LTC4449 datasheet(47/108 Pages) LINER

47 / 108 page

LTC3882-1

Rev A

For more information www.analog.com

APPLICATIONS INFORMATION

fC = crossover frequency =

fPWM

fZ1(ERR) = fLC =

2πR2C1

fZ2(RES) =

2π(R1+ R3)C3

fP1(ERR) = fESR =

2πR2(C1//C2)

fP2(RES) = 5fC =

2πR3C3

Required error amplifier gain at frequency fC is:

≈ 40log 1+

fLC

⎛

⎝⎜

⎞

⎠⎟

– 20log 1+

fESR

⎛

⎝⎜

⎞

⎠⎟

– 15.56

Once the value of resistor R1 (function of selected VOUT

range) and pole/zero locations have been decided, the

value of R2, C1, C2, R3 and C3 can be obtained from the

previous equations.

Compensating a switching power supply feedback loop is

a complex task. The applications shown in this data sheet

providetypicalvalues,optimizedforthepowercomponents

shown.Thoughsimilarpowercomponentsshouldsuffice,

substantially changing even one major power component

may degrade performance significantly. Stability also may

depend on circuit board layout. To verify the calculated

component values, all new circuit designs should be

prototyped and tested for stability.

The LTPowerCAD software tool can be used as a guide

through the entire power supply design process, includ-

ing optimization of circuit component values according to

system requirements.

PCB Layout Considerations

To prevent magnetic and electrical field radiation or high

frequency resonant problems and to ensure correct IC

operation, proper layout of the components connected

to the LTC3882-1 is essential. Refer to Figure 24, which

also illustrates current waveforms typically present in the

circuit branches. RSENSE will be replaced with a dead short

ifDCRsensingisused.Formaximumefficiency,theswitch

Theexternalinductor/outputcapacitorcombinationmakes

a more significant contribution to loop behavior. These

componentscausea2ndorderamplituderoll-offthatfilters

the PWM waveform, resulting in the desired DC output

voltage. But the additional 180° phase shift produced by

this filter causes stability issues in the feedback loop and

must be frequency compensated. At higher frequencies,

the reactance of the output capacitor will approach its

ESR, and the roll-off due to the capacitor will stop, leaving

–20dB/decade and 90° of phase shift.

The transfer function of the Type 3 circuit shown in

Figure 22 is given by the following equation:

VCOMP

VOUT

–(1+ sC1R2)[1+ s(R1+ R3)C3]

sR1(C1+ C2)[1+ s(C1//C2)R2](1+ sC3R3)

The RC network across the error amplifier and the feed-

forward components R3 and C3 introduce two pole-zero

pairs to obtain a phase boost at the system unity-gain

(crossover)frequency,fC.Intheory,thezerosandpolesare

placedsymmetricallyaroundfC,andthespreadbetweenthe

zeros and the poles is adjusted to give the desired phase

boostatfC.However,inpractice,ifthecrossoverfrequency

is much higher than the LC double-pole frequency, this

method of frequency compensation normally generates

a phase dip within the unity bandwidth and creates some

concern regarding conditional stability.

If conditional stability is a concern, move the error ampli-

fier zero to a lower frequency to avoid excessive phase

dip. The following equations can be used to compute the

feedback compensation component values:

fLC =

2π LCOUT

fESR =

2πRESRCOUT

choose:

LTC4449 Datasheet(PDF) 47 Page - Linear Technology

LTC4449 Datasheet(HTML) 47 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL