LTM4604A applicaTions inForMaTion A typical LTM4604A application circuit is shown in Fig- Without considering the inductor current ripple, the RMS ure 15. External component selection is primarily deter- current of the input capacitor can be estimated as: mined by the maximum load current and output voltage. Refer to Table 4 for specific external capacitor requirements I I OUT(MAX) • D•(1–D) for a particular application. CIN(RMS) = η% VIN to VOUT Step-Down Ratios In the above equation, η% is the estimated efficiency of the power module. The bulk capacitor can be a switcher- There are restrictions in the maximum VIN and VOUT step- rated aluminum electrolytic capacitor, OS-CON or down ratio that can be achieved for a given input voltage. polymer capacitor. If a low inductance plane is used to The LTM4604A is 100% duty cycle capable, but the VIN power the device, then no input capacitance is required. to VOUT minimum dropout is a function of the load cur- The two internal 10µF ceramics are typically rated for 2A rent. A typical 0.5V minimum is sufficient (see Typical to 3A of RMS ripple current. The worst-case ripple current Performance Characteristics). for the 4A maximum current is 2A or less. Output Voltage ProgrammingOutput Capacitors The PWM controller has an internal 0.8V reference voltage. The LTM4604A is designed for low output voltage ripple. As shown in the Block Diagram, a 4.99k 0.5% internal The bulk output capacitors defined as C feedback resistor connects the V OUT are chosen OUT and FB pins together. with low enough effective series resistance (ESR) to meet The output voltage will default to 0.8V with no externally the output voltage ripple and transient requirements. C applied feedback resistor. Adding a resistor R OUT FB from the can be a low ESR tantalum capacitor, a low ESR polymer FB pin to GND programs the output voltage: capacitor or an X5R/X7R ceramic capacitor. The typical 4.99k +R output capacitance range is 22µF to 100µF. Additional V FB OUT = 0.8V • output filtering may be required by the system designer RFB if further reduction of output ripple or dynamic transient Table 1. FB Resistor vs Output Voltage spikes is required. Table 4 shows a matrix of different output voltages and output capacitors to minimize the VOUT 0.8V 1V 1.2V 1.5V 1.8V 2.5V 3.3V voltage droop and overshoot during a 2A/µs transient. RFB Open 20k 10k 5.76k 4.02k 2.37k 1.62k The table optimizes the total equivalent ESR and total Input Capacitors bulk capacitance to maximize transient performance. The LTpowerCAD GUI is available for further optimization. The LTM4604A module should be connected to a low ac- impedance DC source. Two 10µF ceramic capacitors are Fault Conditions: Current Limit and Overcurrent included inside the module. Additional input capacitors Foldback are only needed if a large load step is required up to a full 4A level. An input 47µF bulk capacitor is only needed The LTM4604A has current mode control, which inher- if the input source impedance is compromised by long ently limits the cycle-by-cycle inductor current not only inductive leads or traces. in steady-state operation, but also in transient. For a buck converter, the switching duty cycle can be To further limit current in the event of an overload condi- estimated as: tion, the LTM4604A provides foldback current limiting as the output voltage falls. The LTM4604A device has over- V temperature shutdown protection that inhibits switching D = OUT operation around 150°C. VIN 4604afc 10 For more information www.linear.com/LTM4604A Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Decoupling Requirements Operation Applications Information Typical Applications Package Description Revision History Package Photograph Related Parts
