LTC3726 Datasheet, PDF (9/16 Pages) Linear Technology – Secondary-Side Synchronous Forward Controller

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OPERATIO
When the LTC3726 has adequate operating voltage, it will
begin the procedure of assuming control from the primary
side. To do this, it first measures the voltage on the power
supply’s main output and then automatically advances its
own soft-start voltage to correspond to the main output
voltage. This ensures that the output voltage increases
monotonically as the soft-start control is transferred from
primary to secondary. The LTC3726 then begins sending
PWM signals to the LTC3705/LTC3725 on the primary
side through a pulse transformer. When the LTC3705/
LTC3725 has detected a stable signal from the secondary
controller, it transfers control of the primary switches over
to the LTC3726, beginning the secondary-side soft-start
mode. The LTC3726 continues in this mode until the
output voltage has ramped up to its final value. If for any
reason, the LTC3726 either stops sending (or initially fails
to send) PWM information to the LTC3705/LTC3725, the
LTC3705/LTC3725 will detect a FAULT and initiate a soft-
start retry (see the LTC3705/LTC3725 data sheet).
Slope Compensation
Slope compensation is added at the input of the PWM
comparator to improve stability and noise margin of the
peak current control loop. The amount of slope compen-
sation can be selected from one of five preprogrammed
values using the SLP pin as shown in Table 1. Note that the
amount of slope compensation doubles when the duty
cycle exceeds 50%.
Table 1
SLP PIN
SLOPE (D < 0.5)
SLOPE (D > 0.5)
GND
0.05 • I_SMAX • f_OSC
0.1 • I_SMAX • f_OSC
V_CC
None
None
400kΩ to GND
0.1 • I_SMAX • f_OSC
0.2 • I_SMAX • f_OSC
200kΩ to GND
0.15 • I_SMAX • f_OSC
0.3 • I_SMAX • f_OSC
100kΩ to GND
0.25 • I_SMAX • f_OSC
0.5 • I_SMAX • f_OSC
50kΩ to GND
0.5 • I_SMAX • f_OSC
1.0 • I_SMAX • f_OSC
In Table 1 above, I_SMAX is the maximum current limit, and f_OSC is the
switching frequency.
Current Sensing and Current Limit
For current sensing, the LTC3726 supports either a cur-
rent sense resistor or a current sense transformer. The
current sense resistor may either be placed in series with
LTC3726
the inductor (either high side or ground lead sensing), or
in the source of the “forward” switch. If a current sense
transformer is used, the I_S– input should be tied to V_CC and
the I_S+ pin to the output of the current sense transformer.
This causes the gain of the internal current sense amplifier
to be reduced by a factor of 16, so that the maximum
current sense voltage (current limit) is increased from
78mV to 1.28V. An internal, adaptive leading edge blank-
ing circuit ensures clean operation for “switch” current
sensing applications.
Current limit is achieved in the LTC3726 by limiting the
maximum voltage excursion of the error signal (I_TH volt-
age). Note that if slope compensation is used, the precise
value at which current limit occurs will be a function of
duty cycle (see Typical Performance Characteristics). If a
short circuit is applied, an independent overcurrent com-
parator may be tripped. In this case, the LTC3726 will enter
a “hiccup” mode using the soft-start circuitry.
Frequency Setting and Synchronization
The LTC3726 uses a single pin to set the operating
frequency, or to synchronize the internal oscillator to a
reference clock with an on-chip phase-locked loop (PLL).
The FS pin may be tied to GND, V_CC or have a single resistor
to GND to set the switching frequency. If a clock signal
(>2V) is detected at the FS pin, the LTC3726 will automati-
cally synchronize to the rising edge of the reference clock.
Table 2 summarizes the operation of the FS pin.
For synchronization between multiple LTC3726s, the PT⁺
pin of one LTC3726 can be used as a master clock
reference and tied to the FS pin of the other LTC3726s.
Table 2
FS PIN
GND
V_CC
R_FS to GND
Reference Clock
SWITCHING FREQUENCY
200kHz
300kHz
f_OSC (Hz) = 4R_FS – 200k
f_OSC = f_REF (75kHz to 500kHz)
This will cause all LTC3726s to operate at the same
frequency and phase. The LTC3726 can also be used as a
“Slave” in a PolyPhase application. In this case, the phase
angle of each LTC3726 can be set by using the FB/PHASE
3726fb
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