LTC3901 Linear Technology, LTC3901 Datasheet - Page 9

• Manufacturer Part Number: LTC3901
• Description: Secondary Side Synchronous Driver for Push-Pull and Full-Bridge Converters
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
output goes high; this is to avoid any ringing immediately
after the MOSFETs are switched on.
Under no/light load conditions, if the inductor average
current is less than half of its peak-to-peak ripple current,
the inductor current will reverse into MOSFETs during a
portion of the free-wheeling period, forcing CSX
above CSX
old is set at 10.5mV to prevent tripping under this normal
no load condition. If at no load, the product of the inductor
negative peak current and MOSFET R
10.5mV; this may trip the comparator and force the
LTC3901 to operate in discontinuous mode. Figure 7
shows the LTC3901 operating in discontinuous mode; the
driver’s output goes low before the next SYNC transition
edge when the inductor current goes negative. In push-
pull topology, both MOSFETs conduct the same amount of
current during the free-wheeling period; this will trip both
comparators at the same time. Discontinuous mode is
sometimes undesirable because if the load current sud-
CURRENT
CURRENT
SDRA
SDRB
SYNC
SYNC
Figure 7a. Discontinuous Mode Operation at No Load
ME
ME
MF
MF
L1
L1
–
Figure 7b. Continuous Mode Operation
with Adjusted Current Sense Threshold
. The current sense comparator input thresh-
U
COMPARATOR TRIP
CURRENT SENSE
ADJUSTED CURRENT SENSE THRESHOLD
U
W
DS(ON)
is higher than
U
+
to rise
3901 F06
0V
0V
0V
0V
denly increases when the MOSFETs are off, it creates a
large output voltage drop. To overcome this, add a resistor
divider, R
10.5mV threshold so that the LTC3901 operates in con-
tinuous mode at no load.
The LTC3901 CSX
clamp circuit (Z
The clamp circuit, together with the external series resis-
tor R
drain voltage in the power delivery cycle. During the power
delivery cycle, one of the MOSFETs (ME or MF) is off. The
drain voltage of the MOSFET that is off is determined by the
primary input voltage and the transformer turn ratio. This
voltage can be high and may damage the internal circuit if
CSX
current sinking capability of the clamp circuit is 5mA
minimum.
The value of the resistorsR
be calculated using the following formulas to meet both
the clamp and threshold voltage requirements:
where:
If the LTC3901 still operates in discontinuous mode with
the calculated resistance value, increase the value of
R
R
trace capacitance forms an R-C delay; this slows down the
response time of the comparators. The resistors and CSX
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
CSX1
CSX2
k = {48 • I
R
R
R
If k = 0 or less than zero, R
I
R
V
voltage
N
RIPPLE
IN(MAX)
+
CSX2
CSX1
CSX3
DS(ON)
S
CSX 1
/N
is connected directly to the drain of its MOSFET. The
= R
and the CSX
to raise the threshold. The resistors R
P
= {200 • V
= k • R
= {R
, protects the CSX
CSX3
= Power transformer T1, turn ratio
CSX1
= Inductor peak-to-peak ripple current
= On-resistance of MOSFET at I
= Primary side main supply maximum input
RIPPLE
CSX1
= {V
and R
CSX
CSX2
IN(MAX)
• R
+
+
• R
IN(MAX)
) that clamps the pin to around 11V.
pins input capacitance plus the PCB
CSX2
pin has an internal current sinking
CSX2
DS(ON)
at the CSX
} / {R
• (N
CSX1
• N
+
CSX2
} –1
pins from the high MOSFET
S
S
CSX1
, R
/N
/N
is not needed and R
P
P
CSX2
) – 11V} / 5mA
–2200 • (1 + k)} /k
+ R
+
pin to increase the
and R
LTC3901
CSX2
RIPPLE
}
CSX3
CSX1
/2
should
CSX1
9
and
3901f
+