LTC3124 datasheet(24/36 Pages) AD

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LT3120

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

bandwidth may be realizable while still maintaining stabil-

ity and good transient response. In this example, we will

use cross over frequency equal to one sixth of the RHPZ

frequency (see Equation 28).

fCC =

fRHPZ

≅6.2kHz

(28)

The RHPZ will have a negligible effect on the gain at the

loop crossover, however it will have a phase contribution

that must be considered (see Equation 29).

ϕRHPZ = –1• tan–1

fcc

fRHPZ

⎛

⎝⎜

⎞

⎠⎟

= –1• tan–1

⎛

⎝⎜

⎞

⎠⎟ =

–8.1

° (29)

Since the converter will be operating in boost mode, the

GCS term must be scaled to represent the commanded

output current.

LookingintheTypicalCurvessection,wefindtheefficiency

to be roughly 80%. Using this information, the effective

output current gain can be calculated using Equation 30.

GCS(OUT) = GCS •

VIN

2VOUT

•Eff

⎛

⎝⎜

⎞

⎠⎟

= 3.265A V

(30)

Using this information, the gain and phase contributions

from the output filter are calculated using Equation 31.

GOUT =GCS(OUT) •

RLOAD2

fCC

fP1

⎛

⎝

⎞

⎠

= 1.098

ϕP1= –1• tan–1

fcc

fP1

⎛

⎝⎜

⎞

⎠⎟

= –1• tan–1

6200

1273

⎛

⎝⎜

⎞

⎠⎟ =

–78.4°

(31)

Choosing a phase margin of 50 degrees, the required

phaseboostfromthecompensationnetworkisdetermined

by summing together the phase contributions that were

calculated in Equation 31. A phase contribution of –90°

is assumed for P2.

ϕZ1= 50–ϕP2 –ϕP1–ϕRHPZ –180 = 46.5°

(32)

The compensation network gain is used to adjust the

loop gain to crossover at the desired frequency. Using the

feedback divider gain and output gain, the compensation

network gain is calculated using Equation 33

GCOMP =

VREF

VOUT

•GOUT

⎛

⎝⎜

⎞

⎠⎟

−1

= 5.727

(33)

The compensation network resistor is then found using

the error amplifier transconductance and the required

compensation gain found in Equation 33.

RZ =

GCOMP

5.727

120µs

= 47.7kΩ

(34)

With the value of RZ now known, the compensation ca-

pacitor can be chosen to place the zero Z1 in the correct

location.

CP1=

tan

ϕZ1

( )

π • fCC •RZ

= 565pF

(35)

Selecting standard value components, values of RZ =

47.5kΩ and CP1 = 560pF are used.

PCB Layout Considerations

The LT3120 buck-boost converter switches large currents

at high frequencies. Special attention should be paid to the

PC board layout to ensure a stable, noise-free and efficient

application circuit. Figure 11 shows a representative PCB

to outline some of the primary considerations. A few key

guidelines are provided below.

1. The parasitic inductance and resistance of all circulat-

ing high current paths should be minimized. This can

be accomplished by keeping the routes to all bold com-

ponents in Figures 11 as short and as wide as possible.

Capacitor ground connections should via down to the

ground plane by way of the shortest route possible. The

bypass capacitors on PVIN, PVOUT and VCC should be

placed as close to the IC as possible and should have

the shortest possible paths to ground.

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