Datasheet LTC3548-1 (Analog Devices) - 10

LTC3548-1
APPLICATIONS INFORMATION
can induce ringing at the VIN pin. At best, this ringing can margin. In addition, a feed-forward capacitor, CFF, is added couple to the output and be mistaken as loop instability. externally to improve the high frequency response. Capaci- At worst, the ringing at the input can be large enough to tor CFF provides phase lead by creating a high frequency damage the part. zero with R1, which improves the phase margin. Since the ESR of a ceramic capacitor is so low, the input The output voltage settling behavior is related to the stability and output capacitor must instead fulfi ll a charge storage of the closed-loop system and will demonstrate the actual requirement. During a load step, the output capacitor must overall supply performance. For a detailed explanation of instantaneously supply the current to support the load optimizing the compensation components, including a re- until the feedback loop raises the switch current enough view of control loop theory, refer to Application Note 76. to support the load. The time required for the feedback In some applications, a more severe transient can be loop to respond is dependent on the compensation and caused by switching loads with large (>1μF) load input the output capacitor size. Typically, 3-4 cycles are required capacitors. The discharged load input capacitors are ef- to respond to a load step, but only in the fi rst cycle does fectively put in parallel with C the output drop linearly. The output droop, V OUT, causing a rapid drop in DROOP, is V usually about 2-3 times the linear drop of the fi rst cycle. OUT. No regulator can deliver enough current to prevent this problem, if the switch connecting the load has low Thus, a good place to start is with the output capacitor resistance and is driven quickly. The solution is to limit size of approximately: the turn-on speed of the load switch driver. A Hot Swap™ ΔI controller is designed specifi cally for this purpose and C ≈ 2.5 OUT OUT f • V usually incorporates current limiting, short-circuit protec- O DROOP tion, and soft-starting. More capacitance may be required depending on the duty cycle and load step requirements.
Effi ciency Considerations
In most applications, the input capacitor is merely required The percent effi ciency of a switching regulator is equal to to supply high frequency bypassing, since the impedance the output power divided by the input power times 100%. to the supply is very low. A 10μF ceramic capacitor is It is often useful to analyze individual losses to determine usually enough for these conditions. what is limiting the effi ciency and which change would produce the most improvement. Percent effi ciency can
Checking Transient Response
be expressed as: The regulator loop response can be checked by look- % Effi ciency = 100% – (L1 + L2 + L3 + ...) ing at the load transient response. Switching regulators take several cycles to respond to a step in load current. where L1, L2, etc. are the individual losses as a percent- When a load step occurs, VOUT immediately shifts by an age of input power. amount equal to ΔILOAD • ESR, where ESR is the effective Although all dissipative elements in the circuit produce series resistance of COUT. ΔILOAD also begins to charge losses, four main sources usually account for most of or discharge COUT, generating a feedback error signal the losses in LTC3548-1 circuits: 1) V used by the regulator to return V IN quiescent current, OUT to its steady-state 2) switching losses, 3) I2R losses, 4) other losses. value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability 1. The VIN current is the DC supply current given in the problem. Electrical Characteristics which excludes MOSFET driver and control currents. V The initial output voltage step may not be within the band- IN current results in a small (<0.1%) loss that increases with V width of the feedback loop, so the standard second-order IN, even at no load. overshoot/DC ratio cannot be used to determine phase Hot Swap is a trademark of Linear Technology Corporation. 35481fc 10