LTC3900_1 Datasheet, PDF (10/20 Pages) Linear Technology – Synchronous Rectifier Driver for Forward Converters

LTC3900
Applications Information
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.
SYNC
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.
C_SG
220pF
PRIMARY
T2
CONTROLLER
SG
(S_OUT)
T2: COILCRAFT Q4470B
OR PULSE P0926
LTC3900
SYNC
R_SYNC
470Ω
3900 F07
Figure 7. SYNC Input Circuit
FG
CG
INDUCTOR
CURRENT
0A
ADJUSTED CURRENT
SENSE THRESHOLD 3900 F06b
Figure 6b. Continuous Current Mode Operation
with Adjusted Current Sense Threshold
3900fb
10