Datasheet LTC3863 (Analog Devices) - 8

LTC3863
PIN FUNCTIONS PLLIN/MODE (Pin 1):
External Reference Clock Input
ITH (Pin 6):
Current Control Threshold and Controller and Burst Mode Enable/Disable. When an external clock Compensation Point. This pin is the output of the error is applied to this pin, the internal phase-locked loop will amplifier and the switching regulator’s compensation synchronize the turn-on edge of the gate drive signal with point. The voltage ranges from 0V to 2.9V, with 0.8V the rising edge of the external clock. When no external corresponding to zero sense voltage (zero current). RITH clock is applied, this input determines the operation during should never be less than 10K to avoid large signal com- light loading. Floating this pin selects low IQ (50µA) Burst pensation issues. For further detail please see the section Mode operation. Pulling to ground selects pulse-skipping Large Signal Effects on ITH. mode operation.
VFBN (Pin 7):
Feedback Input for an Inverting PWM Control-
FREQ (Pin 2):
Switching Frequency Setpoint Input. The ler. Connect VFBN to the center of a resistor divider between switching frequency is programmed by an external set- the output and VFB. The resistor divider is comprised of point resistor RFREQ connected between the FREQ pin and RFB1, placed from the VOUT pin to VFBN pin and RFB2, placed signal ground. An internal 20µA current source creates from the VFBN pin to VFB. It is recommended to set RFB2 a voltage across the external setpoint resistor to set the to be greater than 8K, please see the Output Voltage Pro- internal oscillator frequency. Alternatively, this pin can gramming section for further detail. The VFBN threshold is be driven directly by a DC voltage to set the oscillator 0V. To defeat the inverting amplifier and use the LTC3863 frequency. Grounding selects a fixed operating frequency as an LTC3864 (noninverting buck), tie VFBN > 2V. of 350kHz. Floating selects a fixed operating frequency of 535kHz.
RUN (Pin 8):
Digital Run Control Input. A RUN voltage above the 1.26V threshold enables normal operation, while
SGND (Pin 3):
Ground Reference for Small-Signal Analog a voltage below the threshold shuts down the controller. Component (Signal Ground). Signal ground should be An internal 0.4µA current source pulls the RUN pin up to used as the common ground for all small-signal analog about 3.3V. The RUN pin can be connected to an external inputs and compensation components. Connect the signal power supply up to 60V. ground to the power ground (ground reference for power components) only at one point using a single PCB trace.
CAP (Pin 9):
Gate Driver (–) Supply. A low ESR ceramic bypass capacitor of at least 0.1µF or 10X the effective
SS (Pin 4):
Soft-Start and External Tracking Input. The CMILLER of the P-channel power MOSFET, is required LTC3863 regulates the feedback voltage to the smaller of from VIN to this pin to serve as a bypass capacitor for the 0.8V or the voltage on the SS pin. An internal 10μA pull-up internal regulator. To ensure stable low noise operation, the current source is connected to this pin. A capacitor to bypass capacitor should be placed adjacent to the VIN and ground at this pin sets the ramp time to the final regulated CAP pins and connected using the same PCB metal layer. output voltage. Alternatively, another voltage supply con- nected through a resistor divider to this pin allows the
SENSE (Pin 10):
Current Sense Input. A sense resistor, output to track the other supply during start-up. RSENSE, from the VIN pin to the SENSE pin sets the maxi- mum current limit. The peak inductor current limit is equal
VFB (Pin 5):
Output Feedback Sense. A resistor divider to 95mV/RSENSE. For accuracy, it is important that the VIN from the regulated output point to this pin sets the output pin and the SENSE pin route directly to the current sense voltage. The LTC3863 will nominally regulate VFB to the resistor and make a Kelvin (4-wire) connection. internal reference value of 0.8V. If VFB is less than 0.4V, the switching frequency will linearly decrease and fold
VIN (Pin 11):
Chip Power Supply. A minimum bypass back to about one-fifth of the internal oscillator frequency capacitor of 0.1µF is required from the VIN pin to power to reduce the minimum duty cycle. ground. For best performance use a low ESR ceramic capacitor placed near the VIN pin. Rev. B 8 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts