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I
RMS
+ I
OUT(max)
V
OUT
V
IN
ǒ
1 *
V
OUT
V
IN
Ǔ
Ǹ
(11)
I
RMS
+
I
OUT
2
(12)
DC-DC CONVERTER OUTPUT CAPACITOR SELECTION
I
RMS(Cout)
+ V
OUT
ǒ
1 *
V
OUT
V
IN
Ǔ
L f
1
2 3
Ǹ
(13)
DV
OUT
+ V
OUT
ǒ
1 *
V
OUT
V
IN
Ǔ
L f
ǒ
1
8 C
OUT
f
) ESR
Ǔ
(14)
bq25015
bq25017
SLUS721A – DECEMBER 2006 – REVISED MARCH 2007
APPLICATION INFORMATION (continued)
If ceramic output capacitors are used, the capacitor RMS ripple current rating ensures the application
requirements. For completeness, the RMS ripple current is calculated as:
The worst case RMS ripple current occurs at D=0.5 and is calculated as:
Ceramic capacitors perform well because of the low ESR value, and they are less sensitive to voltage transients
and spikes compared to tantalum capacitors. The input capacitor should be placed as close as possible to the
BAT/OUT pin of the device for best performance. Refer to Table 1 for recommended components.
The advanced fast response voltage mode control scheme of the bq25015/7 allows the use of tiny ceramic
capacitors without having large output voltage under and overshoots during heavy load transients. Ceramic
capacitors having low ESR values have the lowest output voltage ripple and are therefore recommended. If
required, tantalum capacitors may be used as well (refer to Table 1 for recommended components). If ceramic
output capacitors are used, the capacitor RMS ripple current rating always meets the application requirements.
For completeness, the RMS ripple current is calculated as:
At nominal load current the device operates in PWM mode and the overall output voltage ripple is the sum of the
voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and discharging
the output capacitor:
where the output voltage ripple occurs at the highest input voltage V
IN
.
At light load currents the device operates in power save mode, and the output voltage ripple is independent of
the output capacitor value. The output voltage ripple is set by the internal comparator thresholds. The typical
output voltage ripple is 1% of the output voltage V
OUT
.
18
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