LTC3813 Datasheet, PDF(24/32 Page) Linear Technology – 100V Current Mode Synchronous Step-Up Controller

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LTC3813 Datasheet, PDF (24/32 Pages) Linear Technology – 100V Current Mode Synchronous Step-Up Controller

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LTC3813

APPLICATIONS INFORMATION

the appropriate parasitic values are known, simulated or

generated from the modulator transfer function. Mea-

surement will give more accurate results, but simulation

or transfer function can often get close enough to give

a working system. To measure the modulator gain and

phase directly, wire up a breadboard with an LTC3813

and the actual MOSFETs, inductor and input and output

capacitors that the ï¬nal design will use. This breadboard

should use appropriate construction techniques for high

speed analog circuitry: bypass capacitors located close

to the LTC3813, no long wires connecting components,

appropriately sized ground returns, etc. Wire the feedback

ampliï¬er with a 0.1Î¼F feedback capacitor from ITH to FB

and a 10k to 100k resistor from VOUT to FB. Choose the

bias resistor (RB) as required to set the desired output

voltage. Disconnect RB from ground and connect it to

a signal generator or to the source output of a network

analyzer to inject a test signal into the loop. Measure the

gain and phase from the ITH pin to the output node at the

positive terminal of the output capacitor. Make sure the

analyzerâs input is AC coupled so that the DC voltages

present at both the ITH and VOUT nodes do not corrupt

the measurements or damage the analyzer.

If breadboard measurement is not practical, mathemat-

ical software such as MATHCAD or MATLAB can be used

to generate plots from the transfer function given in

equation 1. A SPICE simulation can also be used to gener-

ate approximate gain/phase curves. Plug the expected

capacitor, inductor and MOSFET values into the following

SPICE deck and generate an AC plot of VOUT/ VITH with gain

in dB and phase in degrees. Refer to your SPICE manual

for details of how to generate this plot.

*This ï¬le simulates a simpliï¬ed model of

the LTC3813 for generating a v(out)/(vith)

or a v(out)/v(outin) bode plot

.param vout=24

.param vin=12

.param L=10u

.param cout=270u

.param esr=.018

.param rload=24

.param rdson=0.02

.param Vrng=1

.param vsnsmax={0.173*Vrng-0.026}

.param K={vsnsmax/rdson/1.2}

.param wz={1/esr/cout}

.param wp={2/rload/cout}

* Feedback Ampliï¬er

rfb1 outin vfb 29k

rfb2 vfb 0 1k

eithx ithx 0 laplace {0.8-v(vfb)} =

{1/(1+s/1000)}

eith ith 0 value={limit(1e6*v(ithx),0,2.4)}

cc1 ith vfb 100p

cc2 ith x1 0.01Î¼

rc x1 vfb 100k

* Modulator/Output Stage

eout out 0 laplace {v(ith)} =

{0.5*K*Rload*vin/vout *(1+s/wz)/(1+s/wp)

*(1-s*L/Rload*vout*vout/vin/vin)}

rload out 0 {rload}

vstim out outin dc=0 ac=10m; ac stimulus

.ac dec 100 10 10meg

.probe

.end

With the gain/phase plot in hand, a loop crossover fre-

quency can be chosen. Usually the curves look something

like Figure 10. Choose the crossover frequency about 25%

of the switching frequency for maximum bandwidth. Al-

though it may be tempting to go beyond fSW/4, remember

that signiï¬cant phase shift occurs at half the switching

frequency that isnât modeled in the above H(s) equation

and PSPICE code. Note the gain (GAIN, in dB) and phase

(PHASE, in degrees) at this point. The desired feedback

ampliï¬er gain will be âGAIN to make the loop gain at 0dB

at this frequency. Now calculate the needed phase boost,

assuming 60Â° as a target phase margin:

BOOST = â (PHASE + 30Â°)

If the required BOOST is less than 60Â°, a Type 2 loop can

be used successfully, saving two external components.

BOOST values greater than 60Â° usually require Type 3

loops for satisfactory performance.

3813fb

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