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LTM4604AIV-PBF Datasheet(PDF) 9 Page - Linear Technology |
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LTM4604AIV-PBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LTM4604A 9 4604af APPLICATIONS INFORMATION A typical LTM4604A application circuit is shown in Figure 15. External component selection is primarily determined by the maximum load current and output voltage. Refer to Table 4 for specific external capacitor requirements for a particular application. VIN to VOUT Step-Down Ratios There are restrictions in the maximum VIN and VOUT step- down ratio that can be achieved for a given input voltage. The LTM4604A is 100% duty cycle, but the VIN to VOUT minimum dropout is a function of the load current. A typi- cal 0.5V minimum is sufficient (see Typical Performance Characteristics). Output Voltage Programming The PWM controller has an internal 0.8V reference volt- age. As shown in the Block Diagram, a 4.99k 0.5% internal feedback resistor connects the VOUT and FB pins together. The output voltage will default to 0.8V with no feedback resistor. Adding a resistor RFB from the FB pin to GND programs the output voltage: VV kR R OUT FB FB = + 0 8 499 .• . Table 1. FB Resistor vs Output Voltage VOUT 0.8V 1.2V 1.5V 1.8V 2.5V 3.3V RFB Open 10k 5.76k 4.02k 2.37k 1.62k Input Capacitors The LTM4604A module should be connected to a low ac- impedance DC source. Two 10μF ceramic capacitors are included inside the module. Additional input capacitors are only needed if a large load step is required up to a full 4A level. An input 47μF bulk capacitor is only needed if the input source impedance is compromised by long inductive leads or traces. For a buck converter, the switching duty cycle can be estimated as: D V V OUT IN = Without considering the inductor current ripple, the RMS current of the input capacitor can be estimated as: I CIN(RMS) = I OUT(MAX) η% •D • 1– D () In the above equation, η% is the estimated efficiency of the power module. The bulk capacitor can be a switcher- rated electrolytic aluminum capacitor, OS-CON capacitor for bulk input capacitance due to high inductance traces or leads. If a low inductance plane is used to power the device, then no input capacitance is required. The two internal 10μF ceramics are typically rated for 2A to 3A of RMS ripple current. The worst-case ripple current for the 4A maximum current is 2A or less. Output Capacitors The LTM4604A is designed for low output voltage ripple. The bulk output capacitors defined as COUT are chosen with low enough effective series resistance (ESR) to meet the output voltage ripple and transient requirements. COUT can be a low ESR tantalum capacitor, a low ESR polymer capacitor or an X5R/X7R ceramic capacitor. The typical output capacitance range is 22μF to 100μF. Additional output filtering may be required by the system designer if further reduction of output ripple or dynamic transient spike is required. Table 4 shows a matrix of different output voltages and output capacitors to minimize the voltage droop and overshoot during a 2A/μs transient. The table optimizes the total equivalent ESR and total bulk capacitance to maximize transient performance. The Linear Technology μModule Power Design Tool can be provided for further optimization. Fault Conditions: Current Limit and Overcurrent Foldback The LTM4604A has current mode control, which inher- ently limits the cycle-by-cycle inductor current not only in steady-state operation, but also in transient. To further limit current in the event of an overload condi- tion, the LTM4604A provides foldback current limiting as the output voltage falls. The LTM4604A device has over- temperature shutdown protection that inhibits switching operation around 150°C. |
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