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LTC3726IGN Datasheet(PDF) 9 Page - Linear Technology |
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LTC3726IGN Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page 9 LTC3726 3726fb 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 • ISMAX • fOSC 0.1 • ISMAX • fOSC VCC None None 400k Ω to GND 0.1 • ISMAX • fOSC 0.2 • ISMAX • fOSC 200k Ω to GND 0.15 • ISMAX •fOSC 0.3 • ISMAX •fOSC 100k Ω to GND 0.25 • ISMAX • fOSC 0.5 • ISMAX • fOSC 50k Ω to GND 0.5 • ISMAX • fOSC 1.0 • ISMAX • fOSC In Table 1 above, ISMAX is the maximum current limit, and fOSC 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 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 IS– input should be tied to VCC and the IS+ 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 (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 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, VCC 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 SWITCHING FREQUENCY GND 200kHz VCC 300kHz RFS to GND fOSC (Hz) = 4RFS – 200k Reference Clock fOSC = fREF (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 OPERATIO |
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