Datasheet LTC3403 (Analog Devices) - 8

LTC3403
U OPERATIO (Refer to Functional Diagram) Main Control Loop
ing the BURST comparator to trip and turn the top switch on. This process repeats at a rate that is dependent on the The LTC3403 uses a constant frequency, current mode load demand. step-down architecture. The main (P-channel MOSFET), synchronous (N-channel MOSFET) and bypass (P-chan-
Controlling the Output Voltage
nel MOSFET) switches are internal. During normal opera- tion, the internal main switch is turned on each cycle when The output voltage can be dynamically programmed from the oscillator sets the RS latch, and turned off when the 0.3V to 3.5V using the REF input. Because the gain to VOUT current comparator, I from REF is internally set to 3, the corresponding input COMP, resets the RS latch. The peak inductor current at which I range at REF is 0.1V to 1.167V. V COMP resets the RS latch, is OUT can be modulated controlled by the output of error amplifier EA. When the during operation by driving REF with an external DAC. load current increases, it causes a slight decrease in the When REF exceeds 1.2V, an internal bypass P-channel feedback voltage, FB, relative to the external reference, MOSFET connects VIN to VOUT, dramatically reducing the which in turn, causes the EA amplifier’s output voltage to drop across the inductor and the main switch. increase until the average inductor current matches the new load current. While the main switch is off, the syn-
Dropout Operation
chronous switch is turned on until the beginning of the If the reference voltage would cause V next clock cycle. OUT to exceed VIN, the LTC3403 enters dropout operation. During dropout, In forced continuous mode the inductor current is con- the main switch remains on continuously and operates at stantly cycled. In this mode, the output voltage can re- 100% duty cycle. If the voltage at REF is less than 1.2V, the spond quickly to the external reference voltage by sourc- bypass P-channel MOSFET will stay off even in dropout ing or sinking current as needed. operation. The output voltage is then determined by the input voltage minus the voltage drop across the main switch
Burst Mode Operation
and the inductor. The LTC3403 is capable of Burst Mode operation in which An important detail to remember is that at low input the internal power switches operate intermittently based supply voltages, the RDS(ON) of the P-channel switch on load demand. increases (see Typical Performance Characteristics). In Burst Mode operation, the peak current of the inductor Therefore, the user should calculate the power dissipa- is set to approximately 200mA regardless of the output tion when the LTC3403 is used at 100% duty cycle with load. Each burst event can last from a few cycles at light low input voltage (See Thermal Considerations in the loads to almost continuously cycling with short sleep Applications Information section). intervals at moderate loads. In between these burst events, the power switches and any unneeded circuitry are turned
Low Supply Operation
off, reducing the quiescent current to 20µA. In this sleep The LTC3403 will operate with input supply voltages as state, the load current is being supplied solely from the low as 2.5V, but the maximum allowable output current is output capacitor. As the output voltage droops, the EA reduced at this low voltage. Figure 2 shows the reduction amplifier’s output rises above the sleep threshold signal- in the maximum output current as a function of input voltage for various output voltages. 3403f 8