Rainbow-electronics MAX1870A Manual do Utilizador descarregar pdf (Página 21)

MAX1870A

Step-Up/Step-Down

Li+ Battery Charger

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STATE B

STATE A

PRECALCULATED STEP-DOWN ON-TIME

VALLEY REGULATED OFF-TIME

- V

OUT

> 1.4 x V

BATT

DUTY = V

/ V

OUT

Figure 6. MAX1870A Step-Down Inductor Current Waveform

signal based on the integrated error of the input cur-

rent, charge current, and battery voltage. The error sig-

nal, determined by the lowest voltage clamp (LVC),

sets the threshold for current-mode regulation. The fol-

lowing comparators are used for regulation:

IMIN: The MAX1870A operates in discontinuous con-

duction if LVC is below 0.15V, and does not initiate

another step-down on-time. In discontinuous step-up

conduction, the peak current is set by IMIN. The peak

inductor current in discontinuous step-up mode is:

where V

IMIN

is the IMIN comparator threshold, 0.15V,

and A

CSI

is the charge current-sense amplifier gain,

18V/V.

CCMP: CCMP compares the current-mode control

AxRS

IMIN

CSI

STATE B

STATE C

PRECALCULATED OFF-TIME

PEAK REGULATED ON-TIME

- V

OUT

> 0.9 x V

BATT

DUTY = 1 - V

/ V

OUT

Figure 7. Step-Up Inductor-Current Waveform

1 2 ... 16 17 18 19 20 21 22 23 24 25 26 ... 31 32

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