Datasheet LTM8021 (Analog Devices) - 10

LTM8021
applicaTions inForMaTion Soft-Start
your system can tolerate a few milliamps in this state. If The RUN/SS pin can be used to soft-start the LTM8021, the RUN/SS pin is grounded, the internal power switch reducing the maximum input current during start-up. current will drop to essentially zero. However, if the VIN pin The RUN/SS pin is driven through an external RC filter is grounded while the output is held high, then parasitic to create a voltage ramp at this pin. Figure 2 shows the diodes inside the LTM8021 can pull large currents from soft-start circuit. By choosing a large RC time constant, the output through the internal power switch and the VIN the peak start-up current can be reduced to the current pin. Figure 3 shows a circuit that will run only when the that is required to regulate the output, with no overshoot. input voltage is present and that protects against a shorted Choose the value of the resistor so that it can supply 80µA or reversed input. when the RUN/SS pin reaches 2V.
PCB Layout
RUN Most of the problems associated with the PCB layout 15k have been alleviated or eliminated by the high level of RUN/SS integration of the LTM8021. The LTM8021 is nevertheless 0.22µF GND a switching power supply, and care must be taken to minimize EMI and ensure proper operation. Even with the 8021 F02 high level of integration, one may fail to achieve a specified
Figure 2. To Soft-Start the LTM8021, Add a
operation with a haphazard or poor layout. See Figure 4
Resistor and Capacitor to the RUN/SS Pin
for a suggested layout.
Shorted Input Protection
Ensure that the grounding and heatsinking are acceptable. A few rules to keep in mind are: Care needs to be taken in systems where the output will be held high when the input to the LTM8021 is absent. 1. Place the CIN capacitor as close as possible to the VIN This may occur in battery charging applications or in and GND connection of the LTM8021. battery backup systems where a battery or some other 2. Place the COUT capacitor as close as possible to the supply is diode ORed with the LTM8021’s output. If the VOUT and GND connection of the LTM8021. VIN pin is allowed to float and the RUN/SS pin is held high (either by a logic signal or because it is tied to V 3. Place the CIN and COUT capacitors such that their ground IN), then the LTM8021’s internal circuitry will pull its quiescent currents flow directly adjacent to, or underneath the current through its internal power switch. This is fine if LTM8021. VIN GND PLANE VOUT LTM8021 VIN VIN VOUT VOUT CIN COUT 4V TO 36V RUN/SS BIAS GND RT CIN C R OUT ADJ BIAS FB RADJ 8021 F03 RUN/SS
Figure 3. The Input Diode Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output. It Also Protects
8021 F04
the Circuit from a Reversed Input. The LTM8021 Runs Only When Figure 4. Layout Showing Suggested External Components, the Input is Present GND Plane and Thermal Vias
8021fd 10 For more information www.linear.com/LTM8021 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