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LTC3726 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3726 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LTC3726 3726fb pin (see Slave Mode Operation). The Phase angle cannot be adjusted when the FB/PHASE pin is being used for voltage loop regulation. Soft-Start The soft-start circuitry has four functions: 1) to provide a shutdown, 2) to provide a smooth ramp on the output voltage during start-up, 3) to limit the output current in a short-circuit situation by entering a hiccup mode, and 4) to communicate fault and shutdown information between multiple LTC3726s in a PolyPhase application. When the RUN/SS pin is pulled to GND, the chip is placed into shutdown mode. If this pin is released, the RUN/SS pin is initially charged with a 50 µAcurrentsource.Afterthe RUN/SS pin gets above 0.5V, the chip is enabled. At the instant that the LTC3726 is first enabled, the RUN/SS voltage is rapidly preset to a voltage that will correspond to the main output voltage of the DC/DC converter. (See the Self-Starting Architecture section.) After this preset interval has completed, the normal soft-start interval begins and the charging current is reduced to 5 µA. The external soft-start voltage is used to internally ramp up the 0.6V reference (positive) input to the error amplifier. When fully charged, the RUN/SS voltage remains at 3V. In the event that the sensed switch or inductor current exceeds the overcurrent trip threshold, an internal fault latch is tripped. When such a fault is detected, the LTC3726 immediately goes to zero duty cycle and initiates a soft- start retry. Prior to discharging the soft-start capacitor, however, the LTC3726 first puts a voltage pulse on the RUN/SS pin, which trips the fault latch in any other LTC3726 that shares the RUN/SS. This ensures an orderly shutdown of all phases in a PolyPhase application. After the soft-start capacitor is fully discharged, the LTC3726 attempts a restart. If the fault is persistent, the system enters a “hiccup” mode. Note that in self-starting secondary-side control applica- tions (with the LTC3705 or LTC3725), the presence of the LT3726 bias voltage is dependent upon the regular switch- ing of the primary-side MOSFETs. Therefore, depending on the details of the application circuit, the LTC3726 may lose its bias voltage after a fault has been detected and before completing a soft-start retry. In this case, the “hiccup-mode” operation is actually governed by the LTC3705/LTC3725 soft-start circuitry (see the LTC3705/ LTC3725 data sheets). Drive Mode and Maximum Duty Cycle Although the LTC3726 is primarily intended to be used with the LTC3705/LTC3725 in forward converter applica- tions, the MODE pin provides the flexibility to use the LTC3726 in a wide variety of additional applications. This pin can be used to defeat the gate drive encoding scheme, as well as change the maximum duty cycle from its default value of 50%. The use of the MODE pin is summarized in Table 3. When the gate drive encoding scheme is defeated, a standard PWM-style signal will be present at the PT+ pin and a reference clock (in phase with the PWM signal) will be present at the PT– pin. These outputs can be used in “standalone” applications (without the LTC3705/LTC3725) to drive the gates of MOSFETs in a conventional manner. Table 3 PT+/PT– Mode INTENDED MODE PIN (MAX DUTY CYCLE) APPLICATION GND Encoded PWM 2-Switch Forward (DMAX = 50%) with LTC3705 VCC Encoded PWM 1-Switch Forward (DMAX = 75%) with LTC3725 200k Ω to GND Standard PWM 2-Switch Forward (DMAX = 50%) Standalone 100k Ω to GND Standard PWM 1-Switch Forward (DMAX = 75%) Standalone Overvoltage Protection This circuit monitors the voltage on the FB input. If the voltage on the FB pin exceeds 117% of 0.6V (0.7V), an overvoltage (OVP) is detected. For overvoltage protection, the secondary-side synchronous MOSFET is turned on while all other MOSFETs are turned off. This protection mode is not latched, so that the overvoltage detection is cleared if the FB voltage falls below 115% of 0.6V (0.69V). OPERATIO |
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