MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 13

• Manufacturer Part Number: MCP6271R
• Description: 170 ?a, 2 Mhz Rail-to-rail Op Amp
• Manufacturer: Microchip Technology Inc.

Charging in this manner replenishes a deeply depleted battery in
roughly 2.5 to 3 hours.
Advanced chargers employ additional safety features. For
example the charge is suspended if the cell temperature is
outside a specified window, typically 0°C to 45°C.
Li-ion Charging – System Considerations
A high-performance charging system is required to recharge any
battery quickly and reliably. The following system parameters
should be considered in order to ensure a reliable, cost-effective
solution.
Input Source
Many applications use very inexpensive wall cubes for the input
supply. The output voltage is highly dependent on the AC input
voltage and the load current being drawn from the wall cube.
In the US, the AC mains input voltage can vary from 90 VRMS to
132 VRMS for a standard wall outlet. Assuming a nominal input
voltage of 120 VRMS, the tolerance is +10%, -25%. The charger
must provide proper regulation to the battery independent of
its input voltage. The input voltage to the charger will scale in
accordance to the AC mains voltage and the charge current:
R
resistance of the primary winding (RP/ a2).
R
the bridge rectifiers. In addition, transformer core loss will slightly
reduce the output voltage.
Applications that charge from a car adapter can experience a
similar problem. The output voltage of car adapter will have a
typical range of 9V to 18V.
Constant Current Charge Rate and Accuracy
The choice of topology for a given application may be determined
by the desired constant current. Many high constant current, or
multiple cell applications rely on a switch-mode charging solution
for improved efficiency and less heat generation.
Linear solutions are desirable in low to moderate fast
charge current applications for their superior size and cost
considerations. However, a linear solution purposely dissipates
excess power in the form of heat.
The tolerance on the constant current charge becomes extremely
important to a linear system. If the regulation tolerance is loose,
pass transistors and other components will need to be oversized
adding size and cost. In addition, if the constant current charge
is low, the complete charge cycle will be extended.
Figure 2: Capacity Loss vs. Undercharge Voltage.
EQ
PTC
is the resistance of the secondary winding plus the reflected
Charging Lithium Batteries
V
is the resistance of the PTC, and VFD is the forward drop of
O
= √ 2 x V
10%
9%
8%
7%
6%
5%
4%
3%
2%
1%
0%
0.0%
IN
x a - 1o (R
0.2%
Percent Undercharge(%)
0.4%
EQ
+ R
0.6%
PTC
) - 2 x V
0.8%
FD
1.0%
1.2%
Output Voltage Regulation Accuracy
The output voltage regulation accuracy is critical in order to
obtain the desired goal: maximize battery capacity usage. A small
decrease in output voltage accuracy results in a large decrease
in capacity. However, the output voltage can not be set arbitrarily
high because of safety and reliability concerns.
Figure 2 depicts the importance of output voltage regulation
accuracy.
Charge Termination Method
It can not be stressed enough that over charging is the Achilles’
heal of Li-Ion cells. Accurate charge termination methods are
essential for a safe, reliable, charging system.
Cell Temperature Monitoring
The temperature range over which a Li-Ion battery should
be charged is 0°C to 45°C, typically. Charging the battery at
temperatures outside of this range may cause the battery to
become hot. During a charge cycle, the pressure inside the
battery increases causing the battery to swell. Temperature
and pressure are directly related. As the temperature rises,
the pressure can become excessive. This can lead to a
mechanical breakdown inside the battery or venting. Charging
the battery outside of this temperature range may also harm
the performance of the battery or reduce the battery’s life
expectance.
Generally, thermistors are included in Lithium-Ion battery packs
in order to accurately measure the battery temperature. The
charger measures the resistance value of the thermistor between
the thermistor terminal and the negative terminal. Charging is
inhibited when the resistance, and therefore the temperature, is
outside the specified operating range.
Battery Discharge Current Or Reverse Leakage Current
In many applications, the charging system remains connected to
the battery in the absence of input power. The charging system
should minimize the current drain from the battery when input
power is not present. The maximum current drain should be
below a few microamperes and, typically, should be below one
microampere.
Li-ion Charging – Application Examples
Taking the above system considerations into account, an
appropriate charge management system can be developed.
Linear Solutions
Linear charging solutions are generally employed when a well-
regulated input source is available. Linear solutions, in these
applications, offer advantages of ease of use, size and cost.
Due to the low efficiency of a linear charging solution, the most
important factor is the thermal design. The thermal design is a
direct function of the input voltage, charge current and thermal
impedance between the pass transistor and the ambient cooling
air. The worst-case situation is when the device transitions from
the trickle charge stage to the constant current stage. In this
situation, the pass transistor has to dissipate the maximum
power. A trade-off must be made between the charge current,
size, cost and thermal requirements of the charging system.
Take, for example, an application required to charge a
1000 mAh, single Li-Ion cell from a 5V ±5% input at a constant
current charge rate of 0.5C or 1C. Figure 3 depicts Microchip’s
MCP73843 used to produce a low cost, stand-alone solution.
With a few external components, the preferred charge algorithm
is implemented.
The MCP73843 combines high accuracy constant current,
constant voltage regulation with automatic charge termination.
Analog and Interface Guide – Volume 2
11