LTC3901 Linear Technology, LTC3901 Datasheet - Page 10

• Manufacturer Part Number: LTC3901
• Description: Secondary Side Synchronous Driver for Push-Pull and Full-Bridge Converters
• Manufacturer: Linear Technology

LTC3901
APPLICATIO S I FOR ATIO
the resistors to the LTC3901 CSX
possible . Add a series resistor, R
parallel sum of R
connect the other end of R
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 C
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.
V
The V
peak rectifying the transformer secondary winding as
shown in Figure 9. The Zener diode D
voltage (V
hundred ohms), in series with the base of Q
required to surpress high frequency oscillations depend-
ing on Q
by increasing the zener diode value to offset the drop of the
gate-to-source voltage. The V
the PV
driver switching feedthrough. Connect a 1µF bypass ca-
pacitor for the V
linearly with the supply voltage, driver load and clock
frequency. A 4.7µF bypass capacitor for the PV
is sufficient for most applications. Alternatively, the
LTC3901 can be powered directly by V
10
CC
/PV
CC
CC
CC
/PV
REG
input through a 100Ω resistor. This lowers the
Regulator
Z
CONTROLLER
CC
’s selection. A power MOSFET can also be used
– 0.7V). Resistor R
PRIMARY
supply for the LTC3901 can be generated by
Figure 8. SYNC Input Circuit
CC
CSX1
SG
SDRB
SDRA
U
supply. PV
and R
and R
220Ω
0.1µF
R
CSX3
C
U
SG
SG
SYNC
CSX2
CC
directly to the source of the
T2
CC
B
CSX3
should then be adjusted
input is separated from
+
(on the order of a few
/CSX
supply current varies
to the CSX
W
, with value equal to
OUT
R
4.7k
SYNC
–
Z
pins as short as
LTC3901
SYNC
sets the output
if the voltage is
3901 F08
REG
–
U
CC
, may be
pin and
supply
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 V
V
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 (V
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 V
voltage greater than 4.1V should be provided quickly. The
V
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.
CC
CC
SECONDARY
= 1V to 4.1V. The UVLO detector has 0.5V of hyster-
WINDING
supply circuit in Figure 9 will provide power for the
T1
MBR0540
0.1µF
D3
Figure 9. V
CC
< 4.1V. The output remains off from
CC
R
2k
Z
/PV
CC
OPTIONAL
CC
6V
D
Z
R
< 4.1V), the synchronous
Regulator
C
B
4.7µF
PVCC
Q
FZT690B
REG
100Ω
R
VCC
3901 F09
C
1µF
VCC
PV
V
CC
CC
3901f
CC