Datasheet LTC3417A-2 (Analog Devices) - 8

LTC3417A-2
OPERATION
The LTC3417A-2 uses a constant frequency, current To optimize effi ciency, Burst Mode operation can be se- mode architecture. Both channels share the same clock lected. When the load is relatively light, the LTC3417A-2 frequency. The PHASE pin sets whether the channels are automatically switches into Burst Mode operation in which running in-phase or out of phase. The operating frequency the PMOS switches operate intermittently based on load is determined by connecting the FREQ pin to VIN for demand. By running cycles periodically, the switching 1.5MHz operation or by connecting a resistor from FREQ losses, which are dominated by the gate charge losses to ground for a frequency from 0.6MHz to 4MHz. To suit of the power MOSFETs, are minimized. The main control a variety of applications, the SYNC/MODE pin allows the loop is interrupted when the output voltage reaches the user to trade off noise for effi ciency. desired regulated value. The hysteresis voltage comparator trips when I The output voltages are set by external dividers returned TH is below 0.24V, shutting off the switch and reducing the power. The output capacitor and the induc- to the VFB1 and VFB2 pins. An error amplifi er compares the tor supply the power to the load until I divided output voltage with a reference voltage of 0.8V and TH exceeds 0.31V, turning on the switch and the main control loop which adjusts the peak inductor current accordingly. Undervoltage starts another cycle. comparators will pull the PGOOD output low when either output voltage is 6% below its targeted value. For lower output voltage ripple at low currents, pulse skip- ping mode can be used. In this mode, the LTC3417A-2
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 fi xed 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 fl ows through munications since the noise is a constant frequency and is the bottom N-channel MOSFET switch into the load until thus easy to fi lter 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 amplifi er. The mode selection for the LTC3417A-2 is set using the This amplifi er 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 amplifi er 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 Low Supply Operation
Three modes are available to control the operation of the LTC3417A-2 at low currents. Each of the three modes The LTC3417A-2 incorporates an undervoltage lockout automatically switch from continuous operation to the circuit which shuts down the part when the input voltage selected mode when the load current is low. drops below about 2.07V to prevent unstable operation. 3417a2fa 8