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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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CB2016T2R2M Fiches technique(PDF) 10 Page - Linear Integrated Systems |
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CB2016T2R2M Fiches technique(HTML) 10 Page - Linear Integrated Systems |
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10 / 16 page  LTC3547 10 3547fa APPLICATIO S I FOR ATIO Setting the Output Voltage The LTC3547 regulates the VFB1 and VFB2 pins to 0.6V during regulation. Thus, the output voltage is set by a resistive divider according to the following formula: VV R R OUT =+ ⎛ ⎝⎜ ⎞ ⎠⎟ 06 1 2 1 . (4) Keeping the current small (< 5µA) in these resistors maxi- mizes efficiency, but making it too small may allow stray capacitance to cause noise problems or reduce the phase margin of the error amp loop. To improve the frequency response of the main control loop, a feedback capacitor (CF) may also be used. Great care should be taken to route the VFB line away from noise sources, such as the inductor or the SW line. Fixed output versions of the LTC3547 (e.g. LTC3547-1) include an internal resistive divider, eliminating the need for external resistors. The resistor divider is chosen such that the VFB input current is 3µA. For these versions the VFB pin should be connected directly to VOUT. Table 2 lists the fixed output voltages available for the LTC35476-1. Table 2. Fixed Output Voltage Versions PART NUMBER VOUT1 VOUT2 LTC3547 Adjustable Adjustable LTC3547-1 1.8V 1.2V Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ΔILOAD • ESR, where ESR is the effective series resistance of COUT. ΔILOAD also begins to charge or dis- charge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. The initial output voltage step may not be within the band- width of the feedback loop, so the standard second-order overshoot/DC ratio cannot be used to determine the phase margin. In addition, feedback capacitors (CF1 and CF2) can be added to improve the high frequency response, as shown in Figure 1. Capacitor CF provides phase lead by creating a high frequency zero with R2 which improves the phase margin. The output voltage settling behavior is related to the stability of the closed-loop system and will demonstrate the actual overall supply performance. For a detailed explanation of optimizing the compensation components, including a re- view of control loop theory, refer to Application Note 76. In some applications, a more severe transient can be caused by switching in loads with large (>1µF) input ca- pacitors. The discharged input capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the switch connecting the load has low resistance and is driven quickly. The solution is to limit the turn-on speed of the load switch driver. A Hot Swap™ controller is designed specifically for this purpose and usually in- corporates current limiting, short-circuit protection, and soft-starting. Hot Swap is a trademark of Linear Technology Corporation. |
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