LTC3722-2 Linear Technology, LTC3722-2 Datasheet - Page 24

• Manufacturer Part Number: LTC3722-2
• Description: Synchronous Dual Mode Phase Modulated Full Bridge Controllers
• Manufacturer: Linear Technology

LTC3722-1/LTC3722-2
OPERATIO
Synchronous Rectification
The LTC3722-1/LTC3722-2 produces the precise timing
signals necessary to control current doubler secondary
side synchronous MOSFETs on OUTE and OUTF. Syn-
chronous rectifiers are used in place of Schottky or Silicon
diodes on the secondary side of the power supply. As
MOSFET R
ciency improvements can be realized with synchronous
rectification, provided that the MOSFET switch timing is
optimized. An additional benefit realized with synchro-
nous rectifiers is bipolar output current capability. These
characteristics improve transient response, particularly
overshoot, and improve ZVS ability at light loads.
Programming the Synchronous Rectifier Turn-Off
Delay
The LTC3722-1/LTC3722-2 controllers include a feature
to program the turn-off edge of the secondary side syn-
chronous rectifier MOSFETs relative to the beginning of a
24
DS(ON)
levels continue to drop, significant effi-
U
R
SPRG
SPRG
Figure 14. Synchronous Delay Circuitry
V
+
–
2V
new primary side power delivery pulse. This feature pro-
vides optimized timing for the synchronous MOSFETs
which improves efficiency. At higher load currents it
becomes more advantageous to delay the turn-off of the
synchronous rectifiers until the transformer core has been
reset to begin the new power pulse. This allows for
secondary freewheeling current to flow through the syn-
chronous MOSFET channel instead of its body diode.
The turn-off delay is programmed with a resistor from
SPRG to GND, see Figure 14. The nominal regulated
voltage on SPRG is 2V. The external resistor programs a
current which flows out of SPRG. The delay can be
adjusted from approximately 20ns to 200ns, with resistor
values of 10k to 200k. Do not leave SPRG floating. The
amount of delay can also be modulated based on an
external current source that sinks current out of SPRG.
Care must be taken to limit the current out of SPRG to
350 A or less.
+
–
TURN-OFF
SYNC OUT
3722 F0Y
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