PM6686 STMicroelectronics, PM6686 Datasheet - Page 39

• Manufacturer Part Number: PM6686
• Description: Dual Step-down Controller With Adjustable Voltages, Adjustable Ldo And Auxiliary Charge Pump Controller For Notebook
• Manufacturer: STMicroelectronics

PM6686
10.1.2
10.1.3
Table 14.
Input capacitor
In a buck topology converter the current that flows into the input capacitor is a pulsed current
with zero average value. The input RMS current of the two switching sections can be roughly
estimated as follows:
Equation 12
Where D1, D2 are the duty cycles and I1, I2 are the maximum load currents of the two
sections.
Input capacitor should be chosen with an RMS rated current higher than the maximum RMS
current given by both sections.
Tantalum capacitors are good in term of low ESR and small size, but they occasionally can
burn out if subjected to very high current during the charge. Ceramic capacitors have
usually a higher RMS current rating with smaller size and they remain the best choice.
Below there is a list of some ceramic capacitor manufacturers.
Table 15.
Output capacitor
The selection of the output capacitor is based on the ESR value and on the voltage rating
rather than on the capacitor value Cout.
The output capacitor has to satisfy the output voltage ripple requirements. Lower inductor
value can reduce the size of the choke but increases the inductor current ripp le ΔI
Since the voltage ripple V
Equation 13
A low ESR capacitor is required to reduce the output voltage ripple. Switching sections can
work correctly even with 15mV output ripple.
Manufacturer
COILCRAFT
COILCRAFT
COILCRAFT
TAYIO YUDEN
TAYIO YUDEN
Manufacturer
Inductor manufacturer
Input capacitor manufacturer
MSS-1048
MSS-1260
MLC-1550
Series
I
UMK325BJ106 KM-T
CinRMS
GMK325BJ106MN
RIPPLEout
Inductor value (µH)
Doc ID 15281 Rev 4
Series
=
D
is given by:
V
1
RIPPLEout
×
3,3
3,3
2,5
I
1
2
×
1 (
−
=
D
R
Capacitor value (µF)
1
)
out
+
RMS current (A)
D
×
2
Δ
I
×
L
I
10
10
2
2
7,22
16,5
9,7
×
1 (
−
D
2
Application information
)
Saturation current (A)
Rated voltage (V)
11,4
7,6
7
50
35
L
.
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