LTC3721-2 LINER [Linear Technology], LTC3721-2 Datasheet - Page 10

• Manufacturer Part Number: LTC3721-2
• Description: Synchronous Rectifier Driver for Forward Converters
• Manufacturer: LINER [Linear Technology]

LTC3900
applicaTions inForMaTion
The value of the resistors, R
be calculated using the following formulas to meet both
the threshold and clamp voltage requirements:
where:
If the LTC3900 still operates in discontinuous mode with
the calculated resistance value, increase the value of R
to raise the threshold. The resistors R
CS
form an R-C delay; this slows down the response time
of the comparator. The resistors and CS
currents also create an input offset error.
10
k = 48 • I
R
R
R
If k = 0 or less than zero, R
I
R
V
voltage
N
+
RIPPLE
= R
IN(MAX)
CS2
CS1
CS3
DS(ON)
S
pins input capacitance plus the PCB trace capacitance
/N
INDUCTOR
CURRENT
CS3
P
= {200 • V
= k • R
= {R
SYNC
Figure 6b. Continuous Current Mode Operation
with Adjusted Current Sense Threshold
= Power transformer T1, turn ratio
CG
= Inductor peak-to-peak ripple current
FG
= {V
= On-resistance of Q4 at I
RIPPLE
= Primary side main supply maximum input
CS1
CS2
IN(MAX)
• R
IN(MAX)
• R
CS2
DS(ON)
} / {R
• (N
• (N
S
CS1
/N
–1
CS1
CS2
S
P
/N
) – 11V} / 5mA
+ R
, R
is not needed and R
P
ADJUSTED CURRENT
SENSE THRESHOLD
) –2200 • (1 + k)} /k
CS2
CS2
CS1
RIPPLE
}
and R
and R
+
/2
input leakage
CS3
CS2
3900 F06b
, should
and the
0A
CS1
CS1
To minimize this delay and error, do not use resistance
value higher than required and make the PCB trace from
the resistors to the LTC3900 CS
possible. Add a series resistor, R
parallel sum of R
the other end of R
SYNC Input
Figure 7 shows the external circuit for the LTC3900 SYNC
input. With a selected type of pulse transformers, the
values of the C
an optimum SYNC pulse amplitude and width. A bigger
capacitor, C
The peak of this pulse should be much higher than the typi-
cal LTC3900 SYNC threshold of ±1.4V. Amplitudes greater
than ±5V will help to speed up the SYNC comparator and
reduce the SYNC to drivers propagation delay. The pulse
width should be wider than 75ns. Overshoot during the
pulse transformer reset interval must be minimized and
kept below the minimum SYNC threshold of ±1V. The
amount of overshoot can be reduced by having a smaller
R
SYNC
.
SG
, generates a higher and wider SYNC pulse.
T2: COILCRAFT Q4470B
OR PULSE P0926
SG
CONTROLLER
Figure 7. SYNC Input Circuit
CS1
and R
PRIMARY
CS3
(S
OUT
and R
SG
directly to the source of Q4.
)
SYNC
220pF
C
SG
CS2
should be adjusted to obtain
T2
to the CS
+
CS3
/CS
with value equal to
R
470
–
SYNC
–
LTC3900
SYNC
pins as short as
pin and connect
3900 F07
3900fb