CM9112 Datasheet, PDF (10/18 Pages) California Micro Devices Corp – Dual Inputs Dual Outputs High Accurate Fast Charger

PRELIMINARY
CM9112
Application Information (cont’d)
When using conventional floating charger with the sys­
tem load connected directly to the battery, and in the
CC mode, where a higher current limit is available, for
example, the system can draw a continuous load cur­
rent of 300mA. However, since the system is always
tied to the battery, the charger IC has no way to differ­
entiate the system power demand from the battery
charging demand. The charger will limit the total output
current to 300mA for the system and 1A for charging
the battery. If the battery voltage is low, 3.2V for exam­
ple, the charger IC power dissipation will be at its worst
case, or 3.6W. The charger’s junction temperature
rises quickly, triggering the over-temperature (OT) cur­
rent foldback. If the system continues to draw a large
current, the battery will then be supplying part of that
load current; putting the battery is in a discharge mode,
rather than in a charge mode. The battery voltage will
continue to drop, potentially falling back into a Pre-
charge mode condition, and upsetting the charging
sequence or forcing the charger to shut down.
Even the charger power dissipation due to the system
load alone:
PD1= 1.0A× (5V– 3.2V )=1.8W
may already exceed the chip’s thermal limit and cause
OTP to trigger.
The CM9112 Dual Outputs Charge Advantage
To overcome these issues, the CM9112’s Fast-charge
architecture separates the system power output
(LDO1) from the battery charging power output. See
Figure 3. With a separate output, the power dissipation
contributed by LDO1 in a condition similar to the one
above is now only:
PD1= 1.0A× (5V– 4.2V )=0.8W
In other words, the LDO1 can support the system load,
free from the hindrance of the charger, regardless of
the battery voltage level. The user can continue use
the host system, even when the battery charge voltage
is very low, when there is a defective battery, or there is
no battery present.
VIN
LDO1
VSYS
4.2V ^ISYS
CM9112
Charger
VREF
VOUT
BSEN
System
ICH
VBAT
Figure 3. Dual Outputs Charger
Since the CM9112 provides an independent power
path to the system, as soon as an adapter is plugged-
in, the user can use the system power, even if the bat­
tery is dead or in the Precharge mode.
Charging Current Foldback in the Over-
temperature Condition
A limitation of linear chargers is that they are vulnera­
ble to over-temperature conditions. The CM9112 will
throttle down the charging current when the chip junc­
tion temperature reaches 105°C (with 10°C of hys­
terezis). 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.
© 2006 California Micro Devices Corp. All rights reserved.
10 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 lTel: [phone] lFax: [phone] lwww.cmd.com
07/06/06