LTC3706 Datasheet, PDF (10/20 Pages) Linear Technology – Secondary-Side Synchronous Forward Controller with PolyPhase Capability

LTC3706
OPERATION
DUTY CYCLE = 15%
150ns
7V
DUTY CYCLE = 0%
150ns
7V
V_PT1+ – V_PT1–
–7V
1 CLK PER
–7V
3706 F01
1 CLK PER
Figure 1: Gate Drive Encoding Scheme (V_MODE = GND)
When the LTC3706 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 LTC3706 then begins sending
PWM signals to the LTC3705 on the primary side through a
pulse transformer. When the LTC3705 has detected a stable
signal from the secondary controller, it transfers control of
the primary switches over to the LTC3706, beginning the
secondary-side soft-start mode. The LTC3706 continues
in this mode until the output voltage has ramped up to
its final value. If for any reason, the LTC3706 either stops
sending (or initially fails to send) PWM information to the
LTC3705, the LTC3705 will detect a FAULT and initiate a
soft-start retry. (See the LTC3705 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 LTC3706 supports either a current
sense resistor or a current sense transformer. The current
sense resistor may either be placed in series with 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 blanking
circuit ensures clean operation for “switch” current sensing
applications.
Current limit is achieved in the LTC3706 by limiting the
maximum voltage excursion of the error signal (ITH 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
comparator may be tripped. In this case, the LTC3706 will
enter a “hiccup” mode using the soft-start circuitry.
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