LTC3705_15 Datasheet, PDF (8/20 Pages) Linear Technology – 2-Switch Forward Controller and Gate Driver

LTC3705
U
OPERATIO
Mode Setting
The LTC3705 is a controller and gate driver designed for
use in a 2-switch forward converter. When used in con-
junction with the LTC3706 PolyPhase secondary-side
synchronous forward controller it forms a complete
2-switch forward converter with secondary-side regula-
tion, galvanic isolation between input and output, and
synchronous rectification. In this mode, upon start-up,
the FB/IN⁺ and FS/IN^– pins are effectively shorted by one
winding of the pulse transformer. The LTC3705 detects
this short circuit to determine that it is in secondary-side
control mode. Operation in this mode is confirmed when
the LTC3706 begins switching the pulse transformer.
Alternately, the LTC3705 can be used as a standalone
primary-side controller. In this case, the FB/IN⁺ and FS/IN^–
pins operate independently. The FB/IN⁺ pin is connected to
the collector of an optoisolator to provide feedback and the
FS/IN^– pin is connected to the frequency set resistor.
Gate Drive Encoding
In secondary-side control with the LTC3706, after a start-
up sequence, the LTC3706 transmits multiplexed PWM
information through a pulse transformer to the FB/IN⁺ and
FS/IN^– inputs of the LTC3705. In the LTC3705, the PWM
receiver extracts the duty cycle and uses it to control the
top and bottom gate drivers.
Figure 1 shows that the LTC3706 drives the pulse trans-
former in a complementary fashion, with a duty cycle of
approximately 50%. At the appropriate time during the
positive half cycle, the LTC3706 applies a short (150ns)
zero-voltage pulse across the pulse transformer, indicat-
ing the end of the “on” time. Although this scheme allows
DUTY CYCLE = 15%
150ns
150ns
+7V
DUTY CYCLE = 0%
150ns
V_PT1+ – V_PT1–
–7V
1 CLK PER
1 CLK PER
Figure 1. Gate Drive Multiplexing Scheme
the transmission of 0% to 50% duty cycle, it is necessary
to establish a minimum controllable “on” time of approxi-
mately 100ns. This ensures that 0% duty cycle can be
reliably distinguished from 50% duty cycle.
On-Chip Rectifier
Simultaneously with duty-cycle decoding, and through
the same pulse transformer, the near-square-wave gener-
ated by the LTC3706 provides primary-side V_CC gate drive
bias power by way of the LTC3705’s on-chip full-wave
bridge rectifier. No auxiliary bias supply is necessary and
forward converter design and circuitry are considerably
simplified.
External Series Pass Linear Regulator
The LTC3705 features an external series pass linear regu-
lator that eliminates the long start-up time associated with
the conventional trickle charger. The drain of an external
NMOS is connected to the input voltage and the source is
connected to V_CC. The gate of the NMOS is connected to
NDRV. To power the gate, an external pull-up resistor is
connected from the input voltage to NDRV. The NMOS
must be a standard 3V threshold type (i.e. not logic level).
An on-chip circuit manages the start up and operation of
the linear regulator. It takes approximately 45µs for the
linear regulator to charge V_CC to its target value of 8V
(unless limited by a slower rise of V_IN). The LTC3705
begins operating the gate drives when V_CC reaches 7.4V.
Often, the thermal rating of the NMOS prevents it from
operating continuously, and the LTC3705 “times out” the
linear regulator to prevent overheating. This is accom-
plished using the capacitor connected to the SSFLT pin as
described subsequently.
Trickle Charger Shunt Regulator
Alternately, a trickle charger can be implemented by
eliminating the external NMOS and connecting NDRV to
V_CC and using the pull-up resistor to charge V_CC. To allow
extra headroom for starting, the LTC3705 detects this
mode and increases the threshold for starting the gate
drives to 13.4V. An internal shunt regulator limits the
voltage on the trickle charger to 15V.
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