LTC3407A

11

3407afa

APPLICATIONS INFORMATION

shoot 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

phase margin. In addition, a feed-forward capacitor can be

added to improve the high frequency response, as shown

in Figure 1. Capacitors C1 and C2 provide phase lead by

creating high frequency zeros with R2 and R4 respectively,

which improve 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.

Soft-Start

The RUN/SS pins provide a means to separately run or shut

down the two regulators. In addition, they can optionally be

used to externally control the rate at which each regulator

starts up and shuts down. Pulling the RUN/SS1 pin below

1V shuts down regulator 1 on the LTC3407A. Forcing this

at which each regulator turns on and off, connect a resistor

and capacitor to the RUN/SS pins as shown in Figure 1.

The soft-start duration can be calculated by using the

following formula:

(s)

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, 4 main sources usually account for most of the

Electrical Characteristics which excludes MOSFET

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

top and bottom MOSFET switches. The gate charge

will be more pronounced at higher supply voltages.

In continuous mode, the average output current 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 top and

follows:

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