MCP73123_11 Datasheet, PDF (22/34 Pages) Microchip Technology – Lithium Iron Phosphate (LiFePO4) Battery Charge Management Controller with Input Overvoltage Protection

MCP73123/223
6.1 Application Circuit Design
Due to the low efficiency of linear charging, the most
important factors are thermal design and cost, which
are a direct function of the input voltage, output current
and thermal impedance between the battery charger
and the ambient cooling air. The worst-case situation is
when the device has transitioned from the
Preconditioning mode to the Constant Current mode. In
this situation, the battery charger has to dissipate the
maximum power. A trade-off must be made between
the charge current, cost, and thermal requirements of
the charger.
6.1.1 COMPONENT SELECTION
Selection of the external components in Figure 6-1 is
crucial to the integrity and reliability of the charging
system. The following discussion is intended as a guide
for the component selection process.
6.1.1.1 Charge Current
The recommended fast charge current should be
obtained from the battery manufacturer. For exam-
ple, a 1000 mAh battery pack with 2C preferred fast
charge current has a charge current of 1000 mA.
Charging at this rate provides the shortest charge cycle
times without degradation of the battery pack
performance or life.
Note:
Please consult with your battery supplier
or refer to the battery data sheet for the
preferred charge rate.
6.1.1.2 Thermal Considerations
The worst-case power dissipation in the battery
charger occurs when the input voltage is at the
maximum and the device has transitioned from the
Preconditioning mode to the Constant Current mode.
In this case, the power dissipation is calculated using
Equation 6-1.
EQUATION 6-1:
PowerDissipation = VDDMAX – VPTHMIN  IREGMAX
Where:
V_DDMAX = the maximum input voltage
I_REGMAX = the maximum fast charge current
V_PTHMIN = the minimum transition threshold
voltage
Power dissipation with a 5V, ±10% input voltage
source, 500 mA ±10% and preconditioning threshold
voltage at 2V is calculated using Equation 6-2.
EQUATION 6-2:
PowerDissipation = 5.5V – 2V  550mA = 1.925W
This power dissipation with the battery charger in the
DFN-10 package will raise the temperature
approximately 83C above room temperature.
6.1.1.3 External Capacitors
The MCP73123/223 is stable with or without a battery
load. In order to maintain good AC stability in the
Constant Voltage mode, a minimum capacitance of
1 µF is recommended to bypass the V_BAT pin to V_SS.
This capacitance provides compensation when there is
no battery load. In addition, the battery and
interconnections appear inductive at high frequencies.
These elements are in the control feedback loop during
Constant Voltage mode. Therefore, the bypass
capacitance may be necessary to compensate for the
inductive nature of the battery pack.
A minimum of 16V rated 1 µF is recommended for the
output capacitor, and a minimum of 25V rated 1 µF is
recommended for the input capacitor in typical applica-
tions.
TABLE 6-1:
MLCC
Capacitors
X7R
X5R
MLCC CAPACITOR EXAMPLE
Temperature
Range
Tolerance
-55C to +125C
-55C to +85C
±15%
±15%
Virtually any good quality output filter capacitor can be
used, independent of the capacitor’s minimum
Effective Series Resistance (ESR) value. The actual
value of the capacitor (and its associated ESR)
depends on the output load current. A 1 µF ceramic,
tantalum, or aluminum electrolytic capacitor at the
output is usually sufficient to ensure stability.
6.1.1.4 Reverse-Blocking Protection
The MCP73123/223 provides protection from a faulted
or shorted input. Without the protection, a faulted or
shorted input would discharge the battery pack through
the body diode of the internal pass transistor.
DS22191C-page 22
 2011 Microchip Technology Inc.