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LTC1772BES6-TR Datasheet(PDF) 7 Page - Linear Technology |
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LTC1772BES6-TR Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  LTC1772B 7 1772bfa OPERATION that the external P-channel MOSFET will remain on for more than one oscillator cycle since the inductor current has not ramped up to the threshold set by EAMP. Further reduction in input supply voltage will eventually cause the P-channel MOSFET to be turned on 100%, i.e., DC. The output voltage will then be determined by the input volt- age minus the voltage drop across the MOSFET, the sense resistor and the inductor. Undervoltage Lockout To prevent operation of the P-channel MOSFET below safe input voltage levels, an undervoltage lockout is incorpo- rated into the LTC1772B. When the input supply voltage drops below approximately 2.0V, the P-channel MOSFET and all circuitry is turned off except the undervoltage block, which draws only several microamperes. Short-Circuit Protection When the output is shorted to ground, the frequency of the oscillator will be reduced to about 120kHz. This lower frequency allows the inductor current to safely discharge, thereby preventing current runaway. The oscillator’s fre- quency will gradually increase to its designed rate when the feedback voltage again approaches 0.8V. Overvoltage Protection As a further protection, the overvoltage comparator in the LTC1772B will turn the external MOSFET off when the feedback voltage has risen 7.5% above the reference voltage of 0.8V. This comparator has a typical hysteresis of 20mV. Slope Compensation and Inductor’s Peak Current The inductor’s peak current is determined by: I V R PK ITH SENSE = () –. 085 10 when the LTC1772B is operating below 40% duty cycle. However, once the duty cycle exceeds 40%, slope compensation begins and effectively reduces the peak inductor current. The amount of reduction is given by the curves in Figure 3. Figure 3. Maximum Output Current vs Duty Cycle DUTY CYCLE (%) 110 100 90 80 70 60 50 40 30 20 10 1772 F03 0 70 80 90 100 60 10 20 30 40 50 IRIPPLE = 0.4IPK AT 5% DUTY CYCLE IRIPPLE = 0.2IPK AT 5% DUTY CYCLE VIN = 4.2V (Refer to Functional Diagram) |
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