Datasheet LTC3607 (Analog Devices) - 8

LTC3607
operaTion
The LTC3607 uses a constant-frequency, peak current
Low Current Operation
mode architecture. The operating frequency is set at Two discontinuous-conduction modes (DCMs) are avail- 2.25MHz and can be synchronized to an external oscil ator able to control the operation of the LTC3607 at low output between 1MHz and 4MHz. Both channels share the same currents. Both modes, Burst Mode operation and pulse- clock and run in-phase. To suit a variety of applications, skipping, automatical y switch from continuous operation the selectable MODE/SYNC pin al ows the user to trade- to the selected mode when the load current is low. off ripple for efficiency. To optimize efficiency, Burst Mode operation can be se- The output voltage is set by an external divider returned lected by floating the MODE/SYNC pin or setting it to 1V to the VFB pins. An error amplifier compares the divided or greater. When the load is relatively light, the LTC3607 output voltage with a reference voltage of 0.6V and ad- automatical y switches into Burst Mode operation in which justs the peak inductor current accordingly. Overvoltage the PMOS switch operates intermittently based on load and undervoltage comparators will pull the independent demand with a fixed peak inductor current. By running PGOOD outputs low if the output voltage is not within cycles periodical y, the switching losses, which are domi- ±8.5%. The PGOOD outputs will go high 64 clock cycles nated by the gate charge losses of the power MOSFETs, after achieving regulation and will go low 64 cycles after are minimized. The main control loop is interrupted when fal ing out of regulation. the output voltage reaches the desired regulated value. Whether in Burst Mode or pulse-skipping operation, the A voltage comparator trips when the ITH voltage drops overvoltage protection circuit is still enabled when the rest below an internal clamp voltage, shutting off the switch of the regulator is asleep. Hence, if VOUT rises above the and reducing the power. The output capacitor and the overvoltage threshold, the regulator is forced out of sleep. inductor supply the power to the load until ITH exceeds an internal clamp voltage, turning on the switch and the
Main Control Loop
main control loop, which starts another cycle. During normal operation, the top power switch (P-channel To optimize ripple, pulse-skipping mode can be selected MOSFET) is turned on at the beginning of a clock cycle by grounding the MODE/SYNC pin. In the LTC3607, pulse- when the VFB voltage is below the reference voltage. The skipping mode is implemented similarly to Burst Mode current into the inductor and the load increases until the operation with the ITH clamp set to a lower internal clamp current limit is reached. The switch turns off and energy voltage. This results in lower ripple than in Burst Mode stored in the inductor flows through the bottom switch operation with the trade-off being slightly lower efficiency. (N-channel MOSFET) into the load until the next clock cycle. The peak inductor current is control ed by the internal y
Dropout Operation
compensated ITH voltage, which is the output of the error When the input supply voltage decreases toward the output amplifier. This amplifier compares the VFB pin to the 0.6V voltage, the duty cycle increases to 100% which is the internal reference. When the load current increases, the dropout condition. In dropout, the PMOS switch is turned VFB voltage decreases slightly below the reference. This on continuously with the output voltage being equal to the decrease causes the error amplifier to increase the ITH input voltage minus the voltage drops across the internal voltage until the average inductor current matches the P-channel MOSFET and the inductor. new load current. The main control loop is shut down by pul ing the RUN pin to ground. 3607fb 8 For more information www.linear.com/LTC3607 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts