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PDF LTC3900ES8 Data sheet ( Hoja de datos )
| Número de pieza | LTC3900ES8 | |
| Descripción | Synchronous Rectifier Driver for Forward Converters | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Final Electrical Specifications
LTC3900
Synchronous Rectifier
Driver for Forward Converters
November 2003
FEATURES
DESCRIPTIO
s N-Channel Synchronous Rectifier MOSFET Driver
s Programmable Timeout
s Reverse Inductor Current Sense
s Pulse Transformer Synchronization
s Wide VCC supply range: 4.5V to 11V
s 15ns Rise/Fall Times at VCC = 5V, CL = 4700pF
s Undervoltage Lockout
s Small SO-8 Package
U
APPLICATIO S
s 48V Input Isolated DC/DC Converters
s Isolated Telecom Power Supplies
s Distributed Power Step-Down Converters
s Industrial Control System Power Supplies
s Automotive and Heavy Equipment
The LTC®3900 is a secondary-side synchronous rectifier
driver designed to be used in isolated forward converter
power supplies. The chip drives N-channel rectifier
MOSFETs and accepts pulse sychronization from the
primary-side controller via a pulse transformer.
The LTC3900 incorporates a full range of protection for the
external MOSFETs. A programmable timeout function is
included that disables both drivers when the synchroniza-
tion signal is missing or incorrect. Additionally, the chip
senses the output inductor current through the drain-
source resistance of the catch MOSFET, shutting off the
MOSFET if the inductor current reverses. The LTC3900
also shuts off the drivers if the supply voltage is low.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
VIN
36V TO 72V
BG
LTC1693-2
GATE
LT1950
FORWARD CONTROLLER
GND COMP
LTC1693-2
ISOLATION
BARRIER
T1
Q1
Q3
CSG
SG
T2
L0
D3
+
COUT
RCS2
RCS1
Q4
RCS3
CZ
CS+
CG
CS– VCC
LTC3900
TIMER
FG
SYNC GND
RZ
RB
DZ
RTMR
CVCC
CTMR
QREG
RSYNC
270Ω
RF CF
VOUT
3.3V
R1
R2
LT1797
OC1
VR 3900 F01
Figure 1. Simplified Isolated Forward Converter
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-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
3900i
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
Rise/Fall Time vs Load
Capacitance
50
45
TA = 25°C
VCC = 5V
40
35
30
25
20
RISE TIME
15
FALL TIME
10
5
0
0 1 2 3 4 5 6 7 8 9 10
CLOAD (nF)
3900 G13
Undervoltage Lockout Threshold
Voltage vs Temperature
4.5
4.4
4.3
4.2
4.1
4.0
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3.0
–50
RISING EDGE
FALLING EDGE
–25 0 25 50 75
TEMPERATURE (°C)
100 125
3900 G14
VCC Supply Current vs Load
Capacitance
30
TA = 25°C
25
VCC = 11V
20
15
10
VCC = 5V
5
0
0 1 2 3 4 5 6 7 8 9 10
CLOAD (nF)
3900 G16
LTC3900
VCC Supply Current vs
Temperature
20
CLOAD = 4.7nF
18
16 VCC = 11V
14
12
10
8 VCC = 5V
6
4
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3900 G15
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5 Page 
LTC3900
APPLICATIO S I FOR ATIO
quickly. The VCC supply circuit shown in Figure 9 will
provide power for the LTC3900 within the first few switch-
ing pulses of the primary controller, preventing overheat-
ing of the MOSFETs.
MOSFET Selection
The required MOSFET RDS(ON) should be determined
based on allowable power dissipation and maximum re-
quired output current.
The body diodes conduct during the power-up phase,
when the LTC3900 VCC supply is ramping up. The CG and
FG signals stay low and the inductor current flows through
the body diodes. The body diodes must be able to handle
the load current during start-up until VCC reaches 4.1V.
The LTC3900 drivers dissipate power when switching
MOSFETs. The power dissipation increases with switch-
ing frequency, VCC and size of the MOSFETs. To calculate
the driver dissipation, the total gate charge QG is used.
This parameter is found on the MOSFET manufacturers
data sheet.
The power dissipated in each LTC3900 MOSFET driver is:
PDRIVER = QG • VCC • fSW
where fSW is the switching frequency of the converter.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3900 for your layout:
1. Connect the 4.7µF bypass capacitor as close as possible
to the VCC and GND pins.
2. Connect the two MOSFET drain terminals directly to the
transformer. The two MOSFET sources should be as close
together as possible.
3. Keep the timer, SYNC and VCC regulator circuit away
from the high current path of Q3, Q4 and T1.
4. Place the timer capacitor, CTMR as close as possible to
the LTC3900.
5. Keep the PCB trace from the resistors RCS1, RCS2 and
RCS3 to the LTC3900 CS+/CS– pins as short as possible.
Connect the other ends of the resistors directly to the drain
and source of the MOSFET, Q4.
PACKAGE DESCRIPTIO
.050 BSC
S8 Package
8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.045 ±.005
.189 – .197
(4.801 – 5.004)
NOTE 3
8 765
.245
MIN .160 ±.005
.030 ±.005
TYP
RECOMMENDED SOLDER PAD LAYOUT
.010
(0.254
–
–
.020
0.508)
×
45°
NOTE:
1. DIMENSIONS
IN
INCHES
(MILLIMETERS)
2. DRAWING NOT TO SCALE
3. THESE DIMENSIONS DO NOT INCLUDE
MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS
SHALL NOT EXCEED .006" (0.15mm)
.008 – .010
(0.203 – 0.254)
.016 – .050
(0.406 – 1.270)
.228 – .244
(5.791 – 6.197)
.150 – .157
(3.810 – 3.988)
NOTE 3
0°– 8° TYP
1
.053 – .069
(1.346 – 1.752)
2
34
.004 – .010
(0.101 – 0.254)
.014 – .019
(0.355 – 0.483)
TYP
.050
(1.270)
BSC
SO8 0303
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| PDF Descargar | [ Datasheet LTC3900ES8.PDF ] | |
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