LT1912
36V, 2A, 500kHz Step-Down
Switching Regulator
FEATURES
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DESCRIPTION
The LT
®
1912 is an adjustable frequency (200kHz to 500kHz)
monolithic step-down switching regulator that accepts
input voltages up to 36V. A high efficiency 0.25Ω 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 LT1912 allows the use of
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). The
LT1912 is available in 10-pin MSOP and 3mm
×
3mm DFN
packages with exposed pads for low thermal resistance.
L,
LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
Wide Input Range: Operation From 3.6V to 36V
2A Maximum Output Current
Adjustable Switching Frequency: 200kHz to 500kHz
Low Shutdown Current: I
Q
< 1µA
Integrated Boost Diode
Synchronizable Between 250kHz to 500kHz
Saturating Switch Design: 0.25Ω On-Resistance
0.790V Feedback Reference Voltage
Output Voltage: 0.79V to 20V
Soft-Start Capability
Small 10-Pin Thermally Enhanced MSOP and
(3mm
×
3mm) DFN Packages
APPLICATIONS
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Automotive Battery Regulation
Set-Top Box
Distributed Supply Regulation
Industrial Supplies
Wall Transformer Regulation
TYPICAL APPLICATION
3.3V Step-Down Converter
V
IN
4.5V TO 36V
V
IN
OFF ON
20k
4.7µF
470pF
68.1k
RUN/SS
BD
BOOST
EFFICIENCY (%)
0.47µF
V
C
R
T
316k
100k
47µF
LT1912
SW
6.8µH
80
70
60
50
0
1912 TA01
Efficiency
V
OUT
3.3V
2A
100
90
V
OUT
= 5V
V
OUT
= 3.3V
SYNC
GND
FB
V
IN
= 12V
L = 6.8µF
F = 500kHz
0.5
1.0
1.5
LOAD CURRENT (A)
2
1912 TA01b
1912fa
1
LT1912
ABSOLUTE MAXIMUM RATINGS
(Note 1)
V
IN
, RUN/SS Voltage .................................................36V
BOOST Pin Voltage ...................................................56V
BOOST Pin Above SW Pin.........................................30V
FB, RT, V
C
Voltage.......................................................5V
BD, SYNC Voltage .....................................................30V
Operating Junction Temperature Range (Note 2)
LT1912E ............................................ –40°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
MSE Only .......................................................... 300°C
PIN CONFIGURATION
TOP VIEW
BD
BOOST
SW
V
IN
RUN/SS
1
2
3
4
5
11
GND
10 R
T
9 V
C
8 FB
7 N/C
6 SYNC
TOP VIEW
BD
BOOST
SW
V
IN
RUN/SS
1
2
3
4
5
11
GND
10
9
8
7
6
R
T
V
C
FB
N/C
SYNC
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
JA
= 45°C/W,
JC
= 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
MSE PACKAGE
10-LEAD PLASTIC MSOP
JA
= 45°C/W,
JC
= 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT1912EDD#PBF
LT1912EMSE#PBF
TAPE AND REEL
LT1912EDD#TRPBF
LT1912EMSE#TRPBF
PART MARKING
LDJT
LTDJS
PACKAGE DESCRIPTION
10-Lead (3mm
×
3mm) Plastic DFN
10-Lead Plastic MSOP
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Input Voltage
Quiescent Current from V
IN
Quiescent Current from BD
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 10V, V
RUN/SS
= 10V, V
BOOST
= 15V, V
BD
= 3.3V unless otherwise
noted. (Note 2)
CONDITIONS
l
MIN
TYP
3
0.01
450
1.3
0.01
0.9
1
2.7
MAX
3.6
0.5
600
1.7
0.5
1.3
5
3
UNITS
V
µA
µA
mA
µA
mA
µA
V
V
RUN/SS
= 0.2V
V
BD
= 3V, Not Switching
V
BD
= 0, Not Switching
V
RUN/SS
= 0.2V
V
BD
= 3V, Not Switching
V
BD
= 0, Not Switching
l
l
Minimum Bias Voltage (BD Pin)
1912fa
2
LT1912
ELECTRICAL CHARACTERISTICS
PARAMETER
Feedback Voltage
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 10V, V
RUN/SS
= 10V V
BOOST
= 15V, V
BD
= 3.3V unless otherwise
noted. (Note 2)
CONDITIONS
MIN
780
775
TYP
790
790
7
0.002
25
1000
45
45
3.5
2
R
T
= 187k
l
MAX
800
805
30
0.01
UNITS
mV
mV
nA
%/V
µMho
µA
µA
A/V
V
FB Pin Bias Current (Note 3)
FB Voltage Line Regulation
Error Amp g
m
Error Amp Gain
V
C
Source Current
V
C
Sink Current
V
C
Pin to Switch Current Gain
V
C
Clamp Voltage
Switching Frequency
Minimum Switch Off-Time
Switch Current Limit
Switch V
CESAT
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 4)
BOOST Pin Current
RUN/SS Pin Current
RUN/SS Input Voltage High
RUN/SS Input Voltage Low
SYNC Low Threshold
SYNC High Threshold
SYNC Pin Bias Current
V
FB
= 0.8V, V
C
= 0.4V
4V < V
IN
< 36V
l
160
3.2
200
60
3.7
500
0.02
240
150
4.2
2
2.1
35
10
2.5
kHz
nS
A
mV
µA
V
mA
µA
V
V
V
Duty Cycle = 5%
I
SW
= 2A
V
SW
= 10V, V
BD
= 0V
l
1.5
22
5
0.2
0.5
I
SW
= 1A
V
RUN/SS
= 2.5V
0.7
V
SYNC
= 0V
0.1
V
µA
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 LT1912E is guaranteed to meet performance specifications
from 0°C to 125°C. Specifications over the –40°C to 125°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3:
Bias current flows out of the FB pin.
Note 4:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
1912fa
3
LT1912
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency
100
90
EFFICIENCY (%)
80
V
IN
= 24V
70
60
50
0
V
OUT
= 5V
L: NEC PLC-0745-5R6
f: 500kHz
V
IN
= 12V
V
IN
= 34V
90
85
80
EFFICIENCY (%)
75
70
65
60
55
50
0
V
OUT
= 3.3V
L: NEC PLC-0745-5R6
f: 500kHz
V
IN
= 24V
V
IN
= 12V
LOAD CURRENT (A)
V
IN
= 34V
T
A
= 25°C, unless otherwise noted.
Efficiency
V
IN
= 7V
4.0
3.5
3.0
2.5
Maximum Load Current
TYPICAL
MINIMUM
2.0
1.5
1.0
V
OUT
= 3.3V
L = 4.7µH
f = 500kHz
5
10
20
15
INPUT VOLTAGE (V)
25
30
1912 G03
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
LOAD CURRENT (A)
1912 G01
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
LOAD CURRENT (A)
1912 G02
Maximum Load Current
3.5
TYPICAL
SWITCH CURRENT LIMIT(A)
3.0
LOAD CURRENT (A)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Switch Current Limit
4.5
4.0
SWITCH CURRENT LIMIT (A)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
20
60
40
DUTY CYCLE (%)
80
100
1912 G05
Switch Current Limit
DUTY CYCLE = 10 %
2.5
MINIMUM
2.0
DUTY CYCLE = 90 %
1.5
1.0
V
OUT
= 5V
L = 4.7µH
f = 500kHz
5
10
20
15
INPUT VOLTAGE (V)
25
30
1912 G04
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1912 G06
Switch Voltage Drop
700
600
BOOST PIN CURRENT (mA)
VOLTAGE DROP (mV)
500
400
300
200
100
0
80
70
60
50
40
30
20
10
0
500
1500
1000
2000
SWITCH CURRENT (mA)
2500
1912 G07
Boost Pin Current
0
0
500
1000
1500
2000
SWITCH CURRENT (mA)
2500
1912 G08
1912fa
4
LT1912
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Voltage
840
1.20
1.15
820
FREQUENCY (NORMALIZED)
FEEDBACK VOLTAGE (mV)
1.10
1.05
1.00
0.95
0.90
0.85
760
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1912 G09
T
A
= 25°C, unless otherwise noted.
Switching Frequency
1.2
SWITCHING FREQUENCY (NORMALIZED)
1.0
0.8
0.6
0.4
0.2
0
Frequency Foldback
800
780
0.80
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1912 G10
0
100 200 300 400 500 600 700 800 900
FB PIN VOLTAGE (mV)
1912 G11
Minimum Switch On-Time
140
MINIMUM SWITCH ON TIME (ns)
120
SWITCH CURRENT LIMIT (A)
100
80
60
40
20
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (˚C)
1912 G12
Soft-Start
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
0.5
2.5
2
1.5
RUN/SS PIN VOLTAGE (V)
1
3
3.5
1912 G13
RUN/SS Pin Current
12
10
8
6
4
2
0
RUN/SS PIN CURRENT (µA)
0
5
20
30
15
25
10
RUN/SS PIN VOLTAGE (V)
35
1912 G14
Boost Diode
1.4
1.2
BOOST DIODE V
f
(V)
V
C
PIN CURRENT (µA)
0
0.5
1.0
1.5
BOOST DIODE CURRENT (A)
2.0
1912 G15
Error Amp Output Current
50
40
30
20
10
0
–10
–20
–30
–40
–50
–200
–100
100
0
FB PIN ERROR VOLTAGE (V)
200
1912 G16
1.0
0.8
0.6
0.4
0.2
0
1912fa
5
PCB设计
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