LTC3705 LINER [Linear Technology], LTC3705 Datasheet - Page 8

• Manufacturer Part Number: LTC3705
• Description: 2-Switch Forward Controller and Gate Driver
• Manufacturer: LINER [Linear Technology]

LTC3705
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
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
pins operate independently. The FB/IN
the collector of an optoisolator to provide feedback and the
FS/IN
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
FS/IN
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
V
8
PT1
+
– V
–7V
–
–
+7V
PT1
pin is connected to the frequency set resistor.
inputs of the LTC3705. In the LTC3705, the PWM
–
+
DUTY CYCLE = 15%
Figure 1. Gate Drive Multiplexing Scheme
and FS/IN
150ns
150ns
1 CLK PER
U
–
pins are effectively shorted by one
DUTY CYCLE = 0%
+
pin is connected to
150ns
1 CLK PER
+
and FS/IN
+
and
–
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
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
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
(unless limited by a slower rise of V
begins operating the gate drives when V
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
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.
CC
and using the pull-up resistor to charge V
CC
. The gate of the NMOS is connected to
CC
to its target value of 8V
IN
CC
). The LTC3705
reaches 7.4V.
CC
CC
gate drive
. To allow
3705fb