CM9100 Datasheet, PDF (8/11 Pages) California Micro Devices Corp – Basic Compact Cost-effective Fast-Charger

PRELIMINARY
CM9100
Application Information (cont’d)
Most battery manufactures recommend an optimal
charging current for their battery. This is typically a time
ratio related to the battery capacity, with a value of .7C
to 1C, once the battery is above the Precharge voltage
level. For example, a 750mAh capacity battery with
recommended charge of .7C could have I_CC set for
about 525mA, with R_ISET equal to 4.75kΩ, 1%.
The actual Fast-charge current might be further limited
by either the maximum chip temperature limit, deter-
mined by the power dissipation on the CM9100 chip,
the ambient temperature (T_A), and the junction-to-
ambient thermal resistance, Rth_(JA).
When the battery terminal voltage, sensed at V_OUT,
approaches 4.2V, the CM9100 enters the Termination
(CV) mode. The charger then regulates its output volt-
age at 4.20V, and the charging current gradually
decreases as the battery’s internal voltage, V_OC, rises
toward 4.2V. The actual charging current is now deter-
mined by the differential voltage (4.20V – V_OC) and the
internal impedance, R_internal, of the Li-ion battery-pack.
The CM9100 ends the charging process when charg-
ing current drops below 5% of the Fast-charge (CC)
mode current level. Once terminated, the charge cur-
rent is completely stopped and no trickle charge is
applied. Trickle (or float) charging is not required due to
the minimal self-discharge of the Li-ion cells, and they
are unable to absorb overcharge, which causes plating
of metallic lithium and shortens the life of the battery.
Following the Termination mode, the charger will enter
the Standby mode. The status pin will be set to
STAT=V_IN.
If the wall adapter is left plugged-in while in the
Standby mode, the charger will continue to monitor the
battery voltage. It automatically re-charges the battery
when the battery voltage drops below the re-charge
threshold. When the adapter is removed, the CM9100
will drain less than 1µA from the battery.
sis). This protects the charger IC and its nearby
external components from excessive temperature.
The Charger IC junction temperature is determined by
several factors in the following equation:
T_J = T_A + PD + Rth_(JA)
(1)
The Rth_(JA) is usually determined by the IC package
and the thermal resistance between the package and
the PC board. In particular, a SMD IC package relies
on the underlying PC board copper to move the heat
away from the junction. The key to reducing the ther-
mal resistance between the IC package and the under-
lying PC board is using a large copper (Cu) area for
solder attach and a large ground plane underneath the
charger IC to conduct the heat away.
The power dissipation (PD in equation 1) of a linear
charger is the product of input-output voltage differen-
tial and output current.
PD = (V_IN – V_OUT) × I_OUT
Highest power dissipation occurs when the battery at
its lowest level (3.2V), when it just starts in the Fast-
charge (CC) mode. Assuming V_IN = 5.0V, V_BAT = 3.2V,
I_CC = 1A, the PD = (5V-3.2V) x 1A = 1.8W. Assuming
Rth_(JA) = 50°C/W, then -T = 1.8W x 50°C/W = 90°C. If
the ambient temperature (T_A) is 35°C, then the junction
temperature (T_J) could reach 125°C without over-tem-
perature current foldback.
With over-temperature (OT) current foldback, the
CM9100 will throttle down the charging current, allow-
ing the junction temperature will reach steady-state
equilibrium of 105°C, which translates into 1.4W of
power dissipation, or 0.78A of charge current. As the
battery voltage rises during charging, the allowable PD
dissipation is increased. When the battery voltage
reaches 3.6V, a full 1.0A of charging current is allowed.
Charging Current Foldback in the Over-
temperature Condition
A limitation of linear chargers is that they are vulnera-
ble to over-temperature conditions. The CM9100 will
throttle down the charging current when the chip junc-
tion temperature reaches 105°C (with 10°C of hystere-
OTP and OCP
In addition to chip temperature regulation at 105°C, the
CM9100 provides absolute over-temperature shutdown
protection. In the case of a malfunctioning charger con-
trol, high ambient temperature or an unexpectedly high
IC thermal resistance, Rth_(JA) (for example, due to
faulty soldering of the charger IC chip). The CM9100
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06/30/06