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LTC3901 Datasheet(PDF) 10 Page - Linear Technology

Part # LTC3901
Description  Secondary Side Synchronous Driver for Push-Pull and Full-Bridge Converters
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC3901 Datasheet(HTML) 10 Page - Linear Technology

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LTC3901
10
3901f
APPLICATIO S I FOR ATIO
the resistors to the LTC3901 CSX+/CSXpins as short as
possible . Add a series resistor, RCSX3, with value equal to
parallel sum of RCSX1 and RCSX2 to the CSXpin and
connect the other end of RCSX3 directly to the source of the
MOSFET.
SYNC Input
Figure 8 shows the external circuit for the LTC3901 SYNC
input. The gate drive transformer (T2) should be selected
based on the primary switching frequency and SDRA/
SDRB output voltage.
The values of the CSG and RSYNC should then be adjusted
to obtain a optimum SYNC pulse shape and amplitude. The
amplitude of the SYNC pulse should be much higher than
the LTC3901 SYNC threshold of ±1.4V. Amplitudes greater
than ±5V will help to speed up the SYNC comparator and
reduce the propagation delay from SYNC to the drivers.
When SDRA and SDRB lines go low, the resulting under-
shoot or overshoot must not exceed the minimum SYNC
threshold of ±1V.
higher than 4.5V. This reduces the number of external
components needed.
The LTC3901 has an UVLO detector that pulls the drivers’
output low if VCC < 4.1V. The output remains off from
VCC = 1V to 4.1V. The UVLO detector has 0.5V of hyster-
esis to prevent chattering.
In a typical push-pull converter, the secondary side cir-
cuits have no power until the primary side controller starts
operating. Since power for the LTC3901 is derived from
the power transformer T1, the LTC3901 will initially re-
main off. During this period (VCC < 4.1V), the synchronous
MOSFETs ME and MF will remain off and the MOSFETs’
body diodes will conduct. The MOSFETs may experience
very high power dissipation due to a high voltage drop in
the body diodes. To prevent MOSFET damage, a VCC
voltage greater than 4.1V should be provided quickly. The
VCC supply circuit in Figure 9 will provide power for the
LTC3901 within the first few switching pulses of the
primary controller, preventing overheating of the MOSFETs.
Full-Bridge Converter Application
The LTC3901 can be used in full-bridge converter applica-
tions. Figure 10 shows a simplified full-bridge converter
circuit. The LTC3901 circuit and operation is the same as
in the push-pull application (refer to Figure 1). On the pri-
mary side there are four power MOSFETs, MA to MD, driven
by the respective outputs of the primary controller. Trans-
former T3 and T4 step up the gate drives for MA and MC.
Each full cycle of the full-bridge converter includes four
distinct periods which are similar to those found in the
push-pull application. Figure 11 shows the full-bridge
converter switching waveforms. The shaded areas corre-
spond to power delivery periods.
Figure 9. VCC/PVCC Regulator
Figure 8. SYNC Input Circuit
T2
CSG
0.1µF
PRIMARY
CONTROLLER
LTC3901
SYNC
3901 F08
SDRA
SDRB
RSG
220Ω
RSYNC
4.7k
VCC/PVCC Regulator
The VCC/PVCC supply for the LTC3901 can be generated by
peak rectifying the transformer secondary winding as
shown in Figure 9. The Zener diode DZ sets the output
voltage (VZ – 0.7V). Resistor RB (on the order of a few
hundred ohms), in series with the base of QREG, may be
required to surpress high frequency oscillations depend-
ing on QREG’s selection. A power MOSFET can also be used
by increasing the zener diode value to offset the drop of the
gate-to-source voltage. The VCC input is separated from
the PVCC input through a 100Ω resistor. This lowers the
driver switching feedthrough. Connect a 1µF bypass ca-
pacitor for the VCC supply. PVCC supply current varies
linearly with the supply voltage, driver load and clock
frequency. A 4.7µF bypass capacitor for the PVCC supply
is sufficient for most applications. Alternatively, the
LTC3901 can be powered directly by VOUT if the voltage is
3901 F09
D3
MBR0540
T1
SECONDARY
WINDING
0.1µF
RZ
2k
RB
OPTIONAL
QREG
FZT690B
CPVCC
4.7µF
CVCC
1µF
PVCC
VCC
6V
DZ
RVCC
100Ω


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