MAX1758
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 28V Switch
16 ______________________________________________________________________________________
dissipation ratings more critical when using tantalum
capacitors.
The output filter capacitor is used to absorb the induc-
tor ripple current. The output capacitor impedance
must be significantly less than that of the battery to
ensure that it will absorb the ripple current. Both the
capacitance and ESR rating of the capacitor are impor-
tant for its effectiveness as a filter and to ensure stabili-
ty of the PWM circuit. The minimum output capacitance
for stability is:
where:
C
OUT
is the total output capacitance.
V
REF
is the reference voltage (4.2V).
V
BATT
is the maximum battery regulation voltage
(typically 4.2V per cell).
V
DCIN (MIN)
is the minimum source input voltage.
The maximum output capacitor ESR required for stabili-
ty is:
where:
R
ESR
is the output capacitor ESR.
R
CS
is the current-sense resistor from CS to BATT
(100mΩ typ).
Setting the Timers
The MAX1758 contains four timers: a prequalification
timer, fast-charge timer, full-charge timer, and top-off
timer. Connecting a capacitor from TIMER1 to GND
and TIMER2 to GND sets the timer periods. The
TIMER1 input controls the prequalification, full-charge,
and top-off times, while TIMER2 controls the fast-
charge timeout. The typical timeouts for a 1C charge
rate are set to 7.5 minutes for the prequalification timer,
90 minutes for the fast-charge timer, 90 minutes for the
full-charge timer, and 45 minutes for the top-off timer by
connecting 1nF capacitors to TIMER1 and TIMER2.
Each timer period is directly proportional to the capaci-
tance at the corresponding pin (see Typical Operating
Characteristics).
Compensation
Each of the three regulation loops—the input current
limit, the charging current limit, and the charging volt-
age limit—can be compensated separately at the CCS,
CCI, and CCV pins, respectively.
The charge-current loop error amp output is brought
out at CCI. Likewise, the source-current error amplifier
output is brought out at CCS. The current loops in most
charger designs can be compensated by 0.1µF capac-
itors to ground at CCI and CCS. Raising the value of
these capacitors reduces the bandwidth of these loops.
The voltage-regulating loop error amp output is brought
out at CCV. Compensate this loop by connecting a
capacitor in parallel with a series resistor-capacitor (RC)
from CCV to GND. Recommended values are shown in
Figure 1.
Applications Information
Diode Selection
A Schottky rectifier with a rating of at least 1.5A must
be connected from LX to PGND.
VL and REF Bypassing
The MAX1758 uses an internal linear regulator to drop
the input voltage down to 5.4V, which powers the inter-
nal circuitry. The output of the linear regulator is the VL
pin. The internal linear regulator may also be used to
power external circuitry as long as the maximum cur-
rent of the linear regulator is not exceeded.
A 4.7µF bypass capacitor is required at VL to ensure that
the regulator is stable. A 1µF bypass capacitor is also
required between REF and GND to ensure that the inter-
nal 4.2V reference is stable. In both cases, use a low-ESR
ceramic capacitor.
Chip Information
TRANSISTOR COUNT: 5996
