PM6641 STMicroelectronics, PM6641 Datasheet - Page 36

• Manufacturer Part Number: PM6641
• Description: IC MONO VR DDR2/3 PC VFQFPN-48
• Manufacturer: STMicroelectronics

Specifications of PM6641

• Applications: Converter, DDR2, DDR3
• Voltage - Input: 2.7 ~ 5.5 V
• Number Of Outputs: 3
• Voltage - Output: 0.8 ~ 5.5 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 48-VFQFN
• Output Voltage: 0.9 V
• Output Current: 3 A
• Input Voltage: 2.7 V to 5.5 V
• Switching Frequency: 1000 KHz
• Operating Temperature Range: - 40 C to + 125 C
• Mounting Style: SMD/SMT
• For Use With: 497-8425 - KIT EVAL PM6641 CHIPSET/DDR2/3
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Components selection
8.2
36/47
Once the inductor value is determined, the inductor current ripple is then recalculated:
Equation 13
The next step is the computation of the maximum RMS inductor current:
Equation 14
The inductor must have an RMS current greater than I
stability.
Then the calculation of the maximum inductor peak current follows:
Equation 15
I
The saturation current of the inductor should be greater than the maximum between I
and the programmed peak current limit, selected by an external resistor connected between
CSNS pin and AGND (as described in current limit section).
Input capacitor selection
In a buck topology converter the current that flows through the input capacitor is pulsed and
with zero average value. The RMS input current, for each switching regulator, can be
calculated as follows:
Equation 16
Neglecting the second term, the equation is reduced to:
Equation 17
L,PEAK
is important when choosing the inductor, in term of its saturation current.
I
Cin
RMS
I
Δ
, L
RMS
Doc ID 13510 Rev 3
I
, L
I
, L
MAX
=
I
PEAK
Cin
=
I
RMS
LOAD
=
I (
=
V
LOAD
IN
I
≅
2
LOAD
,
MAX
⋅
I
LOAD
fsw
D
,
MAX
⋅
,
MAX
1 (
−
⋅
L
)
−
V
2
D
OUT
D
+
+
⋅
)
1 (
(
Δ
+
Δ
I
−
⋅
L,RMS
12
I
, L
, L
1
V
2
) D
MAX
12
V
MAX
IN
D
OUT
,
MAX
⋅
(
)
in order to assure thermal
Δ
2
I
L
)
2
PM6641
Lpeak