LTC3419

11

3419f

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

or ringing that would indicate a stability problem.

The initial output voltage step may not be within the

bandwidth of the feedback loop, so the standard second

order overshoot/DC ratio cannot be used to determine the

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 capacitors.

The discharged input capacitors are effectively put in paral-

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 incorporates cur-

rent limiting, short-circuit protection, and soft-starting.

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Percent efficiency can

be expressed as:

% Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc., are the individual losses as a percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, four sources usually account for the losses in

Electrical Characteristics which excludes MOSFET

no load.

2. The switching current is the sum of the MOSFET driver

and control currents. The MOSFET driver current re-

sults from switching the gate capacitance of the power

MOSFETs. Each time a MOSFET gate is switched from

low to high to low again, a packet of charge dQ moves

bottom MOSFET switches. The gate charge losses are

pronounced at higher supply voltages.

flows through inductor L, but is “chopped” between

the internal top and bottom switches. Thus, the series

resistance looking into the SW pin is a function of both

(DC) as follows:

APPLICATIONS INFORMATION

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