Datasheet LTC3407A-2 (Analog Devices) - 7

LTC3407A-2
U OPERATIO
The LTC3407A-2 uses a constant frequency, current mode automatically switches into Burst Mode operation in which architecture. The operating frequency is set at 2.25MHz the PMOS switch operates intermittently based on load and can be synchronized to an external oscillator. Both demand with a fixed peak inductor current. By running channels share the same clock and run in-phase. To suit cycles periodically, the switching losses which are domi- a variety of applications, the selectable MODE/SYNC pin nated by the gate charge losses of the power MOSFETs are allows the user to trade-off noise for efficiency. minimized. The main control loop is interrupted when the output voltage reaches the desired regulated value. A The output voltage is set by an external divider returned to voltage comparator trips when I the V TH is below 0.65V, shut- FB pins. An error amplifier compares the divided ting off the switch and reducing the power. The output output voltage with a reference voltage of 0.6V and adjusts capacitor and the inductor supply the power to the load the peak inductor current accordingly. Overvoltage and until I undervoltage comparators will pull the POR output low if TH exceeds 0.65V, turning on the switch and the main control loop which starts another cycle. the output voltage is not within ±8.5%. The POR output will go high after 65,536 clock cycles (about 29ms in pulse For lower ripple noise at low currents, the pulse skipping skipping mode) of achieving regulation. mode can be used. In this mode, the LTC3407A-2 contin- ues to switch at a constant frequency down to very low
Main Control Loop
currents, where it will begin skipping pulses. During normal operation, the top power switch (P-channel MOSFET) is turned on at the beginning of a clock cycle
Dropout Operation
when the VFB voltage is below the reference voltage. The When the input supply voltage decreases toward the current into the inductor and the load increases until the output voltage, the duty cycle increases to 100% which is current limit is reached. The switch turns off and energy the dropout condition. In dropout, the PMOS switch is stored in the inductor flows through the bottom switch (N- turned on continuously with the output voltage being channel MOSFET) into the load until the next clock cycle. equal to the input voltage minus the voltage drops across The peak inductor current is controlled by the internally the internal P-channel MOSFET and the inductor. compensated ITH voltage, which is the output of the error An important design consideration is that the RDS(ON) of amplifier.This amplifier compares the VFB pin to the 0.6V the P-channel switch increases with decreasing input reference. When the load current increases, the VFB volt- supply voltage (See Typical Performance Characteristics). age decreases slightly below the reference. This Therefore, the user should calculate the power dissipation decrease causes the error amplifier to increase the ITH when the LTC3407A-2 is used at 100% duty cycle with low voltage until the average inductor current matches the new input voltage (See Thermal Considerations in the Applica- load current. tions Information Section). The main control loop is shut down by pulling the RUN/SS pin to ground.
Low Supply Operation
The LTC3407A-2 incorporates an undervoltage lockout
Low Current Operation
circuit which shuts down the part when the input voltage Two modes are available to control the operation of the drops below about 1.65V to prevent unstable operation. LTC3407A-2 at low currents. Both modes automatically A general LTC3407A-2 application circuit is shown in switch from continuous operation to the selected mode Figure 1. External component selection is driven by the when the load current is low. load requirement, and begins with the selection of the To optimize efficiency, the Burst Mode operation can be inductor L. Once the inductor is chosen, CIN and COUT can selected. When the load is relatively light, the LTC3407A-2 be selected. 3407a2f 7