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LTM4600 Datasheet(PDF) 9 Page - Linear Technology |
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LTM4600 Datasheet(HTML) 9 Page - Linear Technology |
9 / 24 page LTM4600 9 4600fa APPLICATIO S I FOR ATIO down when QDOWN is on and QUP is off. If the output voltage VO needs to be margined up/down by ±M%, the resistor values of RUP and RDOWN can be calculated from the following equations: ()• •( %) () . RR V M RR k V SET UP O SET UP 1 100 06 + +Ω = RV M Rk R V SET O SET DOWN ••( – %) () . 1 100 06 +Ω = Input Capacitors The LTM4600 µModule should be connected to a low ac-impedance DC source. High frequency, low ESR input capacitors are required to be placed adjacent to the mod- ule. In Figure 20, the bulk input capacitor CIN is selected for its ability to handle the large RMS current into the converter. For a buck converter, the switching duty-cycle can be estimated as: D V V O IN = Without considering the inductor current ripple, the RMS current of the input capacitor can be estimated as: I I DD CIN RMS OMAX () () % ••( ) =− η 1 In the above equation, η% is the estimated efficiency of the power module. C1 can be a switcher-rated electrolytic aluminum capacitor, OS-CON capacitor or high volume ceramic capacitors. Note the capacitor ripple current ratings are often based on only 2000 hours of life. This makes it advisable to properly derate the input capacitor, or choose a capacitor rated at a higher temperature than required. Always contact the capacitor manufacturer for derating requirements. In Figure 16, the input capacitors are used as high fre- quency input decoupling capacitors. In a typical 10A output application, 1-2 pieces of very low ESR X5R or X7R, 10µF ceramic capacitors are recommended. This decoupling capacitor should be placed directly adjacent The typical LTM4600 application circuit is shown in Figure 17. External component selection is primarily determined by the maximum load current and output voltage. Output Voltage Programming and Margining The PWM controller of the LTM4600 has an internal 0.6V±1% reference voltage. As shown in the block diagram, a 100k/0.5% internal feedback resistor connects VOUT and FB pins. Adding a resistor RSET from VOSET pin to SGND pin programs the output voltage: VV kR R O SET SET = + 06 100 .• Table 1 shows the standard vaules of 1% RSET resistor for typical output voltages: Table 1. RSET (kΩ) Open 100 66.5 49.9 43.2 31.6 22.1 13.7 VO (V) 0.6 1.2 1.5 1.8 2 2.5 3.3 5 Voltage margining is the dynamic adjustment of the output voltage to its worst case operating range in production testing to stress the load circuitry, verify control/protec- tion functionality of the board and improve the system reliability. Figure 2 shows how to implement margining function with the LTM4600. In addition to the feedback resistor RSET, several external components are added. Turn off both transistor QUP and QDOWN to disable the margining. When QUP is on and QDOWN is off, the output voltage is margined up. The output voltage is margined Figure 2. PGND SGND 4600 F02 LTM4600 VOUT VOSET RSET RUP QUP 100k 2N7002 RDOWN QDOWN 2N7002 |
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