LTC3706 Datasheet, PDF (12/20 Pages) Linear Technology – Secondary-Side Synchronous Forward Controller with PolyPhase Capability

LTC3706
OPERATION
Note that in self-starting secondary-side control applica-
tions (with the LTC3705), the presence of the LT3706
bias voltage is dependent upon the regular switching of
the primary-side MOSFETs. Therefore, depending on the
details of the application circuit, the LTC3706 may lose
its bias voltage after a fault has been detected and before
completing a soft-start retry. In this case, the “hiccup-
mode” operation is actually governed by the LTC3705
soft-start circuitry. (See the LTC3705 data sheet.)
Drive Mode and Maximum Duty Cycle
Although the LTC3706 is primarily intended to be used with
the LTC3705 in 2-transistor forward applications, the MODE
pin provides the flexibility to use the LTC3706 in a wide
variety of additional applications. This pin can be used to
defeat the gate drive encoding scheme, as well as change
the maximum duty cycle from its default value of 50%.
The use of the MODE pin is summarized in Table 4.
When the gate drive encoding scheme is defeated, a
standard PWM-style signal will be present at the PT⁺ pin
and a reference clock (in phase with the PWM signal) will
be present at the PT^– pin. These outputs can be used in
“standalone” applications (without the LTC3705) to drive
the gates of MOSFETs in a conventional manner.
Table 4
MODE PIN
GND
V_CC
200kΩ to GND
100kΩ to GND
PT⁺/PT^– Mode
(MAX DUTY CYCLE)
Encoded PWM
(D_MAX = 50%)
Encoded PWM
(D_MAX = 75%)
Standard PWM
(D_MAX = 50%)
Standard PWM
(D_MAX = 75%)
INTENDED
APPLICATION
2-Switch Forward
with LTC3705
1-Switch Forward
2-Switch Forward
Standalone
1-Switch Forward
Standalone
Power Good/Overvoltage Protection
This circuit monitors the voltage on the FB input. The
open-drain PGOOD output will be logic high if the voltage
on the FB pin is within +17%/–7% of 0.6V. If the voltage on
the FB pin exceeds 117% of 0.6V (0.7V), an overvoltage
(OVP) is detected. For overvoltage protection, the sec-
ondary-side synchronous MOSFET is turned on while all
other MOSFETs are turned off. This protection mode is
not latched, so that the overvoltage detection is cleared if
the FB voltage falls below 115% of 0.6V (0.69V).
Linear Regulator Operation
The LTC3706 provides a linear regulator controller that
drives an external N-type pass device. This controller is used
to create a 7V DC bias from the peak-charged secondary
bias voltage (8V to 30V). Internal divider resistors are used
to establish a regulation voltage of 7V at the V_CC pin. An
auxiliary bias supply with a regulated voltage greater than
7V may be applied to the V_CC pin to bypass (bootstrap) the
linear regulator. This improves efficiency and also helps to
avoid overheating the linear regulator pass device.
Thermal protection for the linear regulator pass device is
also provided by means of the REGSD pin. A current is
sourced from this pin that is proportional to the voltage
across the linear regulator pass device (V_IN – V_CC). Since
the V_CC load current is essentially constant for a given
switching frequency and choice of power MOSFETs, the
power dissipated in the external pass device will only vary
with the voltage across it. Thus, a single resistor may be
placed between the REGSD pin and GND to develop a volt-
age that is proportional to the power in the external pass
device. An additional parallel capacitor can also be used
to account for the thermal time constant associated with
the external pass device itself. When the voltage on the
REGSD pin exceeds 4V, an overtemperature fault occurs
and the LTC3706 attempts a soft-start retry.
3706fb
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