LT3710
U
W U U
APPLICATIO S I FOR ATIO
Output N-Channel MOSFET Drivers
discharging the output, which would cause the output to
undershoot below ground.
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.
Layout Considerations
For maximum efficiency, the switching rise and fall times
are less than 20ns. To prevent radiation, the power
MOSFETs,SWpinandinputbypasscapacitorleadsshould
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.
Light Load Operation
IftheBGSpinisgrounded,theLT3710staysincontinuous
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 VRS1 drops below 8mV, BGATE
will be turned off(see comparator CA2 of Block Diagram)
and the LT3710 goes into discontinous mode operation.
Output Voltage Programming
Thefeedbackreferencevoltageis0.8V. Theoutputvoltage
can be easily programmed by the resistor divider, R3 and
R4, as shown in the Block Diagram.
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/RS1. 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.
R3
VOUT2 = 0.8• 1+
R4
Filtering on the SYNC Input
It is necessary to add RC filtering on the SYNC input of the
LT3710toeliminatethenegativeglitchattheturnonofthe
top MOSFET. When the top MOSFET M1 turns on, the
transformersecondarycurrentinstantlychangesfromthe
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-
ondaryvoltageVS todropforashortinterval.Iftheleakage
inductance is large enough, the VS 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.
Soft-Start and Shutdown
Duringsoft-start,VSS isthereferencevoltagethatcontrols
the output voltage and the output ramps up following VSS.
The effective range of VSS is from 0V to VREF. The typical
time for the output to reach the programmed level is
(C • 0.8V)/10µA.
During start up, BGATE will stay off until VSS 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 VSS
dropsbelow0.6V.ThispreventsthebottomMOSFETfrom
A filter circuit is needed to ensure proper operation. A
small RC filter with RS = 10k and CS = 10pF are typical.
3710f
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