Rainbow-electronics MAX8730 Manual de usuario descargar pdf (Pagina 23)

MAX8730

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CCS Loop Compensation

The simplified schematic in Figure 8 is sufficient to

describe the operation of the MAX8730 when the input

current-limit loop (CCS) is in control. Since the output

capacitor’s impedance has little effect on the response

of the input current-limit loop, only a single pole is

required to compensate this loop. A

CSS

is the internal

gain of the current-sense amplifier, RS1 = 10mΩ in the

typical application circuits. R

OGMS

is the equivalent

output impedance of the GMS amplifier, which is

greater than 10MΩ. GMS is the charge-current amplifier

transconductance = 1µA/mV. GM

is the DC-DC con-

verter’s input-referred transconductance = GM

OUT

/D =

2.22A/V/D.

The loop-transfer function is given by:

the loop-transfer function simplifies to:

The crossover frequency is given by:

For stability, choose a crossover frequency lower than

1/10 of the switching frequency:

Values for CCS greater than 10 times the minimum

value may slow down the current-loop response exces-

sively. Figure 9 shows the Bode plot of the input cur-

rent-limit-loop frequency response using the values

calculated above.

GMS

OSC

IN MAX

BATT MIN

= 5

2π

GMS

CO CS

IN MAX

BATT MIN

2π

LTF GMS

SR C

xRS RS

OGMS

OGMS CS

+×1

Since GM

ARS

CSS

LTF GM A RSI GMS

SR C

IN CSS

OGMS

OGMS CS

=×××

+×1

FREQUENCY (Hz)

MAGNITUDE (dB)

100k1k10

-20

100

-40

-45

-90

0.1

MAG

PHASE

Figure 7. CCI Loop Response

OGMS

GMS

CSS

CLS

CCS

CSSP

RS1

CSSI

SYSTEM

LOAD

ADAPTER

INPUT

Figure 8. CCI Loop Diagram

FREQUENCY (Hz)

MAGNITUDE (dB)

100k 10M1k10

-20

100

-40

-45

-90

0.1

MAG

PHASE

PHASE (DEGREES)

Figure 9. CCS Loop Response

1 2 ... 18 19 20 21 22 23 24 25 26 27 28 29

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