LINEAR TECHNOLOGY LT3684 Technical data
36V, 2A, 2.8MHz Step-Down
Switching Regulator
FEATURES DESCRIPTION
LT3684
■
Wide Input Range: 3.6V to 34V Operating,
36V Maximum
■
2A Maximum Output Current
■
Adjustable Switching Frequency: 300kHz to 2.8MHz
■
Low Shutdown Current: IQ < 1µA
■
Integrated Boost Diode
■
Power Good Flag
■
Saturating Switch Design: 0.18Ω On-Resistance
■
1.265V Feedback Reference Voltage
■
Output Voltage: 1.265V to 20V
■
Soft-Start Capability
■
Small 10-Pin Thermally Enhanced MSOP and
(3mm × 3mm) DFN Packages
APPLICATIONS
■
Automotive Battery Regulation
■
Power for Portable Products
■
Distributed Supply Regulation
■
Industrial Supplies
■
Wall Transformer Regulation
The LT®3684 is an adjustable frequency (300kHz to 2.8MHz)
monolithic buck switching regulator that accepts input
voltages up to 34V (36V maximum). A high effi ciency
0.18Ω switch is included on the die along with a boost
Schottky diode and the necessary oscillator, control and
logic circuitry. Current mode topology is used for fast
transient response and good loop stability. The LT3684’s
high operating frequency allows the use of small, low cost
inductors and ceramic capacitors resulting in low output
ripple while keeping total solution size to a minimum.
The low current shutdown mode reduces input supply
current to less than 1µA while a resistor and capacitor on
the RUN/SS pin provide a controlled output voltage ramp
(soft-start). A power good fl ag signals when V
reaches
OUT
90% of the programmed output voltage. The LT3684 is
available in 10-Pin MSOP and 3mm × 3mm DFN packages
with Exposed Pads for low thermal resistance.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
3.3V Step-Down Converter
V
IN
4.5V TO
34V
OFF ON
16.2k
4.7µF
330pF
V
RUN/SS BOOST
V
C
RT
PG
60.4k
IN
LT3684
GND
BD
BIAS
SW
Effi ciency
V
OUT
3.3V
2A
0.47µF
FB
200k
4.7µH
324k
22µF
3684 TA01
90
80
70
60
EFFICIENCY (%)
50
40
30
0
VIN = 12V
= 3.3V
V
OUT
L = 4.7µH
f = 800kHz
0.5 1 1.5 2
LOAD CURRENT (A)
3684 TA01b
3684f
1
LT3684
ABSOLUTE MAXIMUM RATINGS
(Note 1)
VIN, RUN/SS Voltage .................................................36V
BOOST Pin Voltage ...................................................56V
BOOST Pin Above SW Pin .........................................30V
FB, RT, V
Voltage .......................................................5V
C
BIAS, PG, BD Voltage ................................................30V
Maximum Junction Temperature .......................... 125°C
Operating Temperature Range (Note 2)
LT3684E ............................................... –40°C to 85°C
LT3684I ............................................. –40°C to 125°C
Storage Temperature Range ................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
(MSE Only) ....................................................... 300°C
PACKAGE/ORDER INFORMATION
TOP VIEW
BD
1
BOOST
2
3
SW
4
V
IN
5
RUN/SS
10-LEAD (3mm × 3mm) PLASTIC DFN
T
JMAX
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
DD PACKAGE
= 125°C, θJA = 45°C/W, θJC = 10°C/W
10
RT
V
9
11
C
FB
8
7
BIAS
6
PG
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
BD
BOOST
SW
V
RUN/SS
T
JMAX
ORDER PART NUMBER DD PART MARKING* ORDER PART NUMBER MSE PART MARKING*
TOP VIEW
10
1
2
11
3
4
IN
5
MSE PACKAGE
10-LEAD PLASTIC MSOP
= 125°C, θJA = 45°C/W, θJC = 10°C/W
RT
9
V
C
FB
8
BIAS
7
PG
6
LT3684EDD
LT3684IDD
LCVT
LCVT
LT3684EMSE
LT3684IMSE
LTCVS
LTCVS
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 specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container.
ELECTRICAL CHARACTERISTICS
The
temperature range, otherwise specifi cations are at T
= 25°C. VIN = 10V, V
A
●
denotes the specifi cations which apply over the full operating
RUNS/SS
= 10V, V
BOOST
= 15V, V
= 3.3V unless otherwise
BIAS
noted. (Note 2)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage
Quiescent Current from V
IN
Quiescent Current from BIAS V
●
V
= 0.2V 0.01 0.5 µA
RUN/SS
= 3V, Not Switching
V
BIAS
= 0, Not Switching 1.2 2.0 mA
V
BIAS
= 0.2V 0.01 0.5 µA
RUN/SS
= 3V, Not Switching
V
BIAS
= 0, Not Switching 00.1 mA
V
BIAS
●
●
3 3.6 V
0.4 0.8 mA
0.85 1.5 mA
2
3684f
LT3684
ELECTRICAL CHARACTERISTICS
The
temperature range, otherwise specifi cations are at T
= 25°C. VIN = 10V, V
A
noted. (Note 2)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Bias Voltage 2.7 3 V
Feedback Voltage
FB Pin Bias Current (Note 3)
FB Voltage Line Regulation 4V < V
Error Amp g
Error Amp Gain 1000
Source Current 75 µA
V
C
Sink Current 100 µA
V
C
Pin to Switch Current Gain 3.5 A/V
V
C
Clamp Voltage 2V
V
C
Switching Frequency R
Minimum Switch Off-Time
Switch Current Limit Duty Cycle = 5% 3.1 3.6 4.0 A
Switch V
Boost Schottky Reverse Leakage V
Minimum Boost Voltage (Note 4)
BOOST Pin Current I
RUN/SS Pin Current V
RUN/SS Input Voltage High 2.5 V
RUN/SS Input Voltage Low 0.2 V
PG Threshold Offset from Feedback Voltage VFB Rising 100 mV
PG Hysteresis 10 mV
PG Leakage V
PG Sink Current V
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 LT3684E is guaranteed to meet performance specifi cations
from 0°C to 85°C. Specifi cations over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3684I specifi cations are
guaranteed over the –40°C to 125°C temperature range.
m
CESAT
IN
= 8.66k
T
R
= 29.4k
T
R
= 187k
T
ISW = 2A 360 mV
= 10V, V
SW
= 1A 18 30 mA
SW
RUN/SS
= 5V 0.1 1 µA
PG
= 0.4V
PG
●
denotes the specifi cations which apply over the full operating
= 10V V
RUNS/SS
< 34V 0.002 0.02 %/V
= 0V 0.02 2 µA
BIAS
= 2.5V 5 10 µA
Note 3: Bias current measured in regulation. Bias current fl ows into the FB
pin.
Note 4: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
BOOST
= 15V, V
●
●
●
●
●
= 3.3V unless otherwise
BIAS
1.25
1.24
2.7
1.25
250
100 300 µA
1.265
1.265
30 100 nA
330 µMho
3.0
1.4
300
100 150 nS
1.6 2.1 V
1.28
1.29
3.3
1.55
350
MHz
MHz
kHz
V
V
3684f
3
LT3684
TYPICAL PERFORMANCE CHARACTERISTICS
Effi ciency (V
100
90
80
70
EFFICIENCY (%)
60
50
0.2 0.4 0.6
0
OUT
0.8 1.0
LOAD CURRENT (A)
Maximum Load Current
4.0
3.5
3.0
= 5.0V)
VIN = 12V
VIN = 24V
L: NEC PLC-0745-4R7
f = 800kHz
1.2 1.4 1.8
TYPICAL
1.6
3684 G01
2.0
Effi ciency (V
90
85
80
75
70
65
EFFICIENCY (%)
60
55
50
0.2 0.4 0.6
0
Maximum Load Current
4.0
3.5
3.0
= 3.3V)
OUT
VIN = 24V
L: NEC PLC-0745-4R7
f = 800kHz
0.8 1.0
LOAD CURRENT (A)
TYPICAL
VIN = 7V
VIN = 12V
1.2 1.4 1.8
(TA = 25°C unless otherwise noted)
Effi ciency
90
85
1.6
3684 G02
2.0
80
75
70
65
EFFICIENCY (%)
60
55
50
0
VIN = 24V
0.5 1 2
SWITCHING FREQUENCY (MHz)
Switch Current Limit
4.0
3.5
3.0
VIN = 12V
1.5
V
= 3.3V
OUT
L = 10µH
LOAD = 1A
2.5 3
3684 G03
2.5
2.0
LOAD CURRENT (A)
1.5
1.0
5
MINIMUM
10 20
15
INPUT VOLTAGE (V)
Switch Current Limit
4.5
4.0
3.0
2.5
2.0
1.5
1.0
SWITCH CURRENT LIMIT (A)
0.5
0
–50
DUTY CYCLE = 10 %
DUTY CYCLE = 90 %
0–25 5025 10075
TEMPERATURE (°C)
V
= 3.3V
OUT
= 25°C
T
A
L = 4.7µH
f = 800kHz
25 30
3684 G04
3684 G07
125
2.5
2.0
LOAD CURRENT (A)
1.5
1.0
5
MINIMUM
10 20
Switch Voltage Drop
700
600
500
400
300
VOLTAGE DROP (mV)
200
100
0
0
500 1000
SWITCH CURRENT (mA)
15
INPUT VOLTAGE (V)
2000 3000 3500
1500 2500
V
= 5V
OUT
= 25°C
T
A
L = 4.7µH
f = 800kHz
25 30
3684 G05
3684 G08
2.5
2.0
SWITCH CURRENT LIMIT (A)
1.5
1.0
20 60
0
40
DUTY CYCLE (%)
Boost Pin Current
90
80
70
60
50
40
30
20
BOOST PIN CURRENT (mA)
10
0
500
0
1500 3500
1000
SWITCH CURRENT (mA)
2000
80 100
3684 G06
2500 3000
3684 G09
4
3684f
LT3684
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Voltage
1.290
1.285
1.280
1.275
1.270
1.265
FEEDBACK VOLTAGE (V)
1.260
1.255
1.250
–50
0 50 100–25 25 75
TEMPERATURE (°C)
125
3684 G10
Minimum Switch On-Time Soft-Start RUN/SS Pin Current
140
120
100
80
60
40
MINIMUM SWITCH ON-TIME (ns)
20
0
–50
–25 0 50 75 100 125
25
TEMPERATURE (˚C)
3684 G13
Switching Frequency Frequency Foldback
1.20
RT = 45.3k
1.15
1.10
1.05
1.00
0.95
FREQUENCY (MHz)
0.90
0.85
0.80
–50
4.0
3.5
3.0
2.5
2.0
1.5
1.0
SWITCH CURRENT LIMIT (A)
0.5
0
0
0 50 100–25 25 75
TEMPERATURE (°C)
0.5 1 2
1.5
RUN/SS PIN VOLTAGE (V)
2.5 3 3.5
(TA = 25°C unless otherwise noted)
1200
RT = 45.3k
1000
800
600
400
SWITCHING FREQUENCY (kHz)
200
0
3684 G11
3684 G14
125
0
200 400
12
10
8
6
4
RUN/SS PIN CURRENT (µA)
2
0
0
510
600 1000
FB PIN VOLTAGE (mV)
15 25
RUN/SS PIN VOLTAGE (V)
800 1200 1400
3684 G12
20 30 35
3684 G15
Boost Diode Error Amp Output Current
1.6
1.4
1.2
(V)
f
1.0
0.8
0.6
BOOST DIODE V
0.4
0.2
0
0
0.5
BOOST DIODE CURRENT (A)
1.0
1.5
2.0
3684 G16
100
80
60
40
20
0
PIN CURRENT (µA)
–20
C
V
–40
–60
–80
1.065
1.165 1.365
1 .265 1.465
FB PIN VOLTAGE (V)
3684 G17
Minimum Input Voltage
4.5
4.0
3.5
3.0
INPUT VOLTAGE (V)
2.5
2.0
0.001
0.01
LOAD CURRENT (A)
0.1
V
= 3.3V
OUT
= 25°C
T
A
L = 4.7µH
f = 800kHz
1
3684 G18
10
3684f
5
LT3684
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Input Voltage
6.5
6.0
5.5
5.0
INPUT VOLTAGE (V)
4.5
4.0
0.001
0.01
LOAD CURRENT (A)
0.5A/DIV
V
= 5V
OUT
= 25°C
T
A
L = 4.7µH
f = 800kHz
0.1
1
10
3684 G19
Switching Waveforms
(Discontinuous Operation)
I
L
VC Voltages
2.50
2.00
CURRENT LIMIT CLAMP
1.50
1.00
THRESHOLD VOLTAGE (V)
0.50
0
SWITCHING THRESHOLD
–25 0 50 100
–50
TEMPERATURE (°C)
25 75 125
0.5A/DIV
(TA = 25°C unless otherwise noted)
Power Good Threshold
1.200
PG RISING
1.180
1.160
1.140
THRESHOLD VOLTAGE (V)
1.120
1.100
3684 G20
–25 0 50 100
–50
TEMPERATURE (°C)
Switching Waveforms
(Continuous Operation)
I
L
25 75 125
3684 G21
V
SW
5V/DIV
V
OUT
10mV/DIV
VIN = 12V, FRONT PAGE APPLICATION
I
= 140mA
LOAD
1µs/DIV
3684 G22
V
RUN/SS
5V/DIV
V
OUT
10mV/DIV
VIN = 12V, FRONT PAGE APPLICATION
I
= 1A
LOAD
1µs/DIV
3684 G23
6
3684f
PIN FUNCTIONS
LT3684
BD (Pin 1): This pin connects to the anode of the boost
Schottky diode.
BOOST (Pin 2): This pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
SW (Pin 3): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
(Pin 4): The VIN pin supplies current to the LT3684’s
V
IN
internal regulator and to the internal power switch. This
pin must be locally bypassed.
RUN/SS (Pin 5): The RUN/SS pin is used to put the
LT3684 in shutdown mode. Tie to ground to shut down
the LT3684. Tie to 2.3V or more for normal operation. If
the shutdown feature is not used, tie this pin to the V
pin. RUN/SS also provides a soft-start function; see the
Applications Information section.
IN
PG (Pin 6): The PG pin is the open collector output of an
internal comparator. PG remains low until the FB pin is
within 10% of the fi nal regulation voltage. PG output is
valid when V
BIAS (Pin 7): The BIAS pin supplies the current to the
LT3684’s internal regulator. Tie this pin to the lowest
available voltage source above 3V (typically V
architecture increases effi ciency especially when the input
voltage is much higher than the output.
FB (Pin 8): The LT3684 regulates the FB pin to 1.265V.
Connect the feedback resistor divider tap to this pin.
(Pin 9): The VC pin is the output of the internal error
V
C
amplifi er. The voltage on this pin controls the peak switch
current. Tie an RC network from this pin to ground to
compensate the control loop.
(Pin 10): Oscillator Resistor Input. Connecting a resistor
RT
to ground from this pin sets the switching frequency.
Exposed Pad (Pin 11): Ground. The Exposed Pad must
be soldered to PCB.
is above 3.5V and RUN/SS is high.
IN
OUT
). This
3684f
7
LT3684
BLOCK DIAGRAM
V
V
IN
R
IN
4
C1
BIAS
7
RUN/SS
5
RT
10
T
SOFT-START
PG
6
INTERNAL 1.265V REF
+
1.12V
–
GND
11 8
R2
ERROR AMP
FB
R1
–
+
Σ
SLOPE COMP
OSCILLATOR
300kHz–2.8MHz
SWITCH
LATCH
R
S
VC CLAMP
Q
+
–
BD
BOOST
SW
1
2
C3
3
V
C
9
L1
D1
C
C
C
R
F
C
C2
3684 BD
V
OUT
8
3684f
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