MAX745 Maxim, MAX745 Datasheet - Page 7

• Manufacturer Part Number: MAX745
• Description: Switch-Mode Lithium-Ion Battery-Charger
• Manufacturer: Maxim

To set currents below full scale without changing
R1, adjust the voltage at SETI according to the follow-
ing formula:
A capacitor at CCI sets the current-feedback loop’s
dominant pole. While the current is in regulation, CCV
voltage is clamped to within 80mV of the CCI voltage.
This prevents the battery voltage from overshooting
when the voltage setting is changed. The converse is
true when the voltage is in regulation and the current
setting is changed. Since the linear range of CCI or
CCV is about 2V (1.5V to 3.5V), the 80mV clamp results
in negligible overshoot when the loop switches from
voltage regulation to current regulation, or vice versa.
The battery-charging current can be externally moni-
tored by placing a scaling resistor (R
IBAT and GND. IBAT is the output of a voltage-con-
trolled current source, with output current given by:
where V
resistor (in millivolts) given by:
The voltage across R
Figure 2. Functional Diagram
V
SENSE
SENSE
V
I
IBAT
I
is the voltage across the current-sense
BAT
= V
CHG
_______________________________________________________________________________________
CELL0
CELL1
CS
VADJ
BATT
SETI
CCV
= 0.9µA/V
CCI
CS
=
= I
IBAT
Monitoring Charge Current
- V
FS
0.9 A
I
CHG
BATT
is then given by:
(V
1 / 4
SETI
LOGIC
SENSE
CURRENT
CELL
= I
SENSE
x
A
V
ON
= 6
/ V
R
CHG
IBAT
R
REF
IBAT
1
x R1
)
IBAT
) between
DCIN
5.4V
REG
GMV
GMI
Switch-Mode Lithium-Ion
CLAMP
R
2V for the maximum charging current. Connect IBAT to
GND if unused.
The battery voltage or current is controlled by a
current-mode, PWM DC/DC converter controller. This
controller drives two external N-channel MOSFETs,
which control power from the input source. The con-
troller sets the switched voltage’s pulse width so that it
supplies the desired voltage or current to the battery.
Total component cost is reduced by using a dual,
N-channel MOSFET.
The heart of the PWM controller is a multi-input com-
parator. This comparator sums three input signals to
determine the switched signal’s pulse width, setting the
battery voltage or current. The three signals are the
current-sense amplifier’s output, the GMV or GMI error
amplifier’s output, and a slope-compensation signal
that ensures that the current-control loop is stable.
The PWM comparator compares the current-sense
amplifier’s output to the lower output voltage of either
the GMV or GMI amplifiers (the error voltage). This cur-
rent-mode feedback reduces the effect of the inductor
on the output filter LC formed by the output inductor
(L1) and C1 (Figure 1). This makes stabilizing the cir-
cuit much easier, since the output filter changes to a
first-order RC from a complex, second-order RLC.
IBAT
REF
4.2
must be chosen to limit V
LOGIC
PWM
Battery Charger
VL
REF
2
REF
IBAT
VL
STATUS
BST
DHI
LX
DLO
PGND
THM/SHDN
GND
PWM Controller
to voltages below
7