Max1870a step-up/step-down li+ battery charger – Rainbow Electronics MAX1870A User Manual
Page 21
MAX1870A
Step-Up/Step-Down
Li+ Battery Charger
______________________________________________________________________________________
21
STATE B
STATE A
PRECALCULATED STEP-DOWN ON-TIME
VALLEY REGULATED OFF-TIME
dl
dt
V
IN
- V
OUT
L
=
dl
dt
V
OUT
L
=
V
IN
> 1.4 x V
BATT
DUTY = V
IN
/ 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
I
V
A
x RS
PK
IMIN
CSI
>
2
STATE B
STATE C
PRECALCULATED OFF-TIME
PEAK REGULATED ON-TIME
dl
dt
V
IN
- V
OUT
L
=
dl
dt
V
OUT
L
=
V
IN
> 0.9 x V
BATT
DUTY = 1 - V
IN
/ V
OUT
Figure 7. Step-Up Inductor-Current Waveform