MAX1873 Datasheet, PDF (12/14 Pages) Maxim Integrated Products – Simple Current-Limited Switch-Mode Li Charger Controller

Simple Current-Limited Switch-Mode
Li+ Charger Controller
I_PEAK = I_CHG(1+ LIR/2₎
For example, for a 4-cell charging current of 3A, a
V_DCIN(MAX) of 24V, and an LIR of 0.5, L is calculated to
be 11.2µH with a peak current of 3.75A. Therefore a
10µH inductor would be satisfactory.
MOSFET Selection
The MAX1873 uses a P-channel power MOSFET
switch. The MOSFET must be selected to meet the effi-
ciency or power dissipation requirements of the charg-
ing circuit as well as the maximum temperature of the
MOSFET. Characteristics that affect MOSFET power
dissipation are drain-source on-resistance (_RDS(ON))
and gate charge. Generally these are inversely propor-
tional.
To determine MOSFET power dissipation, the operating
duty cycle must first be calculated. When the charger is
operating at higher currents, the inductor current will be
continuous (the inductor current will not drop to 0). In
this case, the high-side MOSFET duty cycle (D) can be
approximated by the equation:
D ≈ ^VBATT
V_DCIN
And the catch-diode duty cycle (D') will be 1 - D or:
D'≈ ^{VDCIN − VBATT}
V_DCIN
where V_BATT is the battery-regulation voltage (typically
4.2V per cell) and V_DCIN is the source-input voltage.
For MOSFETs, the worst-case power dissipation due to
on-resistance (P_R) occurs at the maximum duty cycle,
where the operating conditions are minimum source-
voltage and maximum battery voltage. P_R can be
approximated by the equation:
P_R
=
V_BATT(MAX)
V_DCIN(MIN)
× R_DS(ON)
× I_CHG²
Transition losses (P_T) can be approximated by the
equation:
P_T
=
V_DCIN
× I_CHG
3
× f_SW
×
t _TR
where t_TR is the MOSFET transition time and f_SW is the
switching frequency. The total power dissipation of the
MOSFET is then:
P_TOT = P_R + P_T
Diode Selection
A Schottky rectifier with a current rating of at least the
charge current limit must be connected from the MOS-
FET drain to GND. The voltage rating of the diode must
exceed the maximum expected input voltage.
Capacitor Selection
The input capacitor shunts the switching current from
the charger input and prevents that current from circu-
lating through the source, typically an AC wall cube.
Thus the input capacitor must be able to handle the
input RMS current. At high charging currents, the con-
verter will typically operate in continuous conduction. In
this case, the RMS current of the input capacitor can
be approximated with the equation:
I_CIN ≈ I_CHG D − D²
where I_CIN is the input capacitor RMS current, D is the
PWM converter duty cycle (typically V_BATT/V_DCIN), and
I_CHG is the battery-charging current.
The maximum RMS input current occurs at 50% duty
cycle, so the worst-case input-ripple current is 0.5 x
I_CHG. If the input-to-output voltage ratio is such that the
PWM controller will never work at 50% duty cycle, then
the worst-case capacitor current will occur where the
duty cycle is nearest 50%.
The impedance of the input capacitor is critical to pre-
venting AC currents from flowing back into the wall
cube. This requirement varies depending on the wall
cube’s impedance and the requirements of any con-
ducted or radiated EMI specifications that must be met.
Low ESR aluminum electrolytic capacitors may be
used, however, tantalum or high-value ceramic capaci-
tors generally provide better performance.
The output filter capacitor absorbs the inductor-ripple
current. The output-capacitor impedance must be sig-
nificantly less than that of the battery to ensure that it
will absorb the ripple current. Both the capacitance and
the ESR rating of the capacitor are important for its
effectiveness as a filter and to ensure stability of the
PWM circuit. The minimum output capacitance for sta-
bility is:
C_OUT
>

V_{REF }1+

V_BATT
V_DCIN(MIN)


V_BATT × f_SW × R_CSB
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