Datasheet LTC3407A (Analog Devices) - 7

LTC3407A
OPERATION
The LTC3407A uses a constant frequency, current mode the PMOS switch operates intermittently based on load architecture. The operating frequency is set at 1.5MHz demand with a fi xed peak inductor current. By running and can be synchronized to an external oscillator. Both cycles periodically, the switching losses which are domi- channels share the same clock and run in-phase. To suit nated by the gate charge losses of the power MOSFETs a variety of applications, the selectable MODE/SYNC pin are minimized. The main control loop is interrupted when allows the user to trade-off noise for effi ciency. the output voltage reaches the desired regulated value. A voltage comparator trips when I The output voltage is set by an external divider returned TH is below 0.65V, shutting off the switch and reducing the power. The output capaci- to the VFB pins. An error amplifi er compares the divided tor and the inductor supply the power to the load until I output voltage with a reference voltage of 0.6V and adjusts TH exceeds 0.65V, turning on the switch and the main control the peak inductor current accordingly. Overvoltage and loop which starts another cycle. undervoltage comparators will pull the POR output low if the output voltage is not within ±8.5%. The POR output For lower ripple noise at low currents, the pulse skipping will go high after 65,536 clock cycles (about 44ms in pulse mode can be used. In this mode, the LTC3407A continues skipping mode) of achieving regulation. to switch at a constant frequency down to very low cur- rents, where it will begin skipping pulses.
Main Control Loop
During normal operation, the top power switch (P-channel
Dropout Operation
MOSFET) is turned on at the beginning of a clock cycle when When the input supply voltage decreases toward the the VFB voltage is below the reference voltage. The current output voltage, the duty cycle increases to 100% which into the inductor and the load increases until the current is the dropout condition. In dropout, the PMOS switch is limit is reached. The switch turns off and energy stored in turned on continuously with the output voltage being equal the inductor fl ows through the bottom switch (N-channel to the input voltage minus the voltage drops across the MOSFET) into the load until the next clock cycle. internal P-channel MOSFET and the inductor. The peak inductor current is controlled by the internally An important design consideration is that the RDS(ON) compensated ITH voltage, which is the output of the er- of the P-channel switch increases with decreasing input ror amplifi er.This amplifi er compares the VFB pin to the supply voltage (See Typical Performance Characteristics). 0.6V reference. When the load current increases, the Therefore, the user should calculate the power dissipation VFB voltage decreases slightly below the reference. This when the LTC3407A is used at 100% duty cycle with low decrease causes the error amplifi er to increase the ITH input voltage (See Thermal Considerations in the Applica- voltage until the average inductor current matches the tions Information Section). new load current. The main control loop is shut down by pulling the RUN/SS
Low Supply Operation
pin to ground. The LTC3407A incorporates an undervoltage lockout circuit which shuts down the part when the input voltage drops
Low Current Operation
below about 1.65V to prevent unstable operation. Two modes are available to control the operation of the A general LTC3407A application circuit is shown in LTC3407A at low currents. Both modes automatically Figure 1. External component selection is driven by the switch from continuous operation to the selected mode load requirement, and begins with the selection of the when the load current is low. inductor L. Once the inductor is chosen, CIN and COUT To optimize effi ciency, the Burst Mode operation can be can be selected. selected. When the load is relatively light, the LTC3407A automatically switches into Burst Mode operation in which 3407afa 7