LTC3806
Synchronous
Flyback DC/DC Controller
FEATURES
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DESCRIPTIO
High Efficiency at Full Load
Better Cross Regulation Than Nonsynchronous
Converters (Multiple Outputs)
Soft-Start Minimizes Inrush Current
Current Mode Control Provides Excellent
Transient Response
High Maximum Duty Cycle: 89% Typical
±2%
Programmable Undervoltage Lockout Threshold
±1%
Internal Voltage Reference
Micropower Start-Up
Constant Frequency Operation (Never Audible)
3mm
×
4mm 12-Pin DFN Package
The LTC
®
3806 is a current mode synchronous flyback
controller that drives N-channel power MOSFETs and
requires very few external components. It is intended for
medium power applications where multiple outputs are
required. Synchronous rectification provides higher effi-
ciency and improved output cross regulation than
nonsynchronous converters.
The IC contains all the necessary control circuitry includ-
ing a 250kHz oscillator, precision undervoltage lockout
circuit with hysteresis, gate drivers for primary and syn-
chronous switches, current mode control circuitry and
soft-start circuitry.
Programmable soft-start reduces inrush currents. This
makes it easier to design compliant Power Over Ethernet
supplies.
Low start-up current reduces power dissipation in the
start-up resistor and reduces the size of the external start-
up capacitor.
The LTC3806 is available in a 12-pin, exposed pad DFN
package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
APPLICATIO S
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48V Telecom Supplies
12V/42V Automotive
24V Industrial
VoIP Phone
Power Over Ethernet
TYPICAL APPLICATIO
V
IN
36V TO 72V
R1
51k
R2
604k
C7
4.7μF
LTC3806
RUN SENSE
R8
100Ω
I
TH
FB
V
IN
C1
100μF
R3
26.7k
C2
1nF
R4
3.4k
INTV
CC
C3
4.7μF
SS
G2
G1
GND
D1
T1
•
•
•
M1
M2
C4
0.47μF
R5
0.056Ω
R7
12.4k
R6
21k
Figure 1. Multiple Output Flyback Converter for Telecom
3806fb
U
V
OUT1
3.3V
V
OUT2
3A
2.5V
3A
M3
C5
470μF
C6
470μF
3806 F01
U
U
1
LTC3806
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PI CO FIGURATIO
TOP VIEW
RUN
I
TH
FB
NC
V
IN
INTV
CC
1
2
3
13
4
5
6
V
IN
Voltage ............................................................. 25V
INTV
CC
Voltage ......................................................... 8V
INTV
CC
Output Current ........................................ 50mA
G1, G2 Voltages ....................... – 0.3V to V
INTVCC
+ 0.3V
I
TH
, FB, SS Voltages .................................– 0.3V to 2.7V
RUN Voltage ............................................... – 0.3V to 7V
SENSE Pin Voltage ..................................... – 0.3V to 8V
Operating Ambient Temperature Range
(Note 2) .................................................. – 40°C to 85°C
Junction Temperature (Note 3) ............................ 125°C
Storage Temperature Range ................. – 65°C to 125°C
12 SENSE
11 NC
10 SS
9
8
7
G1
G2
GND
DE12 PACKAGE
12-LEAD (4mm
×
3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 34°C/W
EXPOSED PAD (PIN 13) IS GND
MUST BE SOLDERED TO PCB
ORDER I FOR ATIO
LEAD FREE FINISH
LTC3806EDE#PBF
TAPE AND REEL
PART MARKING
3806
PACKAGE DESCRIPTION
12-Lead (4mm
×
3mm) Plastic DFN
LTC3806EDE#TRPBF
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
SYMBOL
V
IN(MIN)
I
Q
PARAMETER
Minimum Input Voltage
Input Voltage Supply Current
Quiescent
Shutdown Mode
Start-Up Mode
Rising RUN Input Threshold Voltage
Falling RUN Input Threshold Voltage
RUN Pin Input Threshold Hysteresis
RUN Input Current
Feedback Voltage
Feedback Pin Input Current
Line Regulation
Load Regulation
Error Amplifier Transconductance
Main Control Loop
The
●
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
IN
= 10V, V
RUN
= 1.5V, unless otherwise specified.
CONDITIONS
(Note 4)
(Note 5)
V
RUN
= 0V
V
RUN
> 1.255V, V
IN
< 7V
V
IN
= 20V
●
MIN
10
V
RUN+
V
RUN–
V
RUN(HYST)
I
RUN
V
FB
I
FB
ΔV
FB
/ΔV
IN
ΔV
FB
/ΔV
ITH
g
m
1.205
1.181
1.116
1.093
45
1.218
1.212
V
IN
= 20V
●
V
IN
= 20V
V
ITH
= 0.75V (Note 6)
●
V
ITH
= 0.75V (Note 6)
10V
≤
V
IN
≤
20V
V
TH
= 0.55V to 0.95V (Note 6)
I
TH
Pin Load =
±5μA
(Note 6)
●
–1
2
U
TEMPERATURE RANGE
–40°C to 85°C
TYP
MAX
UNITS
V
1000
50
80
1.230
1.139
91
1
1.230
18
0.01
–0.1
650
μA
μA
μA
V
V
V
V
mV
nA
V
V
nA
%/V
%
μMho
3806fb
U
U
U
U
W W
W
W
U
90
140
1.255
1.279
1.162
1.185
137
60
1.242
1.248
100
LTC3806
The
●
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
IN
= 10V, V
RUN
= 1.5V, unless otherwise specified.
SYMBOL
I
SENSE(ON)
I
SENSE(OFF)
I
SS
Oscillator
f
OSC
DC(MAX)
Regulator
V
INTVCC
ΔINTV
CC
ΔV
IN
V
LDO(LOAD)
V
UVL+
V
UVL–
Gate Drivers
t
r1
t
f1
t
r2
t
f2
t
DEAD
Gate Driver 1 Output Rise Time
Gate Driver 1 Output Fall Time
Gate Driver 2 Output Rise Time
Gate Driver 2 Output Fall Time
Gate Driver Dead Time
C
L1
= 3300pF
C
L1
= 3300pF
C
L2
= 4700pF
C
L2
= 4700pF
C
L1
= 3300pF, C
L2
= 4700pF
25
18
25
18
100
100
100
100
100
ns
ns
ns
ns
ns
INTV
CC
Regulator Output Voltage
INTV
CC
Regulator Line Regulation
INTV
CC
Load Regulation
Rising V
IN
Threshold Voltage
Falling V
IN
Threshold Voltage
V
IN
= 10V
10V
≤
V
IN
≤
20V
0
≤
I
INTVCC
≤
20mA
–6
14
7.5
–3
15
8
16
8.5
6
6.9
7.8
100
V
mV
%
V
V
Oscillator Frequency
Maximum Duty Cycle
210
84
250
89
290
94
kHz
%
PARAMETER
SENSE Pin Current (G1 High)
SENSE Pin Current (G1 Low)
SS Pin Source Current
CONDITIONS
V
SENSE
= 0V
V
SENSE
= 1V
V
SS
= 1.5V
3
MIN
110
TYP
150
35
0.1
5
MAX
190
50
5
8
UNITS
mV
μA
μA
μA
ELECTRICAL CHARACTERISTICS
V
SENSE(MAX)
Maximum Current Sense Input Threshold
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 LTC3806E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the – 40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3:
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
• 34°C/W)
Note 4:
The minimum operating voltage is allowed once operation begins.
To begin operation, V
IN
must be above the rising undervoltage lockout
threshold with V
RUN
above the rising RUN input threshold.
Note 5:
The dynamic input supply current is higher due to power MOSFET
gate charging (Q
G
• f
OSC
). See Applications Information.
Note 6:
The LTC3806 is tested in a feedback loop which servos V
FB
to the
reference voltage with the I
TH
pin forced to a voltage between 0V and 1.4V
(the no load to full load operating voltage range for the I
TH
pin is 0.3V to
1.23V).
3806fb
3
LTC3806
TYPICAL PERFOR A CE CHARACTERISTICS
FB Voltage vs Temperature
1.2400
1.2310
1.2350
FB PIN CURRENT (nA)
FB VOLTAGE (V)
1.2300
FB VOLTAGE (V)
1.2250
1.2200
1.2150
–40
–15
35
10
TEMPERATURE (°C)
Shutdown Mode I
Q
vs V
IN
80
70
SHUTDOWN MODE, I
Q
(μA)
SHUTDOWN MODE, I
Q
(μA)
T
A
= 25°C
75
SOFT-START CURRENT (μA)
60
50
40
30
20
10
0
0
2
4
6
8
10 12 14 16 18 20
V
IN
(V)
3806 G04
G1 Rise and Fall Time vs C
L
250
T
A
= 25°C
120
100
80
200
RUN THRESHOLDS (V)
TIME (ns)
TIME (ns)
150
100
50
0
0
4000
12000
8000
C
L
(pF)
16000
4
U W
60
3806 G07
FB Voltage Line Regulation
30
T
A
= 25°C
FB Pin Current vs Temperature
25
1.2305
20
15
10
5
1.2300
1.2295
1.2290
85
3806 G01
10 11 12 13 14 15 16 17 18 19 20
V
IN
(V)
3806 G02
0
–40
–15
10
35
TEMPERATURE (°C)
60
85
3806 G03
Shutdown Mode I
Q
vs Temperature
80
7.0
Soft-Start Current vs Temperature
6.5
70
65
60
55
50
–40
6.0
5.5
–15
10
35
TEMPERATURE (°C)
60
85
3806 G05
5.0
–40
–15
10
35
TEMPERATURE (°C)
60
85
3806 G06
G2 Rise and Fall Time vs C
L
T
A
= 25°C
RUN Thresholds vs Temperature
1.22
TRIP
1.20
1.18
1.16
1.14
RELEASE
1.12
1.10
–40
60
40
20
0
20000
0
4000
12000
8000
C
L
(pF)
16000
20000
3806 G08
–15
10
35
TEMPERATURE (°C)
60
85
3806 G10
3806fb
LTC3806
TYPICAL PERFOR A CE CHARACTERISTICS
Frequency vs Temperature
260
155
154
MAX SENSE THRESHOLD (mV)
255
SENSE PIN CURRENT (μA)
–15
35
10
TEMPERATURE (°C)
60
85
3806 G12
FREQUENCY (kHz)
250
245
240
–40
–15
35
10
TEMPERATURE (°C)
INTV
CC
Load Regulation
7.020
7.015
INTV
CC
VOLTAGE (V)
T
A
= 25°C
7.010
7.005
7.000
6.995
6.990
0
10
20
30
INTV
CC
LOAD (mA)
40
50
3806 G14
INTV
CC
VOLTAGE (V)
INTV
CC
Dropout Voltage
vs Current, Temperature
2.8
2.7
T
A
= –40°C
DROPOUT VOLTAGE (V)
2.6
2.4
2.3
2.2
2.1
2.0
1.9
0
10
T
A
= 25°C
T
A
= 55°C
T
A
= 85°C
EFFICIENCY (%)
2.5
U W
60
3806 G11
Maximum Sense Threshold
vs Temperature
32.0
SENSE Pin Current
vs Temperature
153
152
151
150
149
148
147
146
31.5
31.0
30.5
85
145
–40
30.0
–40
–15
35
10
TEMPERATURE (°C)
60
85
3806 G13
INTV
CC
Line Regulation
7.020
7.015
7.010
7.005
7.000
6.995
6.990
10
12
14
16
V
IN
(V)
18
20
3806 G15
Efficiency vs Output Power
90
FIGURE 8 CIRCUIT
T
A
= 0°C
85
80
75
30
20
INTV
CC
LOAD (mA)
40
50
3806 G16
10
20 30 40 50 60 70 80 90 100
% OF MAXIMUM OUTPUT POWER
3806 G17
3806fb
5
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