LT3505
1.2A, Step-Down
Switching Regulator in
3mm
×
3mm DFN
FEATURES
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!
!
DESCRIPTION
The LT
®
3505 is a current mode PWM step-down DC/DC
converter with an internal 1.4A power switch. The wide
operating input range of 3.6V to 36V (40V maximum)
makes the LT3505 ideal for regulating power from a wide
variety of sources, including unregulated wall transform-
ers, 24V industrial supplies and automotive batteries. The
oscillator can be programmed for high frequency operation
allowing the use of tiny, low cost external components or
it can be programmed for lower frequency operation to
maximize efficiency.
Cycle-by-cycle current limit provides protection against
shorted outputs and soft-start eliminates input current
surge during start-up. The low current (<2µA) shutdown
mode provides output disconnect, enabling easy power
management in battery-powered systems.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
!
!
!
!
!
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Wide Input Range: 3.6V to 36V Operating,
40V Maximum
Up to 1.2A Output Current
Resistor-Programmable Fixed-Frequency Operation
from 200kHz to 3MHz
Output Adjustable Down to 780mV
Short-Circuit Robust
Uses Tiny Capacitors and Inductors
Soft-Start
Low Shutdown Current: <2µA
Low V
CESAT
Switch: 350mV at 1A
Thermally Enhanced, Low Profile 3mm x 3mm
DFN-8 and MSOP-8 Packages
APPLICATIONS
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!
!
!
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Automotive Battery Regulation
Industrial Control Supplies
Wall Transformer Regulation
Distributed Supply Regulation
Battery-Powered Equipment
TYPICAL APPLICATION
750kHz, 3.3V Step-Down Converter
V
IN
4.2V TO 36V
ON OFF
V
IN
SHDN
LT3505
FB
R
T
75.0k
1µF
GND
V
C
69.8k
68pF
11.3k
10µF
BOOST
0.1µF 10µH
SW
36.5k
22pF
V
OUT
3.3V
1.1A, V
IN
> 5V
1.2A, V
IN
> 8V
EFFICIENCY (%)
90
85
80
75
70
65
60 V
IN
= 12V
V
OUT
= 3.3V
55 f
SW
= 750kHz
L = 10 H
50
0.8
0
0.2
0.4
0.6
LOAD CURRENT (A)
Efficiency
3505 TA01
1.0
1.2
3505fc
1
LT3505
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Input Voltage (V
IN
) ....................................................40V
BOOST Pin Voltage ..................................................50V
BOOST Pin Above SW Pin.........................................25V
SHDN Pin ..................................................................40V
FB Pin .........................................................................6V
V
C
Pin .........................................................................3V
R
T
Pin .........................................................................3V
Operating Temperature Range (Note 2)
LT3505E .............................................. –40°C to 85°C
LT3505I ............................................. –40°C to 125°C
Maximum Junction Temperature .......................... 125°C
Storage Temperature Range................... –65°C to 150°C
PIN CONFIGURATION
TOP VIEW
BOOST 1
SW 2
V
IN
3
SHDN 4
9
8
7
6
5
V
C
FB
R
T
GND
BOOST
SW
V
IN
SHDN
1
2
3
4
TOP VIEW
9
8
7
6
5
V
C
FB
R
T
GND
DD PACKAGE
8-LEAD (3mm
×
3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 43°C/W,
θ
JC
= 5°C/W
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
MS8E PACKAGE
8-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 40°C/W,
θ
JC
= 5°C/W
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3505EDD#PBF
LT3505IDD#PBF
LT3505EMS8E#PBF
LT3505IMS8E#PBF
LEAD BASED FINISH
LT3505EDD
LT3505IDD
LT3505EMS8E
LT3505IMS8E
TAPE AND REEL
LT3505EDD#TRPBF
LT3505IDD#TRPBF
LT3505EMS8E#TRPBF
LT3505IMS8E#TRPBF
TAPE AND REEL
LT3505EDD#TR
LT3505IDD#TR
LT3505EMS8E#TR
LT3505IMS8E#TR
PART MARKING
LCHB
LCHC
LTCNX
LTCNY
PART MARKING
LCHB
LCHC
LTCNX
LTCNY
PACKAGE DESCRIPTION
8-Lead (3mm x 3mm) Plastic DFN
8-Lead (3mm x 3mm) Plastic DFN
8-Lead Plastic MSOP
8-Lead Plastic MSOP
PACKAGE DESCRIPTION
8-Lead (3mm x 3mm) Plastic DFN
8-Lead (3mm x 3mm) Plastic DFN
8-Lead Plastic MSOP
8-Lead Plastic MSOP
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 125°C
–40°C to 85°C
–40°C to 125°C
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 125°C
–40°C to 85°C
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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/
3505fc
2
LT3505
ELECTRICAL CHARACTERISTICS
PARAMETER
V
IN
Operating Range
Undervoltage Lockout
Feedback Voltage
FB Pin Bias Current
Quiescent Current
Quiescent Current in Shutdown
Reference Line Regulation
Switching Frequency
V
FB
= Measured V
REF
(Note 4)
Not Switching, R
T
= 75.0k
V
SHDN
= 0V
V
IN
= 5V to 36V
V
FB
= 0.7V, R
T
= 13.7k
V
FB
= 0.7V, R
T
= 75.0k
V
FB
= 0.7V, R
T
= 357k
R
T
= 75.0k
V
FB
= 0.78V
V
FB
= 0.78V
V
FB
= 0V, V
C
= 1.5V
V
FB
= 1V, V
C
= 1.5V
I
OUT
= 0mA
V
FB
= 0V
V
FB
= 0.6V
V
FB
= 0V, R
T
= 75.0k
(Note 3)
I
SW
= 1A
I
SW
= 1A
I
SW
= 1A
2.3
0.3
V
SHDN
= 2.3V (Note 5)
V
SHDN
= 0V
6
0.01
20
0.1
1.4
"
"
"
The
"
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C, V
IN
= 12V, V
BOOST
= 17V, unless otherwise noted. (Note 2).
CONDITIONS
MIN
3.6
3.1
765
3.35
780
55
2.0
0.01
0.007
2.70
675
180
90
3.01
750
200
94
200
400
10
14
0.9
1.7
0.5
50
1.75
350
0.1
1.6
24
2
2.2
50
2.2
3.30
825
220
TYP
MAX
36
3.6
795
150
2.7
2
UNITS
V
V
mV
nA
mA
µA
%/V
MHz
kHz
kHz
%
µA/V
V/V
µA
µA
V
V
V
mV
A
mV
µA
V
mA
V
V
µA
µA
Maximum Duty Cycle
Error Amp Transconductance
Error Amp Voltage Gain
V
C
Source Current
V
C
Sink Current
V
C
Switching Threshold Voltage
V
C
Clamp Voltage
R
T
Bias Voltage
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
Minimum Boost Voltage Above Switch
BOOST Pin Current
SHDN Input Voltage High
SHDN Input Voltage Low
SHDN Bias Current
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 LT3505E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3505I specifications are
guaranteed over the –40°C to 125°C temperature range.
Note 3:
Current limit guaranteed by design and/or correlation to static test.
Slope compensation reduces current limit at higher duty cycle.
Note 4:
Current flows out of pin.
Note 5:
Current flows into pin.
3505fc
3
LT3505
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency (V
OUT
= 5V, L = 10µH,
f
SW
= 750kHz)
95
90
85
EFFICIENCY (%)
EFFICIENCY (%)
80
75
70
65
60
55
50
0
0.2
0.4
0.6
0.8
LOAD CURRENT (A)
V
IN
= 8V
V
IN
= 12V
V
IN
= 24V
1.0
1.2
T
A
= 25°C
95
90
85
EFFICIENCY (%)
80
75
70
65
60
55
50
0
0.2
0.4
0.6
0.8
LOAD CURRENT (A)
V
IN
= 8V
V
IN
= 12V
V
IN
= 24V
1.0
1.2
Efficiency (V
OUT
= 3.3V, L = 10µH,
f
SW
= 750kHz)
T
A
= 25°C
95
90
85
80
75
70
65
60
55
50
Efficiency (V
OUT
= 3.3V,
L = 4.7µH, f
SW
= 2.2MHz)
T
A
= 25°C
V
IN
= 8V
V
IN
= 12V
0
0.2
0.4
0.6
0.8
LOAD CURRENT (A)
1.0
1.2
Efficiency (V
OUT
= 5V, L = 4.7µH,
f
SW
= 2.2MHz)
95
90
85
EFFICIENCY (%)
80
75
70
65
60
55
50
0
0.2
0.4
0.6
0.8
LOAD CURRENT (A)
V
IN
= 8V
V
IN
= 12V
1.0
1.2
T
A
= 25°C
1.7
1.6
OUTPUT CURRENT (A)
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
Max Load Current (V
OUT
= 3.3V,
L = 6.8µH, f
SW
= 750kHz)
T
A
= 25°C
TYPICAL
OUTPUT CURRENT (A)
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
5
7
9
15
INPUT VOLTAGE (V)
11
13
17
19
3505 G05
Max Load Current (V
OUT
= 5V,
f
SW
= 750kHz)
T
A
= 25°C
TYPICAL, L = 22µH
TYPICAL, L = 10µH
MINIMUM
MINIMUM, L = 10µH
*10% DROPOUT
*10% DROPOUT
0.8
6
8 10 12 14 16 18 20 22 24 26 28 30
INPUT VOLTAGE (V)
3505 G06
Max Load Current (V
OUT
= 3.3V,
L = 2.2µH, f
SW
= 2.2MHz)
1.8
1.7
1.6
OUTPUT CURRENT (A)
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
5
6
*10% DROPOUT
MINIMUM
T
A
= 25°C
1.80
1.70
OUTPUT CURRENT (A)
TYPICAL
1.60
Max Load Current (V
OUT
= 5V,
L = 3.3µH, f
SW
= 2.2MHz)
T
A
= 25°C
500
450
400
V
CE(SWITCH)
(mV)
TYPICAL
350
300
250
200
150
100
50
16
18
0
Switch Voltage Drop
1.50
1.40
1.30
1.20
1.10
1.00
0.90
T
A
= 85°C
T
A
= 25°C
T
A
= –45°C
MINIMUM
*10% DROPOUT
9
8
7
10
INPUT VOLTAGE (V)
11
12
0.80
7
8
9
10 11 12 14
INPUT VOLTAGE (V)
0
300
900
1200
600
SWITCH CURRENT (mA)
1500
3505 G09
3505 G07
3505 G08
3505fc
4
LT3505
TYPICAL PERFORMANCE CHARACTERISTICS
Undervoltage Lockout
4.00
3.90
3.80
3.70
UVLO (V)
3.60
3.50
3.40
3.30
3.20
3.10
3.00
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
SWITCHING FREQUENCY (MHz)
2.40
2.20
2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
–50
–25
R
T
= 75.0k
50
75
0
25
TEMPERATURE (°C)
100
125
R
T
= 30.1k
R
T
= 21k
R
T
PIN BIAS VOLTAGE (V)
Switching Frequency
0.6
0.5
0.4
0.3
0.2
0.1
0
Frequency Foldback, R
T
= 75.0k
T
A
= 25°C
0
0.1
0.2
0.3 0.4 0.5 0.6
FB VOLTAGE (V)
0.7
0.8
3505 G10
3505 G12
Soft-Start
2.0
1.8
SWITCH CURRENT LIMIT (A)
1.6
1.4
I
SHDN
(µA)
1.2
1.0
0.8
0.6
0.4
0.2
0
0
0.25 0.50 0.75 1 1.25 1.50 1.75
SHDN PIN VOLTAGE (V)
2
T
A
= 25°C
50
45
40
SHDN Pin Current
T
A
= 25°C
7.2
7.0
6.8
INPUT VOLTAGE (V)
6.6
6.4
6.2
6.0
5.8
5.6
5.4
0
2
4
6
8 10 12 14 16 18 20
V
SHDN
(V)
3505 G14
Typical Minimum Input Voltage,
(V
OUT
= 5V, f
SW
= 750kHz)
T
A
= 25°C
TO START
35
30
25
20
15
10
5
0
TO RUN
5.2
1
10
100
LOAD CURRENT (mA)
1000
3505 G15
Typical Minimum Input Voltage,
(V
OUT
= 3.3V, f
SW
= 750kHz)
5.5
5.3
5.1
INPUT VOLTAGE (V)
4.9
4.7
4.5
4.3
4.1
3.9
3.7
3.5
1
10
100
LOAD CURRENT (mA)
1000
3.5
TO RUN
T
A
= 25°C
TO START
INPUT VOLTAGE (V)
5.0
5.5
Typical Minimum Input Voltage,
(V
OUT
= 3.3V, f
SW
= 2.2MHz)
T
A
= 25°C
TO START
INPUT VOLTAGE (V)
7.0
6.8
6.6
6.4
6.2
6.0
5.8
5.6
5.4
5.2
1
10
100
1000
5.0
Typical Minimum Input Voltage,
(V
OUT
= 5V, f
SW
= 2.2MHz)
TO START
TO RUN
4.5
TO RUN
4.0
T
A
= 25°C
1
10
100
LOAD CURRENT (mA)
1000
LOAD CURRENT (mA)
3505fc
5
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