Datasheet

LTC3900
10
3900fb
The value of the resistors, R
CS1
, R
CS2
and R
CS3
, should
be calculated using the following formulas to meet both
the threshold and clamp voltage requirements:
k = 48 • I
RIPPLE
• R
DS(ON)
–1
R
CS2
= {200 • V
IN(MAX)
• (N
S
/N
P
) –2200 • (1 + k)} /k
R
CS1
= k • R
CS2
R
CS3
= {R
CS1
• R
CS2
} / {R
CS1
+ R
CS2
}
If k = 0 or less than zero, R
CS2
is not needed and R
CS1
= R
CS3
= {V
IN(MAX)
• (N
S
/N
P
) – 11V} / 5mA
where:
I
RIPPLE
= Inductor peak-to-peak ripple current
R
DS(ON)
= On-resistance of Q4 at I
RIPPLE
/2
V
IN(MAX)
= Primary side main supply maximum input
voltage
N
S
/N
P
= Power transformer T1, turn ratio
If the LTC3900 still operates in discontinuous mode with
the calculated resistance value, increase the value of R
CS1
to raise the threshold. The resistors R
CS1
and R
CS2
and the
CS
+
pins input capacitance plus the PCB trace capacitance
form an R-C delay; this slows down the response time
of the comparator. The resistors and CS
+
input leakage
currents also create an input offset error.
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
+
/CS
–
pins as short as
possible. Add a series resistor, R
CS3
with value equal to
parallel sum of R
CS1
and R
CS2
to the CS
–
pin and connect
the other end of R
CS3
directly to the source of Q4.
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
SG
and R
SYNC
should be adjusted to obtain
an optimum SYNC pulse amplitude and width. A bigger
capacitor, C
SG
, generates a higher and wider SYNC pulse.
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
.
SYNC
FG
CG
3900 F06b
INDUCTOR
CURRENT
0A
ADJUSTED CURRENT
SENSE THRESHOLD
Figure 6b. Continuous Current Mode Operation
with Adjusted Current Sense Threshold
Figure 7. SYNC Input Circuit
R
SYNC
470Ω
T2
T2: COILCRAFT Q4470B
OR PULSE P0926
C
SG
220pF
PRIMARY
CONTROLLER
SG
(S
OUT
)
LTC3900
SYNC
3900 F07
applicaTions inForMaTion