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