lt3710 Linear Technology Corporation, lt3710 Datasheet - Page 8

• Manufacturer Part Number: lt3710
• Description: Secondary Side Synchronous Post Regulator
• Manufacturer: Linear Technology Corporation

LT3710
APPLICATIO S I FOR ATIO
Output N-Channel MOSFET Drivers
The LT3710 employs high speed N-channel MOSFET
synchronous drivers to achieve high system efficiency.
GBIAS is the 8V regulator output to bias and supply the
drivers and should be properly bypassed with a low ESR
capacitor to ground plane. A Schottky catch diode is
required on the switch node.
Light Load Operation
If the BGS pin is grounded, the LT3710 stays in continuous
mode independent of load condition except in soft-start
operation (see Soft-Start section). If the BGS pin is left
open, under light load and V
will be turned off(see comparator CA2 of Block Diagram)
and the LT3710 goes into discontinous mode operation.
Current Limit
Current limit is set by the 70mV threshold across CL
CL
external resistor RS1(see Block Diagram), the current
limit is set for 70mV/R
limit loop. If current limit is not used, both CL
should be grounded and the BGS pin should also be
grounded to disable comparator CA2.
Soft-Start and Shutdown
During soft-start, V
the output voltage and the output ramps up following V
The effective range of V
time for the output to reach the programmed level is
(C • 0.8V)/10 A.
During start up, BGATE will stay off until V
1.6V. This prevents the bottom MOSFET from turning on
if the output is precharged.
To shut down the LT3710, the SS pin should be pulled
below 50mV by a VN2222 type N-channel transistor. Note
that during shutdown BGATE will be locked off when V
drops below 0.6V. This prevents the bottom MOSFET from
8
–
, the inputs of the amplifier CA1. By connecting an
SS
U
is the reference voltage that controls
S1
SS
. R6 and C6 stablize the current
U
is from 0V to V
RS1
drops below 8mV, BGATE
W
REF
SS
. The typical
gets up to
+
U
and CL
+
and
SS
SS
–
.
discharging the output, which would cause the output to
undershoot below ground.
Layout Considerations
For maximum efficiency, the switching rise and fall times
are less than 20ns. To prevent radiation, the power
MOSFETs, SW pin and input bypass capacitor leads should
be kept as short as possible. A ground plane should be
used under the switching circuitry to prevent interplane
coupling and to act as a thermal spreading path. Note that
the bottom metal of the package is the heat sink, as well as
the IC signal ground, and must be soldered to the ground
plane.
Output Voltage Programming
The feedback reference voltage is 0.8V. The output voltage
can be easily programmed by the resistor divider, R3 and
R4, as shown in the Block Diagram.
Filtering on the SYNC Input
It is necessary to add RC filtering on the SYNC input of the
LT3710 to eliminate the negative glitch at the turn on of the
top MOSFET. When the top MOSFET M1 turns on, the
transformer secondary current instantly changes from the
original first output inductor current to the sum of two
output inductor currents. The high di/dt on the trans-
former leakage inductance causes the transformer sec-
ondary voltage V
inductance is large enough, the V
the synchronization threshold (about 2.5V), falsely trig-
gering the synchronization. The top MOSFET is turned off
immediately. As a result, the output voltage will not be
regulated properly.
A filter circuit is needed to ensure proper operation. A
small RC filter with R
V
OUT2
0 8 1
. •
S
to drop for a short interval. If the leakage
S
R
R
= 10k and C
3
4
S
dip will be lower than
S
= 10pF are typical.
3710f