Datasheet LTC1779 (Analog Devices) - 6

LTC1779
operaTion (Refer to Functional Diagram)
Comparator OVP guards against transient overshoots
Short-Circuit Protection
> 7.5% by turning off the internal P-channel power When the output is shorted to ground, the frequency of MOSFET and keeping it off until the fault is removed. the oscillator will be reduced to about 100kHz. This lower
Burst Mode Operation
frequency allows the inductor current to safely discharge, thereby preventing current runaway. The oscillator’s fre- The LTC1779 enters Burst Mode operation at low load quency will gradually increase to its designed rate when currents. In this mode, the peak current of the inductor is the feedback voltage again approaches 0.8V. set as if VITH/RUN = 1V (at low duty cycles) even though the voltage at the ITH/RUN pin is at a lower value. If the
Overvoltage Protection
inductor’s average current is greater than the load require- As a further protection, the overvoltage comparator in the ment, the voltage at the ITH/RUN pin will drop. When the LTC1779 will turn the internal MOSFET off when the feed- ITH/RUN voltage goes below 0.85V, the sleep signal goes back voltage has risen 7.5% above the reference voltage high, turning off the internal MOSFET. The sleep signal of 0.8V. This comparator has a typical hysteresis of 30mV. goes low when the ITH/RUN voltage goes above 0.925V and the LTC1779 resumes normal operation. The next
Slope Compensation and Inductor’s Peak Current
oscillator cycle will turn the internal MOSFET on and the switching cycle repeats. The inductor’s peak current is determined by: M(V
Dropout Operation
I ITH/RUN – 0.7) PK = 10 R ( SENSE+2Ω) When the input supply voltage decreases towards the output voltage, the rate of change of inductor current when the LTC1779 is operating below 40% duty cycle. during the ON cycle decreases. This reduction means However, once the duty cycle exceeds 40%, slope com- that the internal P-channel MOSFET will remain on for pensation begins and effectively reduces the peak inductor more than one oscillator cycle since the inductor current current. The amount of reduction is given by the curves has not ramped up to the threshold set by EAMP. Further in Figure 2. reduction in input supply voltage will eventually cause the P-channel MOSFET to be turned on 100%, i.e., DC. The 110 output voltage will then be determined by the input voltage 100 minus the voltage drop across the MOSFET, the sense 90 (%) 80 resistor and the inductor. 70 60
Undervoltage Lockout
/I OUT(MAX) 50 OUT IRIPPLE = 0.4IPK To prevent operation of the P-channel MOSFET below safe 40 AT 5% DUTY CYCLE SF = I input voltage levels, an undervoltage lockout is incorpo- I 30 RIPPLE = 0.2IPK AT 5% DUTY CYCLE rated into the LTC1779. When the input supply voltage 20 VIN = 4.2V drops below approximately 2.0V, the P-channel MOSFET 10 0 10 20 30 40 50 60 70 80 90 100 and all circuitry is turned off except the undervoltage block, DUTY CYCLE (%) 1779 F02 which draws only several microamperes.
Figure 2. Maximum Output Current vs Duty Cycle
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