while VIN is below 4.2V, the input current may rise up to several hundred A and the part may begin to switch while the internal circuitry starts up. Figure 7 shows the startup behavior of a typical application with different programmed VIN(EN). Shorted and Reversed Input Protection If the inductor is chosen so that it won't saturate excessively, a LT3970 buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT3970 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode ORed with the LT3970's output. If the VIN pin is allowed to float and the EN pin is held high (either by a logic signal or because it is tied to VIN), then the LT3970's internal circuitry will pull its quiescent current through its SW pin. This is fine if the system can tolerate a few A in this state. If the EN pin is grounded, the SW pin current will drop to 0.7A. However, if the VIN pin is grounded while the output is held high, regardless of EN, parasitic diodes inside the LT3970 can pull current from the output through the SW pin and the VIN pin. Figure 8 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input.
INPUT CURRENT (A)
120 80 40 0 4
VIN(EN) = 12V R3 = 11M R4 = 1M
OUTPUT VOLTAGE (V)
3 2 1 0 0 2 4 6 8 10 12 14
3970 F07
INPUT VOLTAGE (V)
Figure 7. VIN Start-Up of Front Page Application with VOUT = 3.3V, No-Load Current, and VIN(EN) programmed as in Figure 6.
D4 MBRS140 VIN VIN EN GND
BD BOOST LT3970 SW FB VOUT
+
BACKUP
3970 F08
Figure 8. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output. It Also Protects the Circuit from a Reversed Input. The LT3970 Runs Only when the Input is Present
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LT3970/LT3970-3.3/LT3970-5 APPLICATIONS INFORMATION
PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 9 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3970's VIN and SW pins, the internal catch diode and the input capacitor. The loop formed by these components should be as small as possible. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane below these components. The SW and BOOST nodes should be as small as possible. Finally, keep the FB nodes small so that the ground traces will shield them from the SW and BOOST nodes. The Exposed Pad on the bottom of the DFN package must be soldered to ground so that the pad acts as a heat sink. To keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT3970 to additional ground planes within the circuit board and on the bottom side.
GND GND
with stray inductance in series with the power source, forms an under damped tank circuit, and the voltage at the VIN pin of the LT3970 can ring to twice the nominal input voltage, possibly exceeding the LT3970's rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3970 into an energized supply, the input network should be designed to prevent this overshoot. See Linear Technology Application Note 88 for a complete discussion. High Temperature Considerations For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT3970. The Exposed Pad on the bottom of the DFN package must be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the LT3970. Placing additional vias can reduce thermal resistance further. In the MSOP package, the copper lead frame is fused to GND (Pin 5) so place thermal vias near this pin. The maximum load current should be derated as the ambient temperature approaches the maximum junction rating. Power dissipation within the LT3970 can be estimated by calculating the total power loss from an efficiency measurement and subtracting inductor loss. The die temperature is calculated by multiplying the LT3970 power dissipation by the thermal resistance from junction to ambient. Finally, be aware that at high ambient temperatures the internal Schottky diode will have significant leakage current (see Typical Performance Characteristics) increasing the quiescent current of the LT3970 converter. Other Linear Technology Publications Application Notes 19, 35 and 44 contain more detailed descriptions and design information for buck regulators and other switching regulators. The LT1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. Design Note 100 shows how to generate a bipolar output supply using a buck regulator.
1 EN VIN 2 3 4 5
10 9 8 7 6 PG
GND VIAS TO LOCAL GROUND PLANE VIAS TO VOUT
3970 F09
VOUT
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3970 circuits. However, these capacitors can cause problems if the LT3970 is plugged into a live supply. The low loss ceramic capacitor, combined
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16
LT3970/LT3970-3.3/LT3970-5 TYPICAL APPLICATIONS
3.3V Step-Down Converter
VIN 4.2V TO 40V VIN OFF ON EN PG RT 226k f = 600kHz GND BOOST LT3970 SW BD 22pF C1 2.2F FB R2 576k
3490 TA02
5V Step-Down Converter
VIN 6V TO 40V VIN VOUT 3.3V 350mA R1 1M C2 22F C1 2.2F 226k f = 600kHz OFF ON EN PG RT GND BOOST LT3970 SW BD 22pF FB R2 316k
3490 TA03
C3 0.22F L1 22H
C3 0.22F L1 22H VOUT 5V 350mA R1 1M C2 22F
3.3V Step-Down Converter
VIN 4.2V TO 40V VIN OFF ON EN PG RT 226k f = 600kHz GND BOOST 22H SW BD VOUT VOUT 3.3V 350mA VIN 6V TO 40V
5V Step-Down Converter
0.22F LT3970-3.3
0.22F VIN OFF ON EN PG RT 226k GND BOOST 22H BD VOUT SW VOUT 5V 350mA LT3970-5
2.2F
22F
2.2F
22F
3490 TA03a
f = 600kHz
3490 TA03b
2.5V Step-Down Converter
VIN 4.2V TO 40V VIN OFF ON EN PG RT 226k f = 600kHz GND BOOST LT3970 SW BD 47pF C1 2.2F FB R2 931k
3490 TA04
1.8V Step-Down Converter
VIN 4.2V TO 27V VIN VOUT 2.5V 350mA R1 1M C2 47F C1 2.2F 226k f = 600kHz OFF ON EN BD PG RT GND BOOST LT3970 SW 47pF FB R2 1M
3490 TA05
C3 0.47F L1 15H
C3 0.22F L1 10H VOUT 1.8V 350mA R1 487k C2 47F
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17
LT3970/LT3970-3.3/LT3970-5 TYPICAL APPLICATIONS
12V Step-Down Converter
VIN 14V TO 40V VIN OFF ON EN PG RT 226k f = 600kHz GND BOOST LT3970 SW BD 22pF C1 2.2F FB R2 113k
3490 TA06
5V, 2MHz Step-Down Converter
VIN 8.5V TO 16V TRANSIENTS TO 40V VOUT 12V 350mA R1 1M C2 22F VIN OFF ON EN PG RT 49.9k f = 2MHz GND BOOST LT3970 SW BD 22pF C1 1F FB R2 316k
3490 TA07
C3 0.1F L1 33H
C3 0.1F L1 10H VOUT 5V 350mA R1 1M C2 10F
5V Step-Down Converter with Reduced Input Current During Start-Up
k VIN 6V TO 40V 0.22F VIN BOOST LT3970 EN PG RT 226k f = 600kHz GND SW BD 22pF 2.2F FB 316k
3490 TA08a
+ -
5M
22H
1M
VOUT 5V 350mA 1M 22F
Input Current During Start-Up
4.5 4.0 3.5 INPUT CURRENT (mA) 3.0 2.5 2.0 1.5 1.0 0.5 0 -0.5 0 2 6 8 4 INPUT VOLTAGE (V) 10 12 INPUT CURRENT DROPOUT CONDITIONS FRONT PAGE APPLICATION FRONT PAGE APPLICATION WITH EN PROGRAMMED TO 6V
Start-Up from High Impedance Input Source
EN PROGRAMMED TO 6V VIN 5V/DIV VOUT 2V/DIV
3970 TA08c
5ms/DIV FRONT PAGE APPLICATION VOUT = 5V 1k INPUT SOURCE RESISTANCE 2.5mA LOAD
3970 TA08b
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18
LT3970/LT3970-3.3/LT3970-5 PACKAGE DESCRIPTION
DDB Package 10-Lead Plastic DFN (3mm x 2mm)
(Reference LTC DWG # 05-08-1722 Rev O)
0.64 0.05 (2 SIDES) 0.70 0.05 2.55 0.05 1.15 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.39 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229 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 PIN 1 BAR TOP MARK (SEE NOTE 6) 2.00 0.10 (2 SIDES) 0.64 0.05 (2 SIDES) 5 0.25 0.05 2.39 0.05 (2 SIDES) BOTTOM VIEW--EXPOSED PAD PIN 1 R = 0.20 OR 0.25 45 CHAMFER
(DDB10) DFN 0905 REV O
3.00 0.10 (2 SIDES)
R = 0.05 TYP
R = 0.115 TYP 6
0.40 0.10 10
1
0.200 REF
0.75 0.05
0.50 BSC
0 - 0.05
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LT3970/LT3970-3.3/LT3970-5 PACKAGE DESCRIPTION
MS Package 10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661 Rev E)
0.889 (.035
0.127 .005)
5.23 (.206) MIN
3.20 - 3.45 (.126 - .136) 3.00 0.102 (.118 .004) (NOTE 3)
0.50 0.305 0.038 (.0197) (.0120 .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT
10 9 8 7 6
0.497 0.076 (.0196 .003) REF
0.254 (.010) GAUGE PLANE
DETAIL "A" 0 - 6 TYP
4.90 0.152 (.193 .006)
3.00 0.102 (.118 .004) (NOTE 4)
12345 0.53 0.152 (.021 .006) DETAIL "A" 0.18 (.007) SEATING PLANE 0.17 -0.27 (.007 - .011) TYP 0.1016 (.004 0.0508 .002) 1.10 (.043) MAX 0.86 (.034) REF
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.50 (.0197) BSC
MSOP (MS) 0307 REV E
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20
LT3970/LT3970-3.3/LT3970-5 REVISION HISTORY
REV A DATE 5/10 DESCRIPTION Added LT3970-3.3 and LT3970-5 PAGE NUMBER 1 - 22
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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.
21
LT3970/LT3970-3.3/LT3970-5 RELATED PARTS
PART NUMBER LT3971 LT3991 LT3689 DESCRIPTION 38V, 1.2A (IOUT), IQ = 2.8A High Efficiency Step-Down DC/DC Converter 55V, 1.2A (IOUT), IQ = 2.8A High Efficiency Step-Down DC/DC Converter 36V, 60V Transient Protection, 800mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with POR Reset and Watchdog Timer 36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode(R) Operation 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter 34V with Transient Protection to 36V, 2A (IOUT), 2.8MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation 60V, 1.2A (IOUT), 200/500kHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation COMMENTS VIN: 4.3V to 38V, VOUT(MIN) = 1.2V, IQ = 2.8A, ISD < 1A, MSOP-10E, 3mm x 3mm DFN-10 VIN: 4.3V to 55V, VOUT(MIN) = 1.2V, IQ = 2.8A, ISD < 1A, MSOP-10E, 3mm x 3mm DFN-10 VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 75A, ISD < 1A, 3mm x 3mm QFN-16 VIN: 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 75A, ISD < 1A, 3mm x 3mm DFN-12 VIN: 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70A, ISD < 1A, 3mm x 3mm DFN-10, MSOP-10E VIN: 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70A, ISD < 1A, 3mm x 3mm DFN-10, MSOP-10E VIN: 3.6V to 34V, VOUT(MIN) = 1.26V, IQ = 50A, ISD < 1A, 3mm x 3mm DFN-10, MSOP-10E VIN: 3.3V to 60V, VOUT(MIN) = 1.20V, IQ = 100A, ISD < 1A, TSSOP-16E
LT3682 LT3480 LT3685 LT3481 LT1976/LT1977
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22 Linear Technology Corporation
(408) 432-1900
LT 0510 REV A * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
FAX: (408) 434-0507 www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2009

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