Datasheet
© 2011 Microchip Technology Inc. DS22285A-page 21
MCP16321/2
5.0.7 BOOST CAPACITOR
The boost capacitor is used to supply current for the
internal high-side drive circuitry that is above the input
voltage. The boost capacitor must store enough energy
to completely drive the high-side switch on and off. A
22 nF X5R or X7R capacitor is recommended for all
applications. The boost capacitor maximum voltage is
5.5V, so a 6.3V or 10V rated capacitor is
recommended.
5.0.8 THERMAL CALCULATIONS
The MCP16321/2 is available in a 3x3 QFN-16
package. By calculating the power dissipation and
applying the package thermal resistance (θ
JA
), the
junction temperature is estimated. The maximum
continuous junction temperature rating for the
MCP16321/2 is +125°C.
To quickly estimate the internal power dissipation for
the switching step-down regulator, an empirical
calculation using measured efficiency can be used.
Given the measured efficiency, the internal power
dissipation is estimated in Equation 5-4. This power
dissipation includes all internal and external
component losses. For a quick internal estimate,
subtract the estimated inductor ESR loss from the P
DIS
calculation in Equation 5-4.
EQUATION 5-4: TOTAL POWER
DISSIPATION ESTIMATE
The difference between the first term, input power, and
the second term, power delivered, is the total system
power dissipation. The inductor losses are estimated
by P
L
=I
OUT
2
xL
ESR
.
EXAMPLE 5-5: POWER DISSIPATION –
MCP16321
EXAMPLE 5-6: POWER DISSIPATION –
MCP16322
P
DIS
V
OUT
I
OUT
×
Efficency
-------------------------------V
OUT
I
OUT
×
()–=
V
IN
=12V
V
OUT
=5.0V
I
OUT
=1A
Efficiency = 92.5%
Total System Dissipation = 405 mW
L
ESR
=0.04Ω
P
L
=40 mW
MCP16321 internal power dissipation estimate:
P
DIS
–P
L
=365mW
θ
JA
=38.5°C/W
Estimated Junction = +14.052°C
Note 1: θ
JA
= 38.5°C/W for a 4-layer FR4 Printed
Circuit Board with a 13.5 in
2
, 1 oz internal
copper ground plane.
2: A smaller ground plane will result in a
larger θ
JA
temperature rise.
V
IN
=12V
V
OUT
=3.3V
I
OUT
=2A
Efficiency = 87.5%
Total System Dissipation = 943 mW
L
ESR
=0.04Ω
P
L
= 160 mW
MCP16322 internal power dissipation estimate:
P
DIS
–P
L
=783mW
θ
JA
=38.5°C/W
Estimated Junction = +30.14°C