LTC3726 Datasheet, PDF (6/16 Pages) Linear Technology – Secondary-Side Synchronous Forward Controller

LTC3726
PI FU CTIO S
SG (Pin 1): Gate Drive for the “Synchronous” MOSFET.
FG (Pin 2): Gate Drive for the “Forward” MOSFET.
MODE (Pin 3): Tie to either GND or V_CC to set the
maximum duty cycle at either 50% or 75% respectively.
Tie to ground through either a 200k or 100k resistor (50%
or 75% maximum duty cycle) to disable pulse encoding.
In this mode, normal PWM signals will be generated at the
PT⁺ pin, while a clock signal is generated at the PT^– pin.
FB/PHASE (Pin 4): Inverting input of the main loop Error
Amplifier and Control Input to the Phase Selector. In
PolyPhase^® Slave applications (where voltage feedback is
not needed) this pin is used to determine the phasing of the
controller CLK relative to the synchronizing signal at the
FS/SYNC pin.
ITH (Pin 5): The Output of the Main Loop Error Amplifier.
Place compensation components between the ITH pin
and GND.
RUN/SS (Pin 6): Combination Run Control and Soft-Start
Inputs. A capacitor to ground sets the ramp time of the
output voltage. Holding this pin below 0.4V causes the IC
to shut down all internal circuitry.
SLP (Pin 7): Slope Compensation Input. Place a single
resistor to ground to set the desired amount of slope
compensation.
GND (Pin 8): Signal Ground.
PolyPhase is a registered trademark of Linear Technology Corporation.
FS/SYNC (Pin 9): Combination Frequency Set and SYNC
pin. Tie to GND or V_CC to run at 200kHz and 300kHz
respec-tively. Place a single resistor to ground at this pin
to set the frequency between 75kHz and 500kHz. To
synchronize, drive this pin with a clock signal to achieve
PLL synchronization from 75kHz to 500kHz. Sources
20µA of current.
I_S– (Pin 10): Negative Input to the Current Sense Circuit.
When using current sense transformers, this pin may be
tied to V_CC for single-ended sensing with a 1.28V maxi-
mum current trip level.
I_S+ (Pin 11): Positive Input to the Current Sense Circuit.
Connect to the positive end of a current sense resistor or
to the output of a current sense transformer.
SW (Pin 12): Connect to the drain of the “synchronous”
MOSFET. This input is used for adaptive shoot-through
prevention and leading edge blanking.
PGND (Pin 13): Gate Driver Ground Pin.
PT^–, PT⁺ (Pins 14, 15): Pulse Transformer Driver Out-
puts. For most applications, these connect to a pulse
transformer (with a series DC blocking capacitor). The
PWM information is multiplexed together with DC power
and sent through a single pulse transformer to the primary
side. This information may be decoded by the LTC3705
gate driver and primary-side controller.
V_CC (Pin 16): Main V_CC Input for all Driver and Control
Circuitry.
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