Datasheet LTC3417A-1 (Analog Devices) - 8

LTC3417A-1
OPERATION
The LTC3417A-1 uses a constant frequency, current
Low Current Operation
mode architecture. Both channels share the same clock Three modes are available to control the operation of the frequency. The PHASE pin sets whether the channels are LTC3417A-1 at low currents. Each of the three modes running in-phase or out of phase. The operating frequency automatically switch from continuous operation to the is determined by connecting the FREQ pin to VIN for selected mode when the load current is low. 1.5MHz operation or by connecting a resistor from FREQ to ground for a frequency from 0.6MHz to 4MHz. To suit To optimize effi ciency, Burst Mode operation can be se- a variety of applications, the SYNC/MODE pin allows the lected. When the load is relatively light, the LTC3417A-1 user to trade off noise for effi ciency. automatically switches into Burst Mode operation in which the PMOS switches operate intermittently based on load The output voltages are set by external dividers returned demand. By running cycles periodically, the switching to the VFB1 and VFB2 pins. An error amplifi er compares the losses, which are dominated by the gate charge losses divided output voltage with a reference voltage of 0.8V and of the power MOSFETs, are minimized. The main control adjusts the peak inductor current accordingly. Undervoltage loop is interrupted when the output voltage reaches the comparators will pull the POR output low when either output desired regulated value. The hysteresis voltage comparator voltage is 8% below its targeted value. The POR output trips when I will go high after 212,992 cycles (when FREQ is high) or TH is below 0.24V, shutting off the switch and reducing the power. The output capacitor and the induc- 294,912 cycles (when FREQ is tied to ground through an tor supply the power to the load until I external resistor), or about 150ms, after both regulators TH exceeds 0.31V, turning on the switch and the main control loop which are above -6% of the target output voltage. starts another cycle.
Main Control Loop
For lower output voltage ripple at low currents, pulse skip- ping mode can be used. In this mode, the LTC3417A-1 For each regulator, during normal operation, the P-chan- continues to switch at constant frequency down to very nel MOSFET power switch is turned on at the beginning low currents, where it will begin skipping pulses used to of a clock cycle when the VFB voltage is below the refer- control the power MOSFETs. ence voltage. The current into the inductor and the load increases until the current limit is reached. The switch Finally, in forced continuous mode, the inductor current is turns off and energy stored in the inductor fl ows through constantly cycled creating a fi xed output voltage ripple at all the bottom N-channel MOSFET switch into the load until output current levels. This feature is desirable in telecom- the next clock cycle. munications since the noise is a constant frequency and is thus easy to fi lter out. Another advantage of this mode is The peak inductor current is controlled by the voltage that the regulator is capable of both sourcing current into on the ITH pin, which is the output of the error amplifi er. a load and sinking some current from the output. This amplifi er compares the VFB pin to the 0.8V reference. When the load current increases the VFB voltage decreases The mode selection for the LTC3417A-1 is set using the slightly below the reference. This decrease causes the er- SYNC/MODE pin. The SYNC/MODE pin sets the mode for ror amplifi er to increase the ITH voltage until the average both the1A and the 1.5A step-down DC/DC converters. inductor current matches the new load current. The main control loop is shut down by pulling the RUN pin to ground. A digital soft-start is enabled after shutdown, which will slowly ramp the peak inductor current up over 1024 clock cycles. 3417a1fa 8