Datasheet

MCP1790/MCP1791
DS22075B-page 18 © 2010 Microchip Technology Inc.
5.0 APPLICATION CIRCUITS/
ISSUES
5.1 Typical Application
The MCP1790/MCP1791 is most commonly used as a
voltage regulator. It is a high voltage input capability
and thermal protection make it ideal for automotive and
24V industrial applications.
FIGURE 5-1: Typical Application.
5.1.1 APPLICATION INPUT CONDITIONS
5.2 Power Calculations
5.2.1 POWER DISSIPATION
The internal power dissipation of the MCP1790/
MCP1791 is a function of input voltage, output voltage
and output current. The power dissipation, as a result
of the quiescent current draw, is so low, it is
insignificant (70.0 µA x V
IN
). The following equation
can be used to calculate the internal power dissipation
of the LDO.
EQUATION 5-1:
The maximum continuous operating junction
temperature specified for the MCP1790/MCP1791 is
+125°C. To estimate the internal junction temperature
of the MCP1790/MCP1791, the total internal power
dissipation is multiplied by the thermal resistance from
junction to ambient (Rθ
JA
). The thermal resistance
from junction to ambient for the SOT-223-5 package is
estimated at 62
°C/W.
EQUATION 5-2:
The maximum power dissipation capability for a
package can be calculated given the junction-to-
ambient thermal resistance and the maximum ambient
temperature for the application. The following equation
can be used to determine the package maximum
internal power dissipation.
EQUATION 5-3:
EQUATION 5-4:
EQUATION 5-5:
Package Type = SOT-223-5
Input Voltage Range = 8V to 24V
V
IN
maximum = 24V
V
OUT
typical = 5.0V
I
OUT
=70mA maximum
1
2
3
4
5
C
OUT
4.7 µF Ceramic
+
+
C
IN
1µF
24 VDC
5
1N4002
Ceramic
PWRGD
V
OUT
V
IN
5V@70 mA
10 kΩ
MCP1791
P
LDO
V
IN MAX )()
V
OUT MIN()
–()I
OUT MAX )()
×=
P
LDO
= LDO Pass device internal power
dissipation
V
IN(MAX)
= Maximum input voltage
V
OUT(MIN)
= LDO minimum output voltage
T
JMAX()
P
TOTAL
Rθ
JA
× T
AMAX
+=
T
J(MAX)
= Maximum continuous junction
temperature
P
TOTAL
= Total device power dissipation
Rθ
JA
= Thermal resistance from junction
to ambient
T
AMAX
= Maximum ambient temperature
P
DMAX()
T
JMAX()
T
AMAX()
–()
Rθ
JA
---------------------------------------------------=
P
D(MAX)
= Maximum device power
dissipation
T
J(MAX)
= Maximum continuous junction
temperature
T
A(MAX)
= Maximum ambient temperature
Rθ
JA
= Thermal resistance from junction
to ambient
T
JRISE()
P
TOTAL
Rθ
JA
×=
T
J(RISE)
= Rise in device junction
temperature over the ambient
temperature
P
TOTAL
= Total device power dissipation
Rθ
JA
= Thermal resistance from
junction-to-ambient
T
J
T
JRISE()
T
A
+=
T
J
= Junction Temperature
T
J(RISE)
= Rise in device junction
temperature over the ambient
temperature
T
A
= Ambient temperature