ltc3900 Linear Technology Corporation, ltc3900 Datasheet - Page 9

• Manufacturer Part Number: ltc3900
• Description: Synchronous Rectifier Driver For Forward Converters
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
The LTC3900 CS
clamp circuit (Z
pin to 11V. The clamp circuit is to be used together with the
external series resistor, R
high Q4 drain voltage in the power transfer cycle. During
the power transfer cycle, Q4 is off, the drain voltage of Q4
is determined by the primary input voltage and the trans-
former turns ratio. This voltage can be high and may
damage the LTC3900 if CS
drain of Q4. The current sinking capability of the clamp
circuit is 5mA minimum.
The value of the resistors, R
calculated using the following formulas to meet both the
threshold and clamp voltage requirements:
where:
k = 48 • I
R
R
R
If k = 0 or less than zero, R
I
R
V
voltage
N
RIPPLE
IN(MAX)
CS2
CS1
CS3
DS(ON)
S
/N
INDUCTOR
CURRENT
= R
P
= {200 • V
= k • R
= {R
SYNC
Figure 6b. Continuous Current Mode Operation
with Adjusted Current Sense Threshold
= Power transformer T1, turn ratio
CS3
CG
= Inductor peak-to-peak ripple current
FG
= On-resistance of Q4 at I
RIPPLE
= Primary side main supply maximum input
CS1
= {V
CS2
CS
• R
+
IN(MAX)
IN(MAX)
• R
in the Block Diagram) that clamps the
U
CS2
pin has an internal current sinking
DS(ON)
} / {R
• (N
CS1
U
• (N
CS1
+
CS1
–1
is connected directly to the
to protect the CS
CS2
S
S
, R
/N
/N
+ R
P
CS2
P
ADJUSTED CURRENT
is not needed and R
SENSE THRESHOLD
) – 11V} / 5mA
) –2200 • (1 + k)} /k
CS2
W
and R
RIPPLE
}
CS3
/2
, should be
+
U
3900 F06b
pin from
0A
CS1
An alternative method of generating the SYNC pulse is
shown in Figure 8. This circuit produces square SYNC
pulses with amplitude dependent on the logic supply
voltage. The SYNC pulse width can be adjusted with R1
and C1 without affecting the pulse amplitude.
For nonisolated applications, the SYNC input can be driven
directly by a bipolar square pulse. To reduce the propaga-
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.
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
typical LTC3900 SYNC threshold of ±1.4V. Amplitudes
greater than ±5V will help to speed up the SYNC compara-
tor 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
+
pins input capacitance plus the PCB trace capacitance
.
SG
, generates a higher and wider SYNC pulse.
T2: COILCRAFT Q4470B
OR PULSE P0926
SG
CONTROLLER
Figure 7. SYNC Input Circuit
CS1
PRIMARY
and R
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
CS1
R
470Ω
with value equal to
–
SYNC
LTC3900
SYNC
LTC3900
–
pins as short as
and R
+
3900 F07
pin and connect
input leakage
CS2
and the
9
3900f
CS1