LTC3111 datasheet(20/32 Pages) LINER

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LTC3111

3111fa

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APPLICATIONS INFORMATION

Compensation Of The Voltage Loop

The small-signal models of the LTC3111 reveal that the

transfer function from the error amplifier output, COMP,

to the output voltage is characterized by a set of resonant

poles and a possible zero generated by the ESR of the

output capacitor as shown in the Bode plot of Figure 5.

In boost mode operation, there is an additional right-half-

plane zero that produces phase lag and increasing gain at

higher frequencies. Typically, the compensation network

is designed to ensure that the loop crossover frequency is

low enough that the phase loss from the right-half-plane

zero is minimized. The low frequency gain in buck mode

is a constant, but varies with both VIN and VOUT in boost

mode.

For charging or other applications that do not require an

optimized output voltage transient response, a simple

Type I compensation network as shown in Figure 6 can

be used to stabilize the voltage loop. To ensure sufficient

phase margin, the gain of the error amplifier must be low

enoughthattheresultantcrossoverfrequencyofthecontrol

loop is well below the resonant frequency.

In most applications, the low bandwidth of the Type I com-

pensatedloopwillnotprovidesufficienttransientresponse

performance. To obtain a wider bandwidth feedback loop,

optimize the transient response, and minimize the size of

the output capacitor, a Type III compensation network as

shown in Figure 7 is required.

A Bode plot of the typical Type III compensation network

is shown in Figure 8. The Type III compensation network

provides a pole near the origin which produces a very high

loop gain at DC to minimize any steady-state error in the

regulation voltage. Two zeros located at fZERO1 and fZERO2

provide sufficient phase boost to allow the loop crossover

frequency to be set above the resonant frequency, fO, of

the power stage. The Type III compensation network also

introduces a second and third pole. The second pole, at

frequency fPOLE2, reduces the error amplifier gain to a

zero slope to prevent the loop crossover from extending

0.8V

3111 F06

LTC3111

VOUT

COMP

SGND

–

Figure 6: Error Amplifier with Type I Compensation

0.8V

3111 F07

LTC3111

CFB

CFF

RFF

CPOLE

RFB

VOUT

COMP

SGND

–

Figure 7: Error Amplifier with Type III Compensation

Figure 5: Buck-Boost Converter Bode Plot

GAIN

PHASE

BOOST MODE

BUCK MODE

–20dB/DEC

–40dB/DEC

3111 F05

fRHPZ

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