PM6681A STMicroelectronics, PM6681A Datasheet - Page 32

• Manufacturer Part Number: PM6681A
• Description: Dual synchronous step down controller with adjustable LDO
• Manufacturer: STMicroelectronics

Design guidelines
9.4
9.5
32/47
Table 12.
Input capacitors selection
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 18
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 13.
Power MOSFETs
Logic-level MOSFETs are recommended, since low side and high side gate drivers are
powered by LDO5. Their breakdown voltage VBR
In notebook applications, power management efficiency is a high level requirement. The
power dissipation on the power switches becomes an important factor in switching
selections. Losses of high-side and low-side MOSFETs depend on their working conditions.
The power dissipation of the high-side MOSFET is given by:
Equation 19
Manufacturer
Panasonic
Manufacturer
SANYO
Tayio yuden
TDK
Output capacitor manufacturer
Input capacitor manufacturer
POSCAP TPB,TPD,
SPCAP UD, UE
I
UMK432 X5506MM-T
CinRMS
C3225X5R1E106M
Series
TPE
P
Series
=
DHighSide
D
1
×
I
1
2
=
Capacitor value
×
P
1 (
conduction
100 to 470
100 to 470
−
(uF)
D
Capacitor value (uF)
1
)
DSS
+
+
D
P
2
must be higher than V
switching
×
10
10
I
2
2
Rated voltage (V) ESR max (mΩ)
×
1 (
2.5 to 6.3
−
2 to 6.3
D
2
)
Rated voltage (V)
INmax
50
25
12 to 65
7 to 18
.
PM6681A