LTC3725_15 Datasheet, PDF (8/20 Pages) Linear Technology – Single-Switch Forward Controller and Gate Driver

LTC3725
U
OPERATIO
Mode Setting
The LTC3725 is a controller and gate driver designed
for use in a single-switch forward converter. When used
in conjunction with the LTC3706 PolyPhase secondary-
side synchronous forward controller, it forms a complete
forward converter with secondary-side regulation, gal-
vanic isolation between input and output, and synchro-
nous 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 LTC3725 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 LTC3725 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 LTC3725. In the LTC3725, the PWM
receiver extracts the duty cycle and uses it to control the
gate driver.
Figure 1 shows that the LTC3706 drives the pulse trans-
former in a complementary fashion, with a duty cycle of
approximately 75%. At the appropriate time during the posi-
tive half cycle, the LTC3706 applies a short (150ns) zero-
voltage pulse across the pulse transformer, indicating the
DUTY CYCLE = 15%
150ns
150ns
+7V
DUTY CYCLE = 0%
150ns
V_PT1+ – V_PT1–
–7V
1 CLK PER
1 CLK PER
3725 F01
Figure 1. Gate Drive Multiplexing Scheme
end of the “on” time. Although this scheme allows the trans-
mission of 0% to 75% duty cycle, it is necessary to estab-
lish a minimum controllable “on” time of approximately
100ns. This ensures that 0% duty cycle can be reliably dis-
tinguished from 75% duty cycle.
On-Chip Rectifier
Simultaneously with duty-cycle decoding, and through
the same pulse transformer, the wave generated by the
LTC3706 provides primary-side V_CC gate drive bias power
by way of the LTC3725’s on-chip full-wave bridge rectifier.
No auxiliary bias supply is necessary and forward con-
verter design and circuitry are considerably simplified.
External Series Pass Linear Regulator
The LTC3725 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 LTC3725
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 LTC3725 “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 LTC3725 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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