參數(shù)資料
型號: 3710EFE
廠商: Linear Technology Corporation
英文描述: Secondary Side Synchronous Post Regulator
中文描述: 二次側(cè)同步后穩(wěn)壓器
文件頁數(shù): 8/12頁
文件大?。?/td> 226K
代理商: 3710EFE
LT3710
8
3710f
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
RS1
drops below 8mV, BGATE
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
+
and
CL
, the inputs of the amplifier CA1. By connecting an
external resistor RS1(see Block Diagram), the current
limit is set for 70mV/R
S1
. R6 and C6 stablize the current
limit loop. If current limit is not used, both CL
+
and CL
should be grounded and the BGS pin should also be
grounded to disable comparator CA2.
Soft-Start and Shutdown
During soft-start, V
SS
is the reference voltage that controls
the output voltage and the output ramps up following V
SS
.
The effective range of V
SS
is from 0V to V
REF
. The typical
time for the output to reach the programmed level is
(C 0.8V)/10
μ
A.
During start up, BGATE will stay off until V
SS
gets up to
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
SS
drops below 0.6V. This prevents the bottom MOSFET from
APPLICATIU
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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.
V
R
R
OUT2
0.
1
3
4
=
+
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
S
to drop for a short interval. If the leakage
inductance is large enough, the V
S
dip will be lower than
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
S
= 10k and C
S
= 10pF are typical.
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