LTC3900ES8#TR Linear Technology, LTC3900ES8#TR Datasheet - Page 7

• Manufacturer Part Number: LTC3900ES8#TR
• Description: IC DRIVER RECT SYNC CONV 8SOIC
• Manufacturer: Linear Technology

Specifications of LTC3900ES8#TR

• Configuration: High-Side
• Input Type: Inverting and Non-Inverting
• Delay Time: 60ns
• Current - Peak: 2A
• Number Of Configurations: 2
• Number Of Outputs: 2
• Voltage - Supply: 4.5 V ~ 11 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• High Side Voltage - Max (bootstrap): -

APPLICATIONS INFORMATION
Overview
In a typical forward converter topology, a power trans-
former is used to provide the functions of input/output
isolation and voltage step-down to achieve the required
low output voltage. Schottky diodes are often used on
the secondary-side to provide rectifi cation. Schottky
diodes, though easy to use, result in a loss of effi ciency
due to relatively high voltage drops. To improve effi ciency,
synchronous output rectifi ers utilizing N-channel MOSFETs
can be used instead of Schottky diodes. The LTC3900
provides all of the necessary functions required to drive
the synchronous rectifi er MOSFETs.
Figure 1 shows a simplifi ed forward converter application.
T1 is the power transformer; Q1 is the primary-side power
transistor driven by the primary controller, LT1952 output
(OUT). The pulse transformer T2 provides synchronization
and is driven by LT1952 synchronization signal, S
from the primary controller. Q3 and Q4 are secondary-side
synchronous switches driven by the LTC3900’s FG and CG
output. Inductor L
fi lter to provide a steady DC output voltage for the load.
Also shown in Figure 1 is the feedback path from V
through the optocoupler driver LT4430 and an optocoupler,
back to the primary controller to regulate V
Each full cycle of the forward converter operation con-
sists of two periods. In the fi rst period, Q1 turns on and
the primary-side delivers power to the load through T1.
SG goes high and T2 generates a negative pulse at the
LTC3900 SYNC input. The LTC3900 forces FG to turn on
and CG to turn off, Q3 conducts. Current fl ows to the
O
and capacitor C
(S
(OUT)
SYNC
GATE
OUT
OUT
SG
CG
FG
)
form the output
Figure 2. Synchronization Waveforms
OUT
.
OUT
or SG
OUT
load through Q3, T1 and L
off, SG goes low and T2 generates a positive pulse at the
LTC3900 SYNC input. The LTC3900 forces FG to turn off
and CG to turn on, Q4 conducts. Current continues to
fl ow to the load through Q4 and L
LTC3900 synchronization waveforms.
External MOSFET Protection
A programmable timer and a differential input current sense
comparator are included in the LTC3900 for protection
of the external MOSFET during power down and Burst
Mode
if V
When the primary controller is powering down, the primary
controller shuts down fi rst and the LTC3900 continues to
operate for a while by drawing power from the V
cap, C
SYNC pulse to the LTC3900. The LTC3900 keeps one of
the drivers turned on depending on the polarity of the
last SYNC pulse. If the last SYNC pulse is positive, CG
will remain high and the catch MOSFET, Q4 will stay on.
The inductor current will start falling down to zero and
continue going in the negative direction due to the voltage
that is still present across the output capacitor (the current
now fl ows from C
the inductor current is negative, the inductor current will
produce high voltage across Q4, resulting in a MOSFET
avalanche. Depending on the amount of energy stored in
the inductor, this avalanche energy may damage Q4.
Burst Mode is a registered trademark of Linear Technology Corporation.
CC
®
VCC
< 4.1V.
operation. The chip also shuts off the MOSFETs
. The SG signal stops switching and there is no
3900 F02
OUT
back to L
O
. In the next period, Q1 turns
O
). If Q4 is turned off while
O
. Figure 2 shows the
LTC3900
CC
bypass
3900fa
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