Final Electrical Specifications
LT3804
Secondary Side Dual
Output Controller with Opto Driver
U
June 2003
DESCRIPTIO
FEATURES
The LT®3804 is a high efficiency step-down switching
regulatorwithoptocouplerfeedbackcontrolforregulating
multiple outputs in single-secondary winding isolated
power supplies.
■
Regulates Two Secondary Outputs
Optocoupler Feedback Driver and Second Output
Synchronous Driver Controller
■
■
■
■
■
■
■
True Differential Remote Sensing Regulation
High Switching Frequency: up to 800kHz
Programmable Current Limit
Programmable Soft-Start and Power Good
Automatic Frequency Synchronization
Available in Thermally Enhanced 28-Lead TSSOP
TheLT3804contains anerroramplifierand anoptocoupler
driver to regulate the first (main) output. For the second
output regulation, the LT3804 contains a complete PWM
controller todrivedualsynchronousN-channelMOSFETs.
With leading edge modulation, it operates with either
current or voltage mode control of the primary side. The
LT3804 is synchronized to the falling edge of the trans-
former secondary winding and can be used in single-
ended or double-ended isolated power converter topolo-
gies. A user selectable discontinuous conduction mode
improves light load efficiency.
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APPLICATIO S
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48V Input Isolated DC/DC Converters
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Multiple Output Power Supplies
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Offline Converters
DC/DC Power Modules
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True differential Kelvin sensing is used for each output
feedback amplifier to achieve high regulation accuracy
and design simplicity. Other features include soft start,
current limit and power good flags.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIO
V
OUT1
3.3V
V
IN
+
–
L1
1.8µH
V
V
AT 15A
OS1
36V
0.003Ω
+
+
V
CC
BIAS
TO 72V
C
OUT1
LT3804
OS1
SYNC
CL1N
•
•
L2
1.8µH
Q1
OUT2
TGATE
SW
Q4
CL1P
V
1.8V
AT 15A
OUT2
0.003Ω
LTC1693-1
V
CC
OUT1
IN1
C
OUT2
Q3
x2
Q2
x2
BGATE
PGND
CSET
IN2
390pF
–
+
V
V
GNDS1
OS1
CL2P
CL2N
•
•
•
•
TG
BG
604Ω
V
AOUT2
OS1
V
V
FB1
LT3781
2.74k
3.01k
SG
V
FB2
V
V
1.5k
REF
AOUT1
+
–
V
C
FB
GNDS2
OPTO
3804 F01
C
, C : SANYO POSCAP 4TPE680MF 680µF/4V
OUT1 OUT2
ISOLATION
BOUNDARY
L1, L2: SUMIDA CEP125-IR8MC-H
Q1-Q4: SILICONIX Si7892DP
Figure 1. 250kHz, 3.3V and 1.8V Output Isolated DC/DC Converter (Simplified Schematic)
3804i
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
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