Datasheet LTC3417A (Analog Devices) - 8

LTC3417A
operaTion
The LTC3417A uses a constant frequency, current mode To optimize efficiency, Burst Mode operation can be architecture. Both channels share the same clock frequency. selected. When the load is relatively light, the LTC3417A The PHASE pin sets whether the channels are running automatically switches into Burst Mode operation in which in-phase or out of phase. The operating frequency is de- the PMOS switches operate intermittently based on load termined by connecting the FREQ pin to VIN for 1.5MHz demand. By running cycles periodically, the switching operation or by connecting a resistor from FREQ to ground losses, which are dominated by the gate charge losses for a frequency from 0.6MHz to 4MHz. To suit a variety of of the power MOSFETs, are minimized. The main control applications, the SYNC/MODE pin allows the user to trade loop is interrupted when the output voltage reaches the off noise for efficiency. desired regulated value. The hysteresis voltage comparator The output voltages are set by external dividers returned trips when ITH is below 0.24V, shutting off the switch and to the V reducing the power. The output capacitor and the induc- FB1 and VFB2 pins. An error amplifier compares the divided output voltage with a reference voltage of 0.8V and tor supply the power to the load until ITH exceeds 0.31V, adjusts the peak inductor current accordingly. Undervoltage turning on the switch and the main control loop which comparators will pull the PGOOD output low when either starts another cycle. output voltage is 6% below its targeted value. For lower output voltage ripple at low currents, pulse skipping mode can be used. In this mode, the LTC3417A
Main Control Loop
continues to switch at constant frequency down to very For each regulator, during normal operation, the P-chan- low currents, where it will begin skipping pulses used to nel MOSFET power switch is turned on at the beginning control the power MOSFETs. of a clock cycle when the VFB voltage is below the refer- Finally, in forced continuous mode, the inductor current is ence voltage. The current into the inductor and the load constantly cycled creating a fixed output voltage ripple at all increases until the current limit is reached. The switch output current levels. This feature is desirable in telecom- turns off and energy stored in the inductor flows through munications since the noise is a constant frequency and is the bottom N-channel MOSFET switch into the load until thus easy to filter out. Another advantage of this mode is the next clock cycle. that the regulator is capable of both sourcing current into The peak inductor current is controlled by the voltage a load and sinking some current from the output. on the ITH pin, which is the output of the error amplifier. The mode selection for the LTC3417A is set using the This amplifier compares the VFB pin to the 0.8V reference. SYNC/MODE pin. The SYNC/MODE pin sets the mode for When the load current increases the VFB voltage decreases both the1A and the 1.5A step-down DC/DC converters. slightly below the reference. This decrease causes the er- ror amplifier to increase the ITH voltage until the average
Dropout Operation
inductor current matches the new load current. When the input supply voltage decreases toward the output The main control loop is shut down by pulling the RUN pin voltage, the duty cycle increases to 100%. In this dropout to ground. A digital soft-start is enabled after shutdown, condition, the PMOS switch is turned on continuously with which will slowly ramp the peak inductor current up over the output voltage being equal to the input voltage minus 1024 clock cycles. the voltage drops across the internal P-channel MOSFET and inductor.
Low Current Operation
Three modes are available to control the operation of
Low Supply Operation
the LTC3417A at low currents. Each of the three modes The LTC3417A incorporates an undervoltage lockout circuit automatically switch from continuous operation to the which shuts down the part when the input voltage drops selected mode when the load current is low. below about 2.07V to prevent unstable operation. 3417afc 8 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Package Description Revision History Related Parts