LTC1698
Isolated Secondary
Synchronous Rectifier Controller
FEATURES
s
s
s
s
s
s
s
s
s
s
s
DESCRIPTIO
High Efficiency Over Wide Load Current Range
±0.8%
Output Voltage Accuracy
Dual N-Channel MOSFET Synchronous Drivers
Pulse Transformer Synchronization
Optocoupler Feedback Driver
Programmable Current Limit Protection
±5%
Margin Output Voltage Adjustment
Adjustable Overvoltage Fault Protection
Power Good Flag
Auxiliary 3.3V Logic Supply
Available in 16-Lead SSOP and SO Packages
The LTC
®
1698 is a precision secondary-side forward
converter controller that synchronously drives external
N-channel MOSFETs. It is designed for use with the
LT
®
3781 primary-side synchronous forward converter
controller to create a completely isolated power supply.
The LT3781 synchronizes the LTC1698 through a small
pulse transformer and the LTC1698 drives a feedback
optocoupler to close the feedback loop. Output accuracy
of
±0.8%
and high efficiency over a wide range of load
currents are obtained.
The LTC1698 provides accurate secondary-side current
limit using an external current sense resistor. The input
voltage at the MARGIN pin provides
±5%
output voltage
adjustment. A power good flag and overvoltage input are
provided to ensure proper power supply conditions. An
auxiliary 3.3V logic supply is included that supplies up to
10mA of output current.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
s
s
s
48V Input Isolated DC/DC Converters
Isolated Telecommunication Power Systems
Distributed Power Step-Down Converters
Industrial Control Systems
Automotive and Heavy Equipment
TYPICAL APPLICATIO
V
IN
36V to 72V
L1
+
Q1
D1
V
DD
BIAS
C
OUT
+
1
10
V
DD
CG
FG
I
SNS
R1
R2
T1
Q4
Q2
R
PRISEN
R
SECSEN
11
C
SG
TG BG
C
SYNC
15
T2
D2
Q3
2
16
12
•
PWRGD
8
V
FB
C
FB
V
COMP
6
LTC1698
I
SNSGND
SYNC
I
COMP
OVPIN
13
9
R
CILM
•
SG
R
SYNC
MARGIN
7
V
MARGIN
V
AUX
3.3V
10mA
LT3781
R
K
REF
V
FB
R
E
C
K
5
OPTODRV
PGND
3
V
AUX
GND
4
14
O.1µF
1681 F01
V
C
R
F
+
–
C
F
ISOLATION
BOUNDARY
PLEASE REFER TO FIGURE 12 IN THE TYPICAL APPLICATIONS
SECTION FOR THE COMPLETE 3.3V/15A APPLICATION SCHEMATIC
Figure 1. Simplified 2-Transistor Isolated Forward Converter
1698f
U
V
OUT
R
C
C
C
R5
C
CILM
R4
U
U
•
•
1
LTC1698
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
V
DD
CG
PGND
GND
OPTODRV
V
COMP
MARGIN
V
FB
1
2
3
4
5
6
7
8
16 FG
15 SYNC
14 V
AUX
13 I
COMP
12 I
SNS
11 I
SNSGND
10 PWRGD
9
OVPIN
V
DD
, PWRGD ....................................................... 13.2V
Input Voltage
MARGIN, V
FB
, OVPIN, I
SNSGND
, I
SNS
... – 0.3V to 5.3V
SYNC ..................................................... – 14V to 14V
Output Voltage
V
COMP
, I
COMP
(Note 2) ......................... – 0.3V to 5.3V
Power Dissipation .............................................. 500mW
Operating Temperature Range
LTC1698E (Note 3) ............................ – 40°C to 85°C
LTC1698I ........................................... – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC1698EGN
LTC1698ES
LTC1698IGN
LTC1698IS
GN PART MARKING
1698
1698I
GN PACKAGE
S PACKAGE
16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 130°C/W (GN)
T
JMAX
= 125°C,
θ
JA
= 110°C/W (SO)
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL
V
DD
V
UVLO
I
VDD
PARAMETER
Supply Voltage
Undervoltage Lockout
V
DD
Supply Current
The
q
indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
DD
= 8V, unless otherwise noted. (Note 4)
CONDITIONS
q
MIN
6
TYP
8
4
MAX
12.6
UNITS
V
V
V
FB
, OVPIN, V
ISNS
, V
ISNSGND
= 0V,
C
FG
= C
CG
= 1000pF, C
VAUX
= 0.1µF,
V
SYNC
= 0V
f
SYNC
= 100kHz (Note 5)
q
1.8
5.0
1.223
1.215
1.233
1.233
0.05
1.65
16.5
4
MARGIN and Error Amplifier
V
FB
I
VFB
V
MARGIN
R
MARGIN
∆V
FB
G
ERR
BW
ERR
V
CLAMP
I
VCOMP
OPTODRV
G
OPTO
BW
OPTO
V
OPTOHIGH
I
OPTOSC
Opto Driver DC Gain
Opto Driver Unity-Gain Bandwidth
Opto Driver Output High Voltage
Opto Driver Output Short-Circuit Current
OVPIN, V
ISNS
, V
ISNSGND
= 0V
No Load (Note 6)
V
FB
, OVPIN, V
ISNSGND
= 0V, V
ISNS
= – 50mV,
I
OPTODRV
= –10mA
OVPIN, V
ISNSGND
, V
ISNS
= 0V, V
FB
= 1.233V
q
q
q
Feedback Voltage
Feedback Input Current
MARGIN Voltage
MARGIN Input Resistance
Feedback Voltage Adjustment
Error Amplifier Open-Loop DC Gain
Error Amplifier Unity-Gain Bandwidth
Error Amplifier Output Clamp Voltage
Error Amplifier Source Current
Error Amplifier Sink Current
MARGIN = Open, V
COMP
= 1V (Note 7)
q
1.243
1.251
1
V
FB
= 1.233V
MARGIN = Open
V
MARGIN
= 3.3V
V
MARGIN
= 0V
V
COMP
= 0.8V to 1.2V, Load = 2kΩ, 100pF
No Load (Note 6)
V
FB
= 0V
V
FB
= 0V
V
FB
= 5V, V
COMP
= 1.233V
q
q
q
q
4
–6
65
5
–5
90
2
2
6
–4
q
q
3
4.75
– 25
7
5
1
– 10
5.25
4
– 50
5
– 25
– 10
2
U
W
U
U
W W
W
mA
mA
V
V
µA
V
kΩ
%
%
dB
MHz
V
mA
mA
V/V
MHz
V
mA
1698f
LTC1698
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER
V
AUX
V
AUX
Auxiliary Supply Voltage
CONDITIONS
The
q
indicates specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
DD
= 8V, unless otherwise noted. (Note 4)
MIN
q
q
q
q
TYP
3.320
0.05
0.05
MAX
3.465
1
1
– 23.0
– 22.5
– 120
– 80
280
370
5
UNITS
V
µA
µA
mV
mV
µA
µA
µA
µA
millimho
dB
C
VAUX
= 0.1µF, I
LOAD
= 0mA to 10mA, V
DD
= 7V to 12.6V
V
ISNSGND
= 0V
V
ISNS
= 0V
V
ICOMP
= 2.5V, V
ISNSGND
= 0V
V
ISNSGND
= 0V, V
ISNS
= – 0.3V, V
ICOMP
= 2.5V (Note 8)
3.135
Current Limit Amplifier
I
ISNSGND
I
SNSGND
Input Current
I
ISNS
I
SNS
Input Current
V
ILIMTH
I
ICOMP
Current Limit Threshold
(V
ISNS
– V
ISNSGND
)
I
COMP
Source Current
I
COMP
Sink Current
g
mILIM
G
ICOMP
Current Limit Amplifier
Transconductance
Current Limit Amplifier
Open-Loop DC Gain
Percent Below V
FB
Power Good Sink Current
Power Good Output Low Voltage
– 27.0
– 27.5
– 280
– 370
120
80
2.2
48
– 25
– 25
– 200
– 200
200
200
3.5
60
q
V
ISNSGND
= 0V, V
ISNS
= 0.3V, V
ICOMP
= 2.5V (Note 8)
q
V
ISNSGND
= 0V, V
ICOMP
= 2.5V, I
ICOMP
=
±10µA
V
ICOMP
= 2.5V, No Load
q
q
PWRGD and OVP Comparators
V
PWRGD
I
PWRGD
V
OL
V
FB
↓,
MARGIN = Open (Note 9)
V
FB
= 2V
V
FB
= 0V
I
PWRGD
= 3mA, V
FB
= 0V
V
FB
= V
ISNS
= V
ISNSGND
= 0V, OVPIN
↑
(Note 9)
V
OVPIN
= 1.233V
V
FB
↑
V
FB
↓
V
OVPIN
↑,
C
OPTODRV
= 0.1µF
q
q
q
q
q
q
q
q
q
q
q
–9
10
–6
–3
10
0.4
%
µA
mA
V
V
µA
ms
ms
µs
V
V
µA
kHz
ns
ns
ns
V
OVPREF
OVPIN Threshold
I
OVPIN
OVPIN Input Bias Current
t
PWRGD
t
OVP
Power Good Response Time
Power Bad Response Time
Overvoltage Response Time
1.18
1
0.5
1.233
0.1
2
1
5
1.28
1
5
2.5
20
2.2
–1
50
400
90
40
SYNC and Drivers
V
PT
SYNC Input Positive Threshold
V
NT
I
SYNC
f
SYNC
t
d
t
SYNC
t
r
, t
f
t
DDIS
SYNC Input Negative Threshold
SYNC Input Current
SYNC Frequency Range
SYNC Input to Driver Output Delay
Minimum SYNC Pulse Width
Driver Rise and Fall Time
Driver Disable Time-Out
V
SYNC
=
±10V
C
FG
= C
CG
= 1000pF, V
SYNC
=
±5V
C
FG
= C
CG
= 1000pF, f
SYNC
= 100kHz, V
SYNC
=
±5V
f
SYNC
= 100kHz, V
SYNC
=
±10V
(Note 6)
C
FG
= C
CG
= 1000pF, f
SYNC
= 100kHz, V
SYNC
=
±5V,
10% to 90%
C
FG
= C
CG
= 1000pF, f
SYNC
= 100kHz, V
SYNC
=
±5V
Measured from CG
↑
(Note 10)
1
– 2.2
50
1.6
–1.6
1
40
q
q
q
q
q
75
10
q
10
15
20
µs
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired. All voltages refer to GND.
Note 2:
The LTC1698 incorporates a 5V linear regulator to power internal
circuitry. Driving these pins above 5.3V may cause excessive current flow.
Guaranteed by design and not subject to test.
Note 3:
The LTC1698E 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. For guaranteed performance to
specifications over the –40°C to 85°C range, the LTC1698I is available.
Note 4:
All currents into device pins are positive; all currents out of the
device pins are negative. All voltages are referenced to ground unless
otherwise specified. For applications with V
DD
< 7V, refer to the Typical
Performance Characteristics.
Note 5:
Supply current in active operation is dominated by the current
needed to charge and discharge the external FET gates. This will vary with
the LTC1698 operating frequency, supply voltage and the external FETs
used.
Note 6:
This parameter is guaranteed by correlation and is not tested.
Note 7:
V
FB
is tested in an op amp feedback loop which servos V
FB
to the
internal bandgap voltage.
Note 8:
The current comparator output current varies linearly with
temperature.
Note 9:
The PWRGD and OVP comparators incorporate 10mV of
hysteresis.
Note 10:
The driver disable time-out is proportional to the SYNC period
within the frequency synchronization range.
1698f
3
LTC1698
TYPICAL PERFOR A CE CHARACTERISTICS
V
FB
vs Temperature
1.248
V
DD
= 8V
1.248
1.242
1.236
V
FB
(V)
V
FB
(V)
V
FB
(V)
1.230
1.224
1.218
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1698 G01
I
SNS
Threshold vs Temperature
–22.5
–23.0
–23.5
V
DD
= 8V
–22.5
–23.0
–23.5
I
SNS
THRESHOLD (mV)
I
SNS
THRESHOLD (mV)
–24.5
–25.0
–25.5
–26.0
–24.5
–25.0
–25.5
–26.0
g
mILIM
(millimho)
–24.0
–26.5
–27.0
–27.5
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1698 G04
OVPIN Threshold vs Temperature
1.28
V
DD
= 8V
1.26
1.26
OVPIN THRESHOLD (V)
POWER GOOD THRESHOLD (V)
OVPIN THRESHOLD (V)
1.24
1.22
1.20
1.18
–50 –25
0
25 50
75 100 125 150
TEMPERATURE (°C)
1698 G07
4
U W
V
FB
vs V
DD
T
A
= 25°C
1.295
1.282
V
FB
vs V
MARGIN
V
DD
= 8V
T
A
= 25°C
5
4
3
2
1
0
–1
–2
–3
–4
0 0.33 0.66 0.99 1.32 1.65 1.98 2.31 2.64 2.97 3.3
V
MARGIN
(V)
1698 G03
1.242
1.270
1.258
1.236
1.245
1.233
1.221
1.208
∆V
FB
(%)
1.230
1.224
1.196
1.184
1.218
5
6
7
8
9 10
V
DD
(V)
11
12
13
14
1.171
–5
1698 G02
I
SNS
Threshold vs V
DD
T
A
= 25°C
5.0
4.6
4.2
3.8
3.4
3.0
2.6
Current Limit Amplifier g
m
vs Temperature
V
DD
= 8V
–24.0
–26.5
–27.0
–27.5
5
6
7
8
9 10
V
DD
(V)
11
12
13
14
2.2
–50 –25
0
100 125 150
TEMPERATURE (°C)
1698 G06
25
50
75
1698 G05
OVPIN Threshold vs V
DD
1.28
T
A
= 25°C
1.196
Power Good Threshold
vs Temperature
V
DD
= 8V
–3.0
1.181
–4.2
1.24
1.166
–5.4
∆V
FB
(%)
1.22
1.152
–6.6
1.20
1.137
–7.8
1.18
5
6
7
8
9 10
V
DD
(V)
11
12
13
14
1.122
–50 –25
0
–9.0
25 50
75 100 125 150
TEMPERATURE (°C)
1698 G09
1698 G08
1698f
LTC1698
TYPICAL PERFOR A CE CHARACTERISTICS
V
AUX
vs Temperature
3.465
V
DD
= 8V
I
LOAD
= 0mA
3.424
3.383
V
AUX
(V)
V
AUX
(V)
3.300
3.259
3.218
3.176
3.135
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1698 G10
3.300
3.259
3.218
3.176
3.135
5
6
7
8
9 10
V
DD
(V)
11
12
13
14
V
AUX
(V)
3.341
V
AUX
Short-Circuit Current
vs Temperature
0
V
DD
= 8V
0
V
AUX
SHORT-CIRCUIT CURRENT (mA)
V
AUX
SHORT-CIRCUIT CURRENT (mA)
–10
–10
OPTO DRIVER OUTPUT VOLTAGE (V)
–20
–30
–40
–50
–50 –25
0
25 50
75 100 125 150
TEMPERATURE (°C)
1698 G13
Maximum OPTO Driver Output
Voltage vs Load Current
MAXIMUM OPTO DRIVER OUTPUT VOLTAGE (V)
MAXIMUM OPTO DRIVER OUTPUT VOLTAGE (V)
OPTO DRIVER SHORT-CIRCUIT CURRENT (mA)
8
6
4
2
T
A
= 25°C
V
COMP
= 0V
0
1
2
3 4 5 6 7 8
LOAD CURRENT (mA)
9
10
0
U W
V
DD
= 10V
V
DD
= 8V
V
DD
= 7V
V
DD
= 6V
V
DD
= 5V
V
AUX
vs Line Voltage
3.465
V
DD
= 8V
I
LOAD
= 0mA
3.424
3.383
3.341
3.465
V
AUX
vs Load Current
V
DD
= 8V
T
A
= 25°C
3.424
3.383
3.341
3.300
3.259
3.218
3.176
3.135
0
1
2
3 4 5 6 7 8
LOAD CURRENT (mA)
9
10
1698 G11
1698 G12
V
AUX
Short-Circuit Current
vs V
DD
T
A
= 25°C
3.030
3.024
3.018
3.012
3.006
3.000
2.994
2.988
2.982
2.976
2.970
Opto Driver Load Regulation
V
DD
= 8V
T
A
= 25°C
1.0
0.8
0.6
0.4
PERCENT (%)
–20
0.2
0
–0.2
–0.4
–0.6
–0.8
0
1
2
3 4 5 6 7 8
LOAD CURRENT (mA)
9
10
–1.0
–30
–40
–50
5
6
7
8
10
V
DD
(V)
9
11
12
13
14
1698 G14
1698 G15
Maximum OPTO Driver Output
Voltage vs Temperature
8
V
DD
= 10V
Opto Driver Short-Circuit Current
vs Temperature
–10
V
DD
= 8V
V
OPTODRV
= 1.233V
–15
–20
–25
–30
–35
–40
–45
–50
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1698 G16
6
V
DD
= 8V
V
DD
= 7V
4
V
DD
= 6V
V
DD
= 5V
2
V
COMP
= 0V
I
OPTODRV
= –10mA
0
25 50 75 100 125 150
TEMPERATURE (°C)
1698 G23
0
–50 –25
1698 G22
1698f
5
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