Datasheet LTC3547 (Analog Devices) - 10

LTC3547
U U W U APPLICATIO S I FOR ATIO Setting the Output Voltage
a load step occurs, VOUT immediately shifts by an amount equal to The LTC3547 regulates the V ΔILOAD • ESR, where ESR is the effective series FB1 and VFB2 pins to 0.6V resistance of C during regulation. Thus, the output voltage is set by a OUT. ΔILOAD also begins to charge or dis- charge C resistive divider according to the following formula: OUT generating a feedback error signal used by the regulator to return V ⎛ OUT to its steady-state value. During R2⎞ V 0.6V this recovery time, V OUT = 1+ OUT can be monitored for overshoot ⎝⎜ 1⎠⎟ R (4) or ringing that would indicate a stability problem. Keeping the current small (< 5µA) in these resistors maxi- The initial output voltage step may not be within the band- mizes effi ciency, but making it too small may allow stray width of the feedback loop, so the standard second-order capacitance to cause noise problems or reduce the phase overshoot/DC ratio cannot be used to determine the phase margin of the error amp loop. margin. In addition, feedback capacitors (CF1 and CF2) can be added to improve the high frequency response, as To improve the frequency response of the main control shown in Figure 1. Capacitor CF provides phase lead by loop, a feedback capacitor (CF) may also be used. Great creating a high frequency zero with R2 which improves care should be taken to route the VFB line away from noise the phase margin. sources, such as the inductor or the SW line. The output voltage settling behavior is related to the stability Fixed output versions of the LTC3547 (e.g. LTC3547-1) of the closed-loop system and will demonstrate the actual include an internal resistive divider, eliminating the need overall supply performance. For a detailed explanation of for external resistors. The resistor divider is chosen such optimizing the compensation components, including a re- that the VFB input current is 3µA. For these versions the view of control loop theory, refer to Application Note 76. VFB pin should be connected directly to VOUT. Table 2 lists the fi xed output voltages available for the LTC35476-1. In some applications, a more severe transient can be caused by switching in loads with large (>1µF) input ca-
Table 2. Fixed Output Voltage Versions
pacitors. The discharged input capacitors are effectively
PART NUMBER VOUT1 VOUT2
put in parallel with COUT, causing a rapid drop in VOUT. LTC3547 Adjustable Adjustable No regulator can deliver enough current to prevent this LTC3547-1 1.8V 1.2V problem if the switch connecting the load has low resistance and is driven quickly. The solution is to limit the turn-on
Checking Transient Response
speed of the load switch driver. A Hot Swap™ controller The regulator loop response can be checked by looking is designed specifi cally for this purpose and usually in- at the load transient response. Switching regulators take corporates current limiting, short-circuit protection, and several cycles to respond to a step in load current. When soft-starting. Hot Swap is a trademark of Linear Technology Corporation. 3547fa 10