LTC3705_15 Datasheet, PDF (9/20 Pages) Linear Technology – 2-Switch Forward Controller and Gate Driver

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OPERATIO
Self-Starting Architecture
The LTC3705 is combined with the LTC3706 to form a
complete self-starting DC isolated power supply. When
power is first applied, and when V_CC for the LTC3705 is
above the appropriate threshold, the LTC3705 begins
open-loop operation using its own internal oscillator.
Power is supplied to the secondary by switching the gate
drivers with a gradually increasing duty cycle as controlled
by the rate of rise of the voltage on the SSFLT pin. A peak
detector power supply for the LTC3706 allows it to begin
operation even for small duty cycles. Once adequate
voltage is available for the LTC3706, it provides duty cycle
information and gate drive bias power using the pulse
transformer as shown in Figure 1. The LTC3705 detects
the appearance of this signal and transfers control of the
gate drivers to the LTC3706. Simultaneously, the LTC3705
also enables the on-chip rectifier and turns off the linear
regulator.
Alternately, when the LTC3705 is used as a standalone
primary-side controller, the gradually increasing duty cycle
powers up a secondary-side reference and optoisolator and
feedback is accomplished when the output of the
optoisolator begins pulling down in the FB/IN⁺ pin.
Soft-Start and Fault
These two functions are implemented using the SSFLT
pin. (This pin is also used for linear regulator timeout as
described in the following section.)
Initiating soft-start requires that: 1) the gate drive
undervoltage (UVGD) goes low meaning that adequate
voltage is available on the V_CC pin (7.4V for the linear
regulator or 13.4V for the trickle charger) and 2) the input
undervoltage (UVV_IN) goes low meaning that the voltage
on the UVLO pin has reached the 1.242V rising threshold.
During soft-start, the LTC3705 gradually charges the soft-
start capacitor to ramp up the converter duty cycle. Soft-
start is over when the voltage on the SSFLT pin reaches 2.8V.
In normal operation, at some point before this, the LTC3705
makes a transition to controlling duty cycle using closed-
loop regulation of the converter output voltage.
The SSFLT pin is also used to indicate a fault. The LTC3705
recognizes faults from four origins: 1) an overcurrent fault
LTC3705
caused by the current sense voltage on the IS pin exceed-
ing the 300mV overcurrent threshold, 2) an input
undervoltage fault caused by the UVLO pin falling below
the 1.226V falling threshold, 3) a gate drive undervoltage
fault caused by the voltage on the V_CC pin falling below the
7V threshold, or 4) loss of the gate drive encoding signal
from the LTC3706.
Upon sensing a fault, the LTC3705 immediately turns off
the top and bottom gate drives and indicates a fault by
quickly pulling the voltage on the SSFLT pin to within 1.3V
of the voltage on the V_CC pin. After indicating the fault, the
LTC3705 quickly ramps down the voltage on the SSFLT
pin to approximately 2.8V. Then, to allow complete dis-
charge of the secondary-side circuit, the LTC3705 slowly
ramps down the voltage on the SSFLT pin to about 200mV.
The LTC3705 then attempts a restart.
Linear Regulator Timeout
The thermal rating of the linear regulator’s external NMOS
often cannot allow it to indefinitely supply bias current to
the primary-side gate drives. The LTC3705 has a linear
regulator timeout mechanism that also uses the SSFLT
capacitor.
As described in the prior section, soft-start is over once the
voltage on the SSFLT pin reaches 2.8V. However, the
SSFLT capacitor continues to charge and the linear regu-
lator is turned off when the voltage on the SSFLT pin
reaches 3.9V. The “Applications Information” section de-
scribes linear regulator timeout in more detail.
Volt-Second Limit
The volt-second limit ensures that the power transformer
core does not saturate for any combination of duty cycle
and input voltage. The input of an R-C integrator is
connected to V_IN and its output is connected to the V_SLMT
pin. While the top and bottom gate drives are “off,” the
LTC3705 grounds the V_SLMT pin. When the gate drives are
turned “on” the V_SLMT pin is released and the capacitor is
allowed to charge in proportion to V_IN. If the capacitor
voltage on the V_SLMT pin exceeds 1.25V the two gate
drives are immediately turned “off.” Note that this is not
considered a fault condition and the LTC3705 can run
indefinitely with the switch duty cycle being determined by
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