Datasheet LTC3801, LTC3801B (Analog Devices) - 5

LTC3801/LTC3801B
U OPERATIO (Refer to the Functional Diagram) Main Control Loop (Normal Operation)
and the load will eventually cause the error amplifier out- put to start drifting higher. When the error amplifier output The LTC3801/LTC3801B are constant frequency current rises to 0.225V above its zero current level (approximately mode step-down switching regulator controllers. During 0.925V), the sleep comparator will untrip and normal op- normal operation, an external P-channel MOSFET is turned eration will resume. The next oscillator cycle will turn the on each cycle when the oscillator sets the RS latch and external MOSFET on and the switching cycle will repeat. turned off when the current comparator resets the latch. The peak inductor current at which the current comparator
Low Load Current Operation (LTC3801B Only)
trips is controlled by the voltage on the ITH/RUN pin, which is the output of the error amplifier. The negative input to Under very light load current conditions, the ITH/RUN pin the error amplifier is the output feedback voltage V voltage will be very close to the zero current level of 0.85V. FB which is generated by an external resistor divider con- As the load current decreases further, an internal offset at nected between V the current comparator input will ensure that the current OUT and ground. When the load current increases, it causes a slight decrease in V comparator remains tripped (even at zero load current) FB relative to the 0.8V reference, which in turn causes the I and the regulator will start to skip cycles, as it must, in TH/RUN voltage to increase until the average inductor current matches the order to maintain regulation. This behavior allows the new load current. regulator to maintain constant frequency down to very light loads, resulting in less low frequency noise genera- The main control loop is shut down by pulling the ITH/RUN tion over a wide load current range. pin to ground. Releasing the ITH/RUN pin allows an internal 1µA current source (2µA on LTC3801B) to charge Figure 1 illustrates this result for the circuit on the front up the external compensation network. When the I page of this data sheet using both an LTC3801 (in Burst TH/ RUN pin voltage reaches approximately 0.6V, the main Mode operation) and an LTC3801B (with Burst Mode control loop is enabled and the I operation disabled). At an output current of 100mA, the TH/RUN voltage is pulled up by a clamp to its zero current level of approximately LTC3801 exhibits an output ripple of 81.6mVP-P, whereas one diode voltage drop (0.7V). As the external compensa- the LTC3801B has an output ripple of only 17.6mVP-P. At tion network continues to charge up, the corresponding lower output current levels, the improvement is even peak inductor current level follows, allowing normal op- greater. This comes at a tradeoff of lower efficiency for the eration. The maximum peak inductor current attainable is non Burst Mode part at light load currents (see Figure 2). set by a clamp on the I Also notice the constant frequency operation of the TH/RUN pin at 1.2V above the zero current level (approximately 1.9V). LTC3801B, even at 5% of maximum output current.
Burst Mode Operation (LTC3801 Only) Dropout Operation
The LTC3801 incorporates Burst Mode operation at low When the input supply voltage decreases towards the load currents (<25% of I output voltage, the rate of change of inductor current MAX). In this mode, an internal clamp sets the peak current of the inductor at a level cor- during the on cycle decreases. This reduction means that responding to an I at some input-output differential, the external P-channel TH/RUN voltage 0.3V above its zero current level (approximately 1V), even though the actual MOSFET will remain on for more than one oscillator cycle I (start dropping off-cycles) since the inductor current has TH/RUN voltage is lower. When the inductor’s average current is greater than the load requirement, the voltage at not ramped up to the threshold set by the error amplifier. the I Further reduction in input supply voltage will eventually TH/RUN pin will drop. When the ITH/RUN voltage falls to 0.15V above its zero current level (approximately 0.85V), cause the external P-channel MOSFET to be turned on the sleep comparator will trip, turning off the external 100%, i.e., DC. The output voltage will then be determined MOSFET. In sleep, the input DC supply current to the IC is by the input voltage minus the voltage drop across the reduced to 16µA from 195µA in normal operation. With the sense resistor, the MOSFET and the inductor. switch held off, average inductor current will decay to zero sn3801 3801fs 5