BQ25015 Datasheet, PDF (17/28 Pages) Texas Instruments – SINGLE-CHIP CHARGER AND DC/DC CONVERTER IC FOR PORTABLE APPLICATIONS

bq25015
bq25017
www.ti.com
SLUS721A – DECEMBER 2006 – REVISED MARCH 2007
APPLICATION INFORMATION
ADJUSTABLE OUTPUT VOLTAGE VERSION (bq25015)
When the adjustable output voltage version is being used (bq25015), the output is set by the external resistor
divider, as shown in Figure 13.
The output voltage can be calculated as:
ǒ Ǔ V_OUT + 0.5 V
1
)
R1
R2
(8)
where
R1 + R2 ≤ 1 MΩ
Internal reference voltage V_REF(typ) = 0.5 V
C1 and C2 should be selected as:
C1 + _2p
1
10 kHz
R1
(9)
where
R1 = upper resistor of the voltage divider
C1 = upper capacitor of the voltage divider
For C1, a value should be chosen that comes closest to the calculated result.
C2
+
R1
R2
C1
(10)
where
R2 = lower resistor of the voltage divider
C2 = lower capacitor of the voltage divider
For C2, the selected capacitor value should always be selected larger than the calculated result. For example, in
Figure 13, a 100-pF capacitor is selected for a calculated result of C2 = 86.17 pF.
If quiescent current is not a key design parameter, C1 and C2 can be omitted and a low-impedance feedback
divider must be used with R1 + R2 < 100 kΩ. This design reduces the noise available on the feedback pin (FB)
as well, but increases the overall quiescent current during operation.
FIXED OUTPUT VOLTAGE VERSION (bq25017)
When a fixed output voltage version of the device is being used, no external resistive divider network is
necessary. In this case, the output of the inductor should be connected directly the FB pin, as shown in
Figure 13.
INPUT CAPACITOR SELECTION
In most applications, all that is needed is a high-frequency decoupling capacitor. A 0.1-µF ceramic, placed in
close proximity to AC/USB and VSS pins, works fine. The bq2501x is designed to work with both regulated and
unregulated external DC supplies. If a non-regulated supply is chosen, the supply unit should have enough
capacitance to hold up the supply voltage to the minimum required input voltage at maximum load. If not, more
capacitance has to be added to the input of the charger.
CHARGER OUTPUT CAPACITOR (DC-DC CONVERTER INPUT CAPACITOR) SELECTION
Because the buck converter has a pulsating input current, a low ESR input capacitor is required. This results in
the best input voltage filtering and minimizes the interference with other circuits caused by high input voltage
spikes. Also, the input capacitor must be sufficiently large to stabilize the input voltage during heavy load
transients.
For good input voltage filtering, usually a 4.7-µF input capacitor is sufficient and can be increased without any
limit for better input voltage filtering.
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