Datasheet

MAX1870A
Step-Up/Step-Down
Li+ Battery Charger
22 ______________________________________________________________________________________
• CCMP: CCMP compares the current-mode control
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 inductor 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
inductor 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.
STATE C
STATE B
STATE A
STATE B
STATE A
STATE B
STATE C
STATE B
A)
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
dl
dt
V
BATT
- V
IN
L
=
dl
dt
V
IN
L
=
dl
dt
V
BATT
L
=
Figure 8. MAX1870A Step-Up/Step-Down Inductor-Current Waveform