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LTC3706EGN-PBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC3706EGN-PBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTC3706 10 3706fb OPERATION 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 400kΩ to GND 0.1 • ISMAX • fOSC 0.2 • ISMAX • fOSC 200kΩ to GND 0.15 • ISMAX • fOSC 0.3 • ISMAX • fOSC 100kΩ to GND 0.25 • ISMAX • fOSC 0.5 • ISMAX • fOSC 50kΩ 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 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 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 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. Figure 1: Gate Drive Encoding Scheme (VMODE = GND) –7V 7V –7V 7V 150ns 1 CLK PER DUTY CYCLE = 15% VPT1+ – VPT1– 150ns 3706 F01 1 CLK PER DUTY CYCLE = 0% 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 |
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