LTC3901_15 Datasheet, PDF (7/16 Pages) Linear Technology – Secondary Side Synchronous Driver for Push-Pull and Full-Bridge Converters

LTC3901
APPLICATIO S I FOR ATIO
In the first period, SDRA goes low (followed by DRVA
going high) and T2 generates a positive voltage at the
LTC3901’s SYNC input. The LTC3901’s ME output then
pulls low. Current flows to the load through MOSFET MF,
T1’s secondary and L1.
In the second period, SDRA goes high and T2 provides
approximately 0V at the LTC3901 SYNC input. This causes
the LTC3901’s ME output to go high and both MOSFET ME
and MF to conduct. This is the free-wheeling period with
T1 secondary winding shorted.
In the third period, SDRB goes low (followed by DRVB
going high) and T2 generates a negative voltage at the
LTC3901’s SYNC input. The LTC3901’s MF output then
pulls low. Current flows to the load through MOSFET ME,
T1’s secondary and L1.
The last period is also a free-wheeling period like the
second period. Both SDRA and SDRB are high and the
LTC3901 forces both MOSFETs ME and MF to conduct.
External MOSFET Protection
A programmable timer and two differential input current
sense comparators are included in the LTC3901 for pro-
tection of the external MOSFETs during power down and
Burst Mode^® operation. The chip also shuts off the
MOSFETs if V_CC < 4.1V or if the synchronization sequence
is incorrect.
When the primary controller is powering down, the
LTC3901 continues to operate by drawing power from the
V_CC bypass cap, C_VCC. The primary controller synchro-
nous output stops switching and the LTC3901 SYNC input
goes to 0V. Both ME and MF remain on and the decreasing
inductor current continues to flow into the load. Once the
inductor current decreases to zero, it reverses direction,
discharging the output capacitor C_OUT to GND through
both MOSFETs. At the same time, the C_VCC voltage contin-
ues to drop. When the voltage drops below 4.1V, the
LTC3901 shuts down and pulls both ME and MF low. This
causes the inductor current to stop suddenly and the drain
voltage of both MOSFETs to fly high, due to the buildup of
inductor energy. In the absence of a protection timer, if the
inductor energy is high due to a long period of current
reversal, the drain voltage can go above the MOSFET’s
voltage rating and cause damage to the MOSFET.
Burst Mode is a registered trademark of Linear Technology Corporation.
MOSFETs are also kept on for long periods when the
primary controller enters Burst Mode operation. Both ME
and MF stop switching until the primary controller exits
Burst Mode operation. This would also cause the inductor
current to reverse and the drains to fly high.
In both of these situations, the timer and/or current sense
comparator shuts off the drivers before or immediately
after the inductor current reverses direction. This prevents
the buildup of inductor energy.
Timer
The timer circuit (Figure 3) operates by using an external
R-C charging network to program the timeout period. On
every transition at the SYNC input, the chip generates a
200ns pulse to reset the timer capacitor. If the SYNC signal
is missing or incorrect (allowing the timer capacitor volt-
age to go high) it shuts off both drivers once the voltage
reaches the timeout threshold. Figure 4 shows the timer
waveforms.
V_CC
16
LTC3901
V_CC
TMR
TIMEOUT
R1
180k
TIMER ₇
Z_TMR
0.5 • V_CC
R_TMR
32k
C_TMR
470pF
R2 ^TIMER
45k ^RESET
M_TMR
3901 F03
Figure 3. Timer Circuit
SYNC
0V
ME
MF
TIMER RESET
(INTERNAL)
TIMER
Figure 4. Timer Waveforms
TIMEOUT
THRESHOLD
3901 F02
3901f
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