Datasheet

MCP1640/B/C/D
DS22234A-page 16  2010 Microchip Technology Inc.
5.5 Inductor Selection
The MCP1640/B/C/D is designed to be used with small
surface mount inductors; the inductance value can
range from 2.2 µH to 10 µH. An inductance value of
4.7 µH is recommended to achieve a good balance
between inductor size, converter load transient
response and minimized noise.
Several parameters are used to select the correct
inductor: maximum rated current, saturation current
and copper resistance (ESR). For boost converters, the
inductor current can be much higher than the output
current. The lower the inductor ESR, the higher the
efficiency of the converter, a common trade-off in size
versus efficiency.
Peak current is the maximum or limit, and saturation
current typically specifies a point at which the induc-
tance has rolled off a percentage of the rated value.
This can range from a 20% to 40% reduction in induc-
tance. As inductance rolls off, the inductor ripple cur-
rent increases as does the peak switch current. It is
important to keep the inductance from rolling off too
much, causing switch current to reach the peak limit.
5.6 Thermal Calculations
The MCP1640/B/C/D is available in two different
packages (SOT23-6 and 2x3 DFN8). By calculating the
power dissipation and applying the package thermal
resistance, (
JA
), the junction temperature is esti-
mated. The maximum continuous junction temperature
rating for the MCP1640/B/C/D is +125
o
C.
To quickly estimate the internal power dissipation for
the switching boost regulator, an empirical calculation
using measured efficiency can be used. Given the
measured efficiency, the internal power dissipation is
estimated by Equation 5-3.
EQUATION 5-3:
The difference between the first term, input power, and
the second term, power delivered, is the internal
MCP1640/B/C/D power dissipation. This is an estimate
assuming that most of the power lost is internal to the
MCP1640/B/C/D and not C
IN
, C
OUT
and the inductor.
There is some percentage of power lost in the boost
inductor, with very little loss in the input and output
capacitors. For a more accurate estimation of internal
power dissipation, subtract the I
INRMS
2
*L
ESR
power
dissipation.
5.7 PCB Layout Information
Good printed circuit board layout techniques are
important to any switching circuitry and switching
power supplies are no different. When wiring the
switching high current paths, short and wide traces
should be used. Therefore it is important that the input
and output capacitors be placed as close as possible to
the MCP1640/B/C/D to minimize the loop area.
The feedback resistors and feedback signal should be
routed away from the switching node and the switching
current loop. When possible, ground planes and traces
should be used to help shield the feedback signal and
minimize noise and magnetic interference.
TABLE 5-2: MCP1640/B/C/D
RECOMMENDED INDUCTORS
Part
Number
Value
(µH)
DCR
 (typ)
I
SAT
(A)
Size
WxLxH (mm)
Coiltronics
®
SD3110 4.7 0.285 0.68 3.1x3.1x1.0
SD3112 4.7 0.246 0.80 3.1x3.1x1.2
SD3114 4.7 0.251 1.14 3.1x3.1x1.4
SD3118 4.7 0.162 1.31 3.8x3.8x1.2
SD3812 4.7 0.256 1.13 3.8x3.8x1.2
SD25 4.7 0.0467 1.83 5.0x5.0x2.5
Part
Number
Value
(µH)
DCR

(max)
I
SAT
(A)
Size
WxLxH (mm)
Wurth Elektronik
®
WE-TPC
Type TH
4.7 0.200 0.8 2.8x2.8x1.35
WE-TPC
Type S
4.7 0.105 0.90 3.8x3.8x1.65
WE-TPC
Type M
4.7 0.082 1.65 4.8x4.8x1.8
WE-TPC
Type X
4.7 0.046 2.00 6.8x6.8x2.3
Part
Number
Value
(µH)
DCR

(max)
I
SAT
(A)
Size
WxLxH (mm)
Sumida
®
CMH23 4.7 0.537 0.70 2.3x2.3x1.0
CMD4D06 4.7 0.216 0.75 3.5x4.3x0.8
CDRH4D 4.7 0.09 0.800 4.6x4.6x1.5
EPCOS
®
B82462A2
472M000
4.7 0.084 2.00 6.0x6.0x2.5
B82462G4
472M
4.7 0.04 1.8 6.3x6.3x3.0
V
OUT
I
OUT

Efficiency
-------------------------------


V
OUT
I
OUT
– P
Dis
=