Rainbow-electronics MAX8730 Bedienungsanleitung PDF herunterladen (Seite 22)

MAX8730

Low-Cost Battery Charger

22 ______________________________________________________________________________________

where:

BATT

= 16.8V

GMV = 0.125µA/mV

OUT

= 2.22A/V

OUT

= 10µF

OSC

= 350kHz (minimum occurs at V

= 19V and

BATT

= 16.8V)

= 0.2Ω

CO-CV

= 45kHz

To ensure that the compensation zero adequately can-

cels the output pole, select f

Z_CV

≤ f

P_OUT

≥ (R

/ R

) C

OUT

≥ 200pF

Figure 5 shows the Bode plot of the voltage-loop fre-

quency response using the values calculated above.

CCI Loop Compensation

The simplified schematic in Figure 6 is sufficient to

describe the operation of the MAX8730 when the bat-

tery current loop (CCI) is in control. Since the output

capacitor’s impedance has little effect on the response

of the current loop, only a simple single pole is required

to compensate this loop. A

CSI

is the internal gain of the

current-sense amplifier. RS2 is the charge-current-

sense resistor (30mΩ). R

OGMI

is the equivalent output

impedance of the GMI amplifier, which is greater than

10MΩ. GMI is the charge-current amplifier transcon-

ductance = 1µA/mV. GM

OUT

is the DC-DC converter

transconductance = 2.22A/V.

The loop transfer function is given by:

that describes a single-pole system. Since:

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 C

greater than 10 times the minimum value

may slow down the current-loop response. Choosing

= 10nF yields a crossover frequency of 15.9kHz.

Figure 7 shows the Bode plot of the current-loop fre-

quency response using the values calculated above.

x GMI

GMI

CO CI

2π

LTF GMI

sR C

OGMI

OGMI CI

×1

ARS

OUT

CSI

LTF GM A RS GMI

sR C

OUT CSI

OGMI

OGMI CI

=×××

×1

FREQUENCY (Hz)

MAGNITUDE (dB)

PHASE (DEGREES)

100k10k1k100101

-20

-40

-90

-45

-135

0.1 1M

MAG

PHASE

Figure 5. CCV Loop Response

OGMI

CCI

GMI

CSI

ICTL

OUT

CSIP

RS2

CSIN

Figure 6. CCI Loop Diagram

1 2 ... 17 18 19 20 21 22 23 24 25 26 27 28 29

Low-Cost Battery Charger 1

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