LTC3900_1 Datasheet, PDF (8/20 Pages) Linear Technology – Synchronous Rectifier Driver for Forward Converters

LTC3900
Applications Information
The timer circuit and current sense comparator in LTC3900
are used to prevent reverse current buildup in the output
inductor.
Timer
Figure 3 shows the LTC3900 timer internal and external
circuits. The timer operates by using an external R-C
charging network to program the time-out period. On
every negative transition at the SYNC input, the chip
generates a 200ns pulse to reset the timer cap. If the
SYNC signal is missing or incorrect, allowing the timer
cap voltage to go high, it shuts off both drivers once the
voltage reaches the time-out threshold. Figure 4 shows
the timer waveforms.
A typical forward converter cycle always turns on Q3
and Q4 alternately and the SYNC input should alternate
between positive and negative pulses. The LTC3900 timer
also includes sequential logic to monitor the SYNC input
sequence. If after one negative pulse, the SYNC compara-
tor receives another negative pulse, the LTC3900 will not
reset the timer cap. If no positive SYNC pulse appears,
both drivers are shut off once the timer times out. Once
positive pulses reappear the timer resets and the drivers
start switching again. This is to protect the external com-
ponents in situations where only negative SYNC pulse is
present and FG output remains high. Figure 5 shows the
timer waveforms with incorrect SYNC pulses.
The LTC3900 has two separate SYNC comparators (S⁺ and
S^– in the Block Diagram) to detect the positive and negative
pulses. The threshold voltages of both comparators are
designed to be of the same magnitude (1.4V typical) but
opposite in polarity. In some situations, for example dur-
ing power up or power down, the SYNC pulse magnitude
may be low, slightly higher or lower than the threshold of
the comparators. This can cause only one of the SYNC
comparators to trip. This also appears as incorrect SYNC
pulse and the timer will not reset.
The timeout period is determined by the external R_TMR
and C_TMR values and is independent of the V_CC voltage.
This is achieved by making the timeout threshold a ratio
of V_CC. The ratio is 0.2x, set internally by R1 and R2 (see
Figure 3). The timeout period should be programmed to
be around one period of the primary switching frequency
using the following formula:
TIMEOUT = 0.2 • R_TMR • C_TMR + 0.27E-6
To reduce error in the timeout setting due to the discharge
time, select C_TMR between 100pF and 1000pF. Start with a
C_TMR around 470pF and then calculate the required R_TMR.
C_TMR should be placed as close as possible to the LTC3900
with minimum PCB trace between C_TMR, the TIMER pin
and GND. This is to reduce any ringing caused by the PCB
trace inductance when C_TMR discharges. This ringing may
introduce error to the timeout setting.
The timer input also includes a current sinking clamp
circuit (Z_TMR in Figure 3) that clamps this pin to about
0.5 • V_CC if there is missing SYNC/timer reset pulse. This
clamp circuit prevents the timer cap from getting fully
charged up to the rail, which results in a longer discharge
SG
R2
TMR
TIMEOUT
TIMER
RESET
V_CC
R1
4
R_TMR
7
Z_TMR
C_TMR
SYNC
FG
CG
TIMER RESET
(INTERNAL)
LAST
PULSE
3900 F03
Figure 3. Timer Circuit
TIMER
Figure 4. Timer Waveforms
TIMEOUT
THRESHOLD
3900 F04
3900fb
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