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TIMER FAULT RECOVERY
APPLICATION INFORMATION
THERMAL CONSIDERATIONS
q
JA
+
T
J
* T
A
P
(5)
P +
ǒ
V
IN
* V
I(BAT)
Ǔ
I
O(OUT)
(6)
PCB LAYOUT CONSIDERATIONS
bq24020 , , bq24022 , , bq24023
bq24024 , bq24025 , bq24026
bq24027
SLUS549E – DECEMBER 2002 – REVISED NOVEMBER 2007
As shown in Figure 4 , the bqTINY-II provides a recovery method to deal with timer-fault conditions. The following
discussion summarizes this method:
Condition #1: The charge voltage is above the recharge threshold (V
(RCH)
), and a timeout fault occurs
Recovery method: bqTINY-II waits for the battery voltage to fall below the recharge threshold. This could
happen as a result of a load on the battery, self-discharge or battery removal. When the battery voltage falls
below the recharge threshold, the bqTINY-II clears the fault and starts a new charge cycle. Toggling POR, CE, or
TTE also clears the fault.
Condition #2: The charge voltage is below the recharge threshold (V
(RCH)
), and a timeout fault occurs
Recovery method: In this scenario, the bqTINY-II applies the I
(FAULT)
current. This small current is used to detect
a battery-removal condition and remains on as long as the battery voltage stays below the recharge threshold. If
the battery voltage goes above the recharge threshold, then the bqTINY-II disables the I
(FAULT)
current and
executes the recovery method described for condition #1. When the battery voltage falls below the recharge
threshold, the bqTINY-II clears the fault and starts a new charge cycle. Toggling POR, CE, or TTE also clears
the fault.
The bqTINY-II is packaged in a thermally enhanced MLP package. The package includes a thermal pad to
provide an effective thermal contact between the device and the printed circuit board (PCB). Full PCB design
guidelines for this package are provided in the application note entitled, QFN/SON PCB Attachment Application
Note (TI Literature Number SLUA271 ).
The most common measure of package thermal performance is thermal impedance ( θ
JA
) measured (or modeled)
from the device junction to the air surrounding the package surface (ambient). The mathematical expression for
θ
JA
is:
Where:
• T
J
= device junction temperature
• T
A
= ambient temperature
• P = device power dissipation
Factors that can greatly influence the measurement and calculation of θ
JA
include:
• whether or not the device is board mounted
• trace size, composition, thickness, and geometry
• orientation of the device (horizontal or vertical)
• volume of the ambient air surrounding the device under test and airflow_lus549
• whether other surfaces are in close proximity to the device being tested
The device power dissipation, P, is a function of the charge rate and the voltage drop across the internal power
FET. It can be calculated from the following equation:
Due to the charge profile of Li-xx batteries, the maximum power dissipation is typically seen at the beginning of
the charge cycle when the battery voltage is at its lowest. See Figure 2 .
It is important to pay special attention to the PCB layout. The following provides some guidelines:
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Product Folder Link(s): bq24020 bq24022 bq24023 bq24024 bq24025 bq24026 bq24027