Rainbow-electronics MAX8724 Manual de usuario descargar pdf (Pagina 20)

MAX1908/MAX8724

CCV Loop Definitions

Compensation of the CCV loop depends on the para-

meters and components shown in Figure 5. C

and

are the CCV loop compensation capacitor and

series resistor. R

ESR

is the equivalent series resistance

(ESR) of the charger output capacitor (C

OUT

). R

is the

equivalent charger output load, where R

= V

BATT

CHG

. The equivalent output impedance of the GMV

amplifier, R

OGMV

≥ 10MΩ. The voltage amplifier

transconductance, GMV = 0.125µA/mV. The DC-DC

converter transconductance, GM

OUT

= 3.33A/V:

where A

CSI

= 20, and RS2 is the charging current-

sense resistor in the Typical Application Circuits.

The compensation pole is given by:

The compensation zero is given by:

The output pole is given by:

where R

varies with load according to R

= V

BATT

/ I

CHG.

Output zero due to output capacitor ESR:

The loop transfer function is given by:

Assuming the compensation pole is a very low

frequency, and the output zero is a much higher fre-

quency, the crossover frequency is given by:

To calculate R

and C

values of the circuit of Figure 2:

Cells = 4

OUT

= 22µF

BATT

= 16.8V

CHG

= 2.5A

GMV = 0.125µA/mV

OUT

= 3.33A/V

OGMV

= 10MΩ

f = 400kHz

Choose crossover frequency to be 1/5th the

MAX1908’s 400kHz switching frequency:

Solving yields R

= 26kΩ.

Conservatively set R

= 1kΩ, which sets the crossover

frequency at:

CO_CV

= 3kHz

Choose the output-capacitor ESR such that the output-

capacitor zero is 10 times the crossover frequency:

MHz

Z ESR

ESR OUT

2 412

ESR

CO CV OUT

×× ×

210

024

. Ω

GMV R GM

kHz

CO CV

CV OUT

OUT

××

GMV R GM

CO CV

CV OUT

OUT

××

2π

LTF GM R GMV R

sC R sC R

OUT L OGMV

OUT ESR CV CV

CV OGMV OUT L

=××××

+×

()

+×

()

+×

()

+×

()

Z ESR

ESR OUT

2π

P OUT

L OUT

2π

ZCV

CV CV

2π

PCV

OGMV CV

2π

ARS

OUT

CSI

Low-Cost Multichemistry Battery Chargers

20 ______________________________________________________________________________________

OUT

BATT

CCV

GMV

REF

OGMV

ESR

OUT

Figure 5. CCV Loop Diagram

1 2 ... 15 16 17 18 19 20 21 22 23 24 25 26 27

MAX1908/MAX8724 1
Package Information 27

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