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DC-DC CONVERTER OUTPUT INDUCTOR SELECTION
DI
L
+ V
OUT
ǒ
1 *
V
OUT
V
IN
Ǔ
L f
(15)
CHARGING WHILE UNDER LOAD
THERMAL CONSIDERATIONS
q
JA
+
T
J
* T
A
P
(16)
bq25015
bq25017
SLUS721A – DECEMBER 2006 – REVISED MARCH 2007
APPLICATION INFORMATION (continued)
For high efficiencies, the inductor should have a low DC resistance to minimize conduction losses. Although the
inductor core material has less effect on efficiency than its DC resistance, an appropriate inductor core material
must be used. The inductor value determines the inductor ripple current. The larger the inductor value, the
smaller the inductor ripple current, and the lower the conduction losses of the converter. On the other hand,
larger inductor values causes a slower load transient response. Usually the inductor ripple current, as calculated
below, should be around 30% of the average output current.
In order to avoid saturation of the inductor, the inductor should be rated at least for the maximum output current
of the converter plus the inductor ripple current that is calculated as:
where
f = switching frequency (1 MHz typical, 650 kHz minimal)
L = inductor value
∆ I
L
= peak-to-peak inductor ripple current
I
L(max)
= maximum inductor current
The highest inductor current occurs at maximum V
IN
. A more conservative approach is to select the inductor
current rating just for the maximum switch current of 350 mA. The internal compensator is designed in such a
way that the optimized resonant frequency of the output inductor and capacitor is approximately 16kHz. The
recommended inductor and capacitor values for various output current are given in Table 3 .
Table 3. Recommended Inductor and Capacitor Values
TYPICAL OUTPUT CURRENT INDUCTOR VALUE CAPACITOR VALUE APPLICATION
(mA) (µH) (µF)
30 100 1 For low current, smallest capacitor
60 47 2.2 For low current, small capacitor
80 33 3.3 For medium current, small capacitor
150 22 4.7 For medium current
300 10 10 For highest current, smallest inductor
The bq25015/7 are designed such that maximum charging safety and efficiency can be obtained by suspending
normal operation while the device is actively charging the battery. In this mode of operation, the timeout function
prevents a defective battery from being charged indefinitely. If charging does not terminate normally within five
hours, the device annunciates a fault condition on the STAT1 and STAT2 pins as indicated in Table 2 .
If a load is applied to the device while it is being used to charge a battery, a false fault condition may result due
to a slower rate of charge being applied to the battery. For this reason it is recommended that the load be
disconnected from the bq25015/7 while it is charging a battery.
The bq25015/7 devices are 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 QFN/SON PCB Attachment (SLUA271).
The most common measure of package thermal performance is thermal impedance ( θ
JA
) measured (or
modeled) from the chip junction to the air surrounding the package surface (ambient). The mathematical
expression for θ
JA
is:
19
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