FEATURES
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LT3825
Isolated No-Opto
Synchronous Flyback Controller
with Wide Input Supply Range
DESCRIPTION
The LT
®
3825 is an isolated switching regulator controller
designed for medium power flyback topologies. A typical
application is 10W to 60W with input voltage limited only
by external power path components. A third transformer
winding provides output voltage feedback.
The LT3825 is a current mode controller that regulates
output voltage based on sensing secondary voltage via
a transformer winding during flyback. This allows for
tight output regulation without the use of an optoisolator,
improving dynamic response and reliability. Synchronous
rectification increases converter efficiency and improves
output cross regulation in multiple output converters.
The LT3825 operates in forced continuous conduction mode
which improves cross regulation in multiple winding ap-
plications. Switching frequency is user programmable and
can be externally synchronized. The part also has load com-
pensation, undervoltage lockout and soft-start circuity.
L,
LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 6948466, 5841643.
Senses Output Voltage Directly from Primary Side
Winding—No Optoisolator Required
Synchronous Driver for High Efficiency
Input Voltage Limited Only by External
Power Components
Accurate Output Regulation Without User Trims
Switching Frequency from 50kHz to 250kHz
Synchronizable
Load Compensation
Programmable Undervoltage Lockout
Available in a Thermally Enhanced 16-Lead
TSSOP Package
APPLICATIONS
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Isolated Medium Power (10W to 60W) Supplies
Isolated Telecom, Medical Converters
Instrumentation Power Supplies
Isolated Power over Ethernet Supplies
TYPICAL APPLICATION
EFFICIENCY (%)
Efficiency
92
90
36V
IN
88
48V
IN
72V
IN
48V to 3.3V at 12V Isolated Supply
V
IN+
36V TO 72V
100pF
28.7k
T1
3.01k
402k
2.2μF
UVLO
15k
LT3825
PGDLY
t
ON
SYNC R
CMP
ENDLY
12k
100k
2.1k
150k
47pF
SENSE
–
V
C
OSC GND SFST
C
CMP
10nF
0.22μF
0.1μF
100k
3825 TA01a
86
84
82
80
78
2
3
4
•
T1
470μF
×4
V
OUT
3.3V
12A
+
•
20Ω
47k
20Ω
•
47Ω
5 6 7 8 9
LOAD CURRENT (A)
10 11 12
3825 TA01b
+
47μF
FB
V
CC
SG
SG
PG
SENSE
+
×2
SG
0.02Ω
330Ω
Regulation
3.43
1μF
15Ω
3.38
3.33
OUTPUT (V)
3.28
3.23
3.18
3.13
2
3
4
5 6 7 8 9
LOAD CURRENT (A)
36V
IN
48V
IN
72V
IN
0.1μF 2.2nF
•
•
10k
10 11 12
3825 TA01c
3825fa
1
LT3825
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
SG
V
CC
t
ON
ENDLY
SYNC
SFST
OSC
FB
1
2
3
4
5
6
7
8
17
16 PG
15 PGDLY
14 R
CMP
13 C
CMP
12 SENSE
+
11 SENSE
–
10 UVLO
9
V
C
V
CC
to GND
Low Impedance Source ........................ –0.3V to 18V
Current Fed
(V
CC
has Internal 19.5V Clamp) ...........30mA into V
CC
UVLO, SYNC Pin Voltage ........................... –0.3V to V
CC
SENSE
–
, SENSE
+
Pin Voltage ...................... –0.5V, +0.5V
FB Pin Current........................................................±2mA
V
C
Pin Current....................................................... ±1mA
Operating Junction Temperature Range
(Notes 2, 3) .......................................... –40°C to 125°C
Storage Temperature Range.................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
FE PACKAGE
16-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 40°C/W,
θ
JC
= 10°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3825EFE#PBF
LEAD BASED FINISH
LT3825EFE
TAPE AND REEL
LT3825EFE#TRPBF
TAPE AND REEL
LT3825EFE#TR
PART MARKING
3825EFE
PART MARKING
3825EFE
PACKAGE DESCRIPTION
16-Lead Plastic 4.4mm TSSOP
PACKAGE DESCRIPTION
16-Lead Plastic 4.4mm TSSOP
TEMPERATURE RANGE
–40°C to 125°C
TEMPERATURE RANGE
–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/
ELECTRICAL CHARACTERISTICS
The
l
denotes the specifications which apply over the full operating temperature range, otherwise specifications are at T
A
= 25°C.
V
CC
= 14V; PG, SG Open; V
C
= 1.5V, V
SENSE–
= 0V; R
CMP
= 1k, R
tON
= 90k, R
PGDLY
= 27.4k, R
ENDLY
= 90k, unless otherwise specified.
PARAMETER
Power Supply
V
CC
Turn-On Voltage
V
CC
Turn-Off Voltage
V
CC
Hysteresis
V
CC
Shunt Clamp
V
CC
Supply Current (Note 5) (I
CC
)
V
CC
Start-Up Current
Feedback Amplifier
Feedback Regulation Voltage (V
FB
)
Feedback Pin Input Bias Current
Feedback Amplifier Transconductance
Feedback Amplifier Source or Sink Current
Feedback Amplifier Clamp Voltage
V
FB
= 0.9V
V
FB
= 1.4V
R
CMP
Open
ΔI
C
= ±10μA
●
●
●
l
●
●
●
●
●
CONDITIONS
MIN
14.0
8
4.0
19.5
4
TYP
15.3
9.7
5.6
20.5
6.4
180
MAX
16.0
11
6.5
10
400
1.251
1400
90
UNITS
V
V
V
V
mA
μA
V
nA
μmho
μA
V
V
CC(ON)
– V
CC(OFF)
V
UVLO
= 0V, I
VCC
= 15mA
V
C
= Open
V
CC
= 10V
1.220
700
25
1.237
200
1000
55
2.56
3525fa
2
LT3825
ELECTRICAL CHARACTERISTICS
The
l
denotes the specifications which apply over the full operating temperature range, otherwise specifications are at T
A
= 25°C.
V
CC
= 14V; PG, SG Open; V
C
= 1.5V, V
SENSE–
= 0V; R
CMP
= 1k, R
tON
= 90k, R
PGDLY
= 27.4k, R
ENDLY
= 90k, unless otherwise specified.
PARAMETER
Reference Voltage Line Regulation
Feedback Amplifier Voltage Gain
Soft-Start Charging Current
Soft-Start Discharge Current
Control Pin Threshold (V
C
)
Gate Outputs
PG, SG Output High Level
PG, SG Output Low Level
PG, SG Output Shutdown Strength
PG Rise Time
SG Rise Time
PG, SG Fall Time
Current Amplifier
Switch Current Limit at Maximum V
C
ΔV
SENSE
/ΔV
C
Sense Voltage Overcurrent Fault Voltage
Timing
Switching Frequency (f
OSC
)
Oscillator Capacitor Value (C
OSC
)
Minimum Switch On Time (t
ON(MIN)
)
Flyback Enable Delay Time (t
ED
)
PG Turn-On Delay Time (t
PGDLY
)
Maximum Switch Duty Cycle
SYNC Pin Threshold
SYNC Pin Input Resistance
Load Compensation
Load Comp to V
SENSE
Offset Voltage
Feedback Pin Load Compensation Current
UVLO Function
UVLO Pin Threshold (V
UVLO
)
UVLO Pin Bias Current
V
UVLO
= 1.2V
V
UVLO
= 1.3V
●
●
●
●
●
CONDITIONS
12V ≤ V
CC
≤ 18V
V
C
= 1.2V to 1.7V
V
SFST
= 1.5V
V
SFST
= 1.5V, V
UVLO
= 0V
Duty Cycle = Min
●
MIN
TYP
0.005
1400
MAX
0.02
25
UNITS
%/V
V/V
μA
mA
V
16
0.8
20
1.3
1.0
6.6
7.4
0.01
1.6
11
15
10
8.0
0.05
2.3
V
V
V
ns
ns
ns
V
UVLO
= 0V; I
PG
, I
SG
= 20mA
C
PG
= 1nF
C
SG
= 1nF
C
PG
, C
SG
= 1nF
V
SENSE+
V
SENSE+
, V
SFST
< 1V
C
OSC
= 100pF
(Note 6)
●
●
88
98
0.07
206
110
230
110
200
mV
V/V
mV
kHz
pF
ns
ns
ns
%
●
●
84
33
100
200
265
200
85
88
1.53
40
2.1
V
kΩ
mV
μA
V
RCMP
with V
SENSE+
= 0V
V
SENSE+
= 20mV, V
FB
= 1.230V
1.215
–0.25
–4.50
1
20
1.240
0
–3.4
1.265
±0.25
–2.50
V
μA
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:
This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 3:
The LT3825E 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 4:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
T
J
= T
A
+ (P
D
• 40°C/W)
Note 5:
Supply current does not include gate charge current to the
MOSFETs. See Applications Information.
Note 6:
Component value range guaranteed by design.
3825fa
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LT3825
TYPICAL PERFORMANCE CHARACTERISTICS
V
CC(0N)
and V
CC(OFF)
vs Temperature
16
15
14
I
VCC
(mA)
I
VCC
(μA)
13
V
CC
(V)
12
11
V
CC(OFF)
10
9
8
–50 –25
0
50
75
25
TEMPERATURE (°C)
100
125
50
0
–50 –25
200
150
100
V
CC(ON)
300
250
V
CC
Start-Up Current
vs Temperature
10
9
8
7
6
V
CC
Current vs Temperature
DYNAMIC CURRENT C
PG
= 1nF,
C
SG
= 1nF, f
OSC
= 100kHz
STATIC PART CURRENT
5
4
V
CC
= 14V
3
50
–50 –25
25
75
0
TEMPERATURE (°C)
50
25
75
0
TEMPERATURE (°C)
100
125
100
125
3825 G01
3825 G02
3825 G03
SENSE Voltage vs Temperature
110
108
106
SENSE VOLTAGE (mV)
104
102
100
98
96
94
92
90
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
FB = 1.1V
SENSE = V
SENSE+
WITH V
SENSE–
= 0V
SENSE VOLTAGE (mV)
220
SENSE Fault Voltage
vs Temperature
SENSE = V
SENSE+
–
215 WITH V
SENSE
= 0V
210
f
OSC
(kHz)
205
200
195
190
185
180
–50 –25
0
50
75
25
TEMPERATURE (°C)
100
125
110
108
106
104
102
100
98
96
94
92
Oscillator Frequency
vs Temperature
C
OSC
= 100pF
90
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3825 G04
3825 G05
3825 G06
V
FB
vs Temperature
1.240
1.239
1.238
1.237
V
FB
(V)
1.236
1.235
1.234
1.233
1.232
1.231
1.230
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
FEEDBACK PIN INPUT BIAS (nA)
250
200
150
100
50
300
Feedback Pin Input Bias
vs Temperature
1.04
R
CMP
OPEN
1.03
1.02
V
FB
RESET (V)
1.01
1.00
0.99
0.98
0.97
50
25
75
0
TEMPERATURE (°C)
100
125
V
FB
Reset vs Temperature
0
–50 –25
0.96
–50 –25
0
50
75
25
TEMPERATURE (°C)
100
125
3825 G07
3825 G08
3825 G09
3525fa
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LT3825
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Amplifier Output
Current vs V
FB
70
125°C
50
30
I
VC
(μA)
10
–10
–30
–50
–70
0.9
1
1.1
1.2
V
FB
(V)
1.3
1.4
1.5
3825 G10
Feedback Amplifier Source and
Sink Current vs Temperature
70
25°C
–40°C
60
I
VC
(μA)
55
50
950
45
40
–50
65
SOURCE CURRENT
V
FB
= 1.1V
SINK
CURRENT
V
FB
= 1.4V
1100
Feedback Amplifier g
m
vs Temperature
1050
g
m
(μmho)
1000
–25
50
25
75
0
TEMPERATURE (°C)
100
125
900
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
3825 G11
3825 G12
Feedback Amplifier Voltage Gain
vs Temperature
1700
1650
1600
1550
1500
UVLO (V)
A
V
(V/V)
1450
1400
1350
1300
1250
1200
1150
1100
–50 –25
0
50
75
25
TEMPERATURE (°C)
100
125
1.225
1.240
1.245
1.250
UVLO vs Temperature
3.7
3.6
3.5
I
UVLO
(μA)
3.4
3.3
3.2
3.1
I
UVLO
Hysteresis vs Temperature
1.235
1.230
1.220
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3.0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3825 G13
3825 G14
3825 G15
Soft-Start Charge Current
vs Temperature
23
22
SFST CHARGE CURRENT (μA)
21
TIME (ns)
20
19
18
17
16
15
–50 –25
0
75
50
25
TEMPERATURE (°C)
100
125
80
70
60
50
PG, SG Rise and Fall Times
vs Load Capacitance
T
A
= 25°C
21.5
V
CC
Clamp Voltage
vs Temperature
I
CC
= 10mA
21.0
40
30
RISE TIME
20
10
0
0
1
2
3 4 5 6 7
CAPACITANCE (nF)
8
9
10
V
CC
(V)
FALL TIME
20.5
20.0
19.5
19.0
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3825 G16
3825 G17
3825 G18
3825fa
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