Datasheet LTC3700 (Analog Devices) - 8

LTC3700
U OPERATIO (Refer to Functional Diagram) Main Control Loop (Buck Controller) Dropout Operation
The LTC3700 is a constant frequency current mode switch- When the input supply voltage decreases towards the ing regulator. During normal operation, the external output voltage, the rate of change of inductor current P-channel power MOSFET is turned on each cycle when during the ON cycle decreases. This reduction means that the oscillator sets the RS latch (RS1) and turned off when the external P-channel MOSFET will remain on for more the current comparator (ICMP) resets the latch. The peak than one oscillator cycle since the inductor current has not inductor current at which ICMP resets the RS latch is ramped up to the threshold set by EAMP. Further reduc- controlled by the voltage on the ITH/RUN pin, which is the tion in input supply voltage will eventually cause the output of the error amplifier EAMP. An external resistive P-channel MOSFET to be turned on 100%, i.e., DC. The divider connected between VOUT and ground allows the output voltage will then be determined by the input voltage EAMP to receive an output feedback voltage VFB. When the minus the voltage drop across the MOSFET, the sense load current increases, it causes a slight decrease in VFB resistor and the inductor. relative to the 0.8V reference, which in turn causes the ITH/RUN voltage to increase until the average inductor
Undervoltage Lockout
current matches the new load current. To prevent operation of the P-channel MOSFET below safe The main control loop is shut down by pulling the ITH/RUN input voltage levels, an undervoltage lockout is incorpo- pin low. Releasing ITH/RUN allows an internal 0.5µA rated into the buck input supply. When the input supply current source to charge up the external compensation voltage drops below approximately 2.1V, the P-channel network. When the ITH/RUN pin reaches 0.3V, the main MOSFET and all circuitry is turned off except the under- control loop is enabled with the ITH/RUN voltage then voltage block, which draws only several microamperes. pulled up to its zero current level of approximately 0.7V. As the external compensation network continues to charge
Short-Circuit Protection
up, the corresponding output current trip level follows, When the output is shorted to ground, the frequency of the allowing normal operation. oscillator will be reduced to about 110kHz. This lower Comparator OVP guards against transient overshoots frequency allows the inductor current to safely discharge, > 7.5% by turning off the external P-channel power thereby preventing current runaway. The oscillator’s fre- MOSFET and keeping it off until the fault is removed. quency will gradually increase to its designed rate when the feedback voltage again approaches 0.8V.
Burst Mode Operation Overvoltage Protection
The buck enters Burst Mode operation at low load cur- rents. In this mode, the peak current of the inductor is set As a further protection, the overvoltage comparator in the as if V buck will turn the external MOSFET off when the feedback ITH/RUN = 1V (at low duty cycles) even though the voltage at the I voltage has risen 7.5% above the reference voltage of TH/RUN pin is at a lower value. If the inductor’s average current is greater than the load require- 0.8V. This comparator has a typical hysteresis of 20mV. ment, the voltage at the ITH/RUN pin will drop. When the I
Slope Compensation and Inductor’s Peak Current
TH/RUN voltage goes below 0.85V, the sleep signal goes high, turning off the external MOSFET. The sleep signal The inductor’s peak current is determined by: goes low when the ITH/RUN voltage goes above 0.925V and the buck resumes normal operation. The next oscilla- V – . 0 7 I ITH = tor cycle will turn the external MOSFET on and the switch- PK 10 R ( SENSE) ing cycle repeats. 3700f 8