Datasheet LTC3543 (Analog Devices) - 8

LTC3543
OPERATION (Refer to Functional Diagram) Main Control Loop Pulse Skip Mode Operation
The LTC3543 uses current mode step-down architecture Connecting the MODE pin to VIN places the LTC3543 in with both the main (P-channel MOSFET) and synchronous pulse skip mode. During light loads, the inductor can (N-channel MOSFET) switches internal. During normal reach zero amps or reverse current on each pulse. This operation, the internal top power MOSFET is turned on is caused by the bottom MOSFET being turned off by the each cycle as the oscillator sets the RS latch, and turned current reversal comparator, IRCMP, at which time the switch off when the current comparator, ICOMP, resets the RS voltage will ring. This is discontinuous mode operation, latch. The peak inductor current at which ICOMP resets and is normal behavior for a switching regulator. At very the RS latch, is controlled by the output of error ampli- light loads, the LTC3543 will automatically skip pulses in fi er, EA. When the load current increases, it causes a order to maintain output regulation. slight decrease in the feedback voltage, VFB, relative to an internal reference voltage which, in turn, causes the
Spread Spectrum Operation
EA amplifi er’s output voltage to increase until the average Setting the MODE pin from 0.55V to 0.8V will place the inductor current matches the new load current. While the part in pulse skip mode with spread spectrum; an easy top MOSFET is off, the bottom MOSFET is turned on until way to do this is to connect the MODE pin to the VFB pin. either the inductor current starts to reverse, as indicated In this mode, an external capacitor is required between by the current reversal comparator IRCMP, or the beginning CAP and GND. The external capacitor assists in smoothing of the next clock cycle. frequency transitions.
Burst Mode Operation
The spread spectrum architecture randomly varies the LTC3543’s switching frequency from 2MHz to 3MHz, The LTC3543 is capable of Burst Mode operation in which signifi cantly reducing the peak radiated and conducting the internal power MOSFETs operate intermittently based noise on both the input and output supplies, making it on load demand. Burst Mode operation is enabled by con- easier to comply with electromagnetic interference (EMI) necting the MODE pin to ground. standards. During Burst Mode operation, the LTC3543’s internal Switching regulators can be particularly troublesome in circuits sense when the inductor peak current falls below applications where electromagnetic interference (EMI) is a 100mA. When below this level, the power MOSFETs and any concern. Switching regulators operate on a cycle-by-cycle unneeded circuitry are turned off, reducing the quiescent basis to transfer power to an output. In most cases, the current to 45μA, and holding the peak current reference frequency of operation is either fi xed or constant, based level at 100mA. The LTC3543 remains in this sleep state on the output load. This method of conversion creates until the feedback voltage falls below its internal reference. large components of noise at the frequency of operation Once this occurs, the regulator wakes up and allows the (fundamental) and multiples of the operating frequency inductor to develop 100mA current pulses. In light loads, (harmonics). Figure 1a depicts the output noise spectrum this will cause the output voltage to increase and the internal of a conventional buck switching converter (LTC3543 peak current reference to decrease. When the peak current with spread spectrum operation disabled) with V reference falls to below 100mA, the part re-enters sleep IN = 3.6V, V mode and the cycle is repeated. This process repeats at OUT = 1.5V and IOUT = 300mA. a rate dependent on the load demand. 3543fa 8