Datasheet LTM8021 (Analog Devices) - 7

LTM8021
operaTion
The LTM8021 is a standalone nonisolated step-down has sufficient stability margin and good transient perfor- switching DC/DC power supply. It can deliver up to mance under a wide range of operating conditions with a 500mA of DC output current with only bulk external input wide range of output capacitors, even all ceramic ones (X5R and output capacitors. This module provides a precisely or X7R). Current mode control provides cycle-by-cycle regulated output voltage programmable via one external fast current limit, and automatic current limiting protects resistor from 0.8VDC to 5VDC. The input voltage range is 3V the module in the event of a short circuit or overload fault. to 36V. Given that the LTM8021 is a step-down converter, The LTM8021 is based upon a 1.1MHz fixed frequency make sure that the input voltage is high enough to support PWM current mode controller, equipped with cycle skip the desired output voltage and load current. Please refer capability for low voltage outputs or light loads. A frequency to the simplified Block Diagram. foldback scheme helps to protect internal components from The LTM8021 contains a current mode controller, power overstress under heavy and short-circuit output loads. switching element, power inductor, power Schottky diode The drive circuit for the internal power switching element and a modest amount of input and output capacitance. is powered through the BIAS pin. Power this pin with at With its high performance current mode controller and least 3V. internal feedback loop compensation, the LTM8021 module
applicaTions inForMaTion
For most applications, the design process is straight
Minimum Duty Cycle
forward, summarized as follows: The LTM8021 has a fixed 1.1MHz switching frequency. For 1. Refer to Table 1 for the row that has the desired input any given output voltage, the duty cycle falls as the input range and output voltage. voltage rises. At very large VIN to VOUT ratios, the duty 2. Apply the recommended C cycle can be very small. Because the LTM8021’s internal IN, COUT and RADJ values. controller IC has a minimum on-time, the regulator will 3. Connect BIAS as indicated. skip cycles in order to maintain output voltage regulation. While these component combinations have been tested for This will result in a larger output voltage ripple and possible proper operation, it is incumbent upon the user to verify disturbances during recovery from a transient load step. proper operation over the intended system’s line, load and The component values provided in Table 1 allow for skip environmental conditions. cycle operation, but hold the resultant output ripple to around 50mV, or less. If even less ripple is desired, then If the desired output voltage is not listed in Table 1, set the more output capacitance may be necessary. Adding a feed- output by applying an RADJ resistor whose value is given by the equation, R forward capacitor has been empirically shown to modestly ADJ = 80/(VOUT – 0.80), where RADJ is in k and V extend the input voltage range to where the LTM8021 does OUT is in volts. Verify the LTM8021’s operation over the system’s intended line, load and environmental not skip cycles. Apply the feedforward capacitor between conditions. the VOUT pins and ADJ. This injects perturbations into the control loop, therefore, values larger than 50pF are not recommended. A good value to start with is 12pF. 8021fd For more information www.linear.com/LTM8021 7 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 Package Photo Typical Application Related Parts