Rainbow-electronics MAX1870A Manual de usuario descargar pdf (Pagina 22)

MAX1870A

Step-Up/Step-Down

Li+ Battery Charger

22 ______________________________________________________________________________________

point, LVC, to the inductor current. In step-down mode,

the off-time (state A) is terminated when the inductor

current falls below the current threshold set by LVC. In

step-up mode, the on-time (state C) is terminated when

the inductor current rises above the current threshold

set by LVC.

IMAX: The IMAX comparators provide a cycle-by-cycle

inductor current limit. This circuit compares the induc-

tor current (CSI in step-down mode or CSS in step-up

mode) to the internally fixed cycle-by-cycle current

limit. The current-sense voltage limit is 200mV. With

RS1_ = RS2 = 30mΩ, which corresponds to 6.7A. If the

inductor current-sense voltage is greater than V

IMAX

(200mV), a step-up on-time is terminated or a step-

down on-time is not permitted.

ZCMP: The ZCMP comparator detects when the induc-

tor current crosses zero. If the ZCMP output goes high

during a step-down off-time, the MAX1870A switches to

the idle state (state D) to conserve power.

Switching Frequency

The MAX1870A includes input and output-voltage feed-

forward to maintain pseudo-fixed-frequency (400kHz)

operation. The time in state B is set according to the

input voltage, output voltage, and a time constant. In

step-up/step-down mode the switching frequency is

STATE C

STATE B

STATE A

STATE B

STATE A

STATE B

STATE C

STATE B

MINIMUM

STEP-DOWN

OFF-TIME

PRECALCULATED STEP-UP

OFF-TIME

PRECALCULATED STEP-DOWN

ON-TIME

PRECALCULATED STEP-DOWN

ON-TIME

MINIMUM

STEP-UP

ON-TIME

PEAK REGULATED

STEP-UP

ON-TIME

VALLEY REGULATED

STEP-DOWN

OFF-TIME

BATT

- V

BATT

Figure 8. MAX1870A Step-Up/Step-Down Inductor-Current Waveform

1 2 ... 17 18 19 20 21 22 23 24 25 26 27 ... 31 32

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