Rainbow Electronics MAX8730 - Manual (Page 16)

MAX8730
Low-Cost Battery Charger
16 ______________________________________________________________________________________
Detailed Description
The MAX8730 includes all the functions necessary to
charge Li+, NiMH, and NiCd batteries. A high-efficien-
cy, step-down, DC-DC converter is used to implement
a precision constant-current, constant-voltage charger.
The DC-DC converter drives a p-channel MOSFET and
uses an external free-wheeling Schottky diode. The
charge current and input current-sense amplifiers have
low-input offset errors, allowing the use of small-value
sense resistors for reduced power dissipation. Figure 2
is the functional diagram.
The MAX8730 features a voltage-regulation loop (CCV)
and two current-regulation loops (CCI and CCS). The
loops operate independently of each other. The CCV
voltage-regulation loop monitors BATT to ensure that its
voltage never exceeds the voltage set by VCTL. The
CCI battery current-regulation loop monitors current
delivered to BATT to ensure that it never exceeds the
current limit set by ICTL. The charge-current-regulation
loop is in control as long as the battery voltage is below
the set point. When the battery voltage reaches its set
point, the voltage-regulation loop takes control and
maintains the battery voltage at the set point. A third
loop (CCS) takes control and reduces the charge cur-
rent when the adapter current exceeds the input cur-
rent limit set by CLS.
The ICTL, VCTL, and CLS analog inputs set the charge
current, charge voltage, and input-current limit, respec-
tively. For standard applications, default set points for
VCTL provide 4.2V per-cell charge voltage. The MODE
input selects a 3- or 4-cell mode.
Based on the presence or absence of the AC adapter,
the MAX8730 provides an open-drain logic output sig-
nal (ACOK) and connects the appropriate source to the
system. P-channel MOSFETs controlled from the PDL
and PDS select the appropriate power source. The
MODE input allows the system to perform a battery
relearning cycle. During a relearning cycle, the battery
is isolated from the charger and completely discharged
through the system load. When the battery reaches
100% depth of discharge, PDL turns off and PDS turns
on to connect the adapter to the system and to allow the
battery to be recharged to full capacity.
Setting Charge Voltage
The VCTL input adjusts the battery output voltage, V BATT .
This voltage is calculated by the following equation:
where CELLS is the number of cells selected with the
MODE input (see Table 1). Connect MODE to LDO for 4-
cell operation. Float the MODE input for 3-cell operation.
The battery-voltage accuracy depends on the absolute
value of VCTL, and the accuracy of the resistive volt-
age-divider that sets VCTL. Calculate the battery volt-
age accuracy according to the following equation:
where E
0
is the worst-case MAX8730 battery voltage
error when using 1% resistors (0.83%), I VCTL is the
VCTL input bias current (4µA), and R VCTL is the imped-
ance at VCTL. Connect VCTL to LDO for the default
setting of 4.20V/cell with 0.7% accuracy.
Connect MODE to GND to enter relearn mode, which
allows the battery to discharge into the system while
the adapter is present; see the Relearn Mode Section.
Setting Charge Current
ICTL sets the maximum voltage across current-sense
resistor RS2, which determines the charge current. The
full-scale differential voltage between CSIP and CSIN is
135mV (4.5A for RS2 = 30m
Ω). Set ICTL according to
the following equation:
The input range for ICTL is 0 to 3.6V. To shut down the
charger, pull ICTL below 65mV. Choose a current-sense
resistor (RS2) to have a sufficient power rating to handle
the full-charge current. The current-sense voltage may
be reduced to minimize the power dissipation. However,
this can degrade accuracy due to the current-sense
amplifier’s input offset (±2V). See the Typical Operating
Characteristics to estimate the charge-current accuracy
at various set points. The charge-current error amplifier
(GMI) is compensated at the CCI pin. See the
Compensation section.
V I x RS x
V
mV
ICTL CHG
.
=
2
3 6
135
V E x
I x R
BATT ERROR
VCTL VCTL
_
%
= + −

 

0
100
36
1
V CELLS x V
V
BATT
VCTL
( ) = + 4
9
Table 1. Cell-Count Programming
CELLS CELL COUNT
GND Relearn mode
Float 3
LDO 4