LTC3413 datasheet(13/20 Pages) LINER

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LTC3617

3617fa

applications inForMation

Since the ESR of a ceramic capacitor is so low, the input

and output capacitor must instead fulfill a charge storage

requirement.Duringaloadstep,theoutputcapacitormust

instantaneously supply the current until the feedback loop

raises the switch current enough to support the load. The

timerequiredforthefeedbacklooptorespondisdependent

on the compensation components and the output capaci-

tor size. Typically, 3 to 4 switching cycles are required to

respond to a load step, but only in the first cycle does the

output drop linearly. The output droop, VDROOP, is usually

about2to4timesthelineardropofthefirstcycle;however,

this behavior can vary depending on the compensation

component values. Thus, a good place to start is with the

output capacitor size of approximately:

COUT ≈

3.5 •

∆IOUT

fSW • VDROOP

This is only an approximation; more capacitance may

be needed depending on the duty cycle and load step

requirements.

Inmostapplications,theinputcapacitorismerelyrequired

to supply high frequency bypassing, since the impedance

to the supply is very low.

Output Voltage Programming

In most applications, VOUT is connected directly to VFB.

The output voltage will be equal to one-half of the voltage

on the VDDQIN pin for this case.

VOUT =

VDDQIN

If a different output relationship is desired, an external

resistor divider from VOUT to VFB can be used. The output

voltagewillthenbesetaccordingtothefollowingequation:

VOUT =

VDDQIN

• 1









Internal and External Compensation

The regulator loop response can be checked by looking at

the load current transient response. Switching regulators

take several cycles to respond to a step in DC load current.

When a load step occurs, VOUT shifts by an amount equal

∆ILOAD • ESR, where ESR is the effective series resis-

tance of COUT. ∆ILOAD also begins to charge or discharge

COUT, generating the feedback error signal that forces the

regulator to adapt to the current change and return VOUTto

its steady-state value. During this recovery time VOUT can

be monitored for excessive overshoot or ringing, which

would indicate a stability problem. The availability of the

ITH pin allows the transient response to be optimized over

a wide range of output capacitance.

The ITH external components (RC and CC) shown in Fig-

ure 1 provide adequate compensation as a starting point

for most applications. The values can be modified slightly

to optimize transient response once the final PCB layout

is done and the particular output capacitor type and value

have been determined. The output capacitors need to be

selected because the various types and values determine

the loop gain and phase. The gain of the loop will be in-

creased by increasing RC and the bandwidth of the loop

will be increased by decreasing CC. If RC is increased by

the same factor that CC is decreased, the zero frequency

will be kept the same, thereby keeping the phase shift the

same in the most critical frequency range of the feedback

loop. The output voltage settling behavior is related to the

stability of the closed-loop system. The external capaci-

tor, CC1, (Figure 1) is not needed for loop stability, but it

helps filter out any high frequency noise that may couple

onto that node.

LTC3617

VFB

VOUT

SGND

3617 F03

Figure 3. Setting the Output Voltage

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