TC1017_13 Datasheet, PDF (13/26 Pages) Microchip Technology – 150 mA, Tiny CMOS LDO With Shutdown

160
140
120
100
80
60
40
20
0
-40
V_IN - V_OUT = 1V
-15 10 35 60 85 110
Ambient Temperature (°C)
FIGURE 5-2:
Maximum Current vs.
Ambient Temperature (SC-70 package).
5.3 Power Dissipation: SOT-23
The TC1017 is also available in a SOT-23 package for
improved thermal performance. The thermal resistance
for the SOT-23 package is approximately 255°C/W
when the copper area used in the printed circuit board
layout is similar to the JEDEC J51-7 low thermal
conductivity standard or semi-G42-88 standard. For
applications with a larger or thicker copper area, the
thermal resistance can be lowered. See AN792, “A
Method to Determine How Much Power a SOT-23 Can
Dissipate in an Application” (DS00792), for a method to
determine the thermal resistance for a particular
application.
The TC1017 power dissipation capability is dependant
upon several variables: input voltage, output voltage,
load current, ambient temperature and maximum
junction temperature. The absolute maximum steady-
state junction temperature is rated at +125°C. The
power dissipation within the device is equal to:
EQUATION 5-4:
P_D = V_IN – V_OUT  I_LOAD + V_IN  I_GND
The V_IN x I_GND term is typically very small when
compared to the (V_IN–V_OUT) x I_LOAD term, simplifying the
power dissipation within the LDO to be:
EQUATION 5-5:
P_D = V_IN – V_OUT  I_LOAD
To determine the maximum power dissipation
capability, the following equation is used:
TC1017
EQUATION 5-6:
P_DMAX
=
_---T_{----J-_---M----A---X-----}–_-----T_{---A---_---M----A---X-----}
R_JA
Where:
T_{J_MAX} = the maximum junction
temperature allowed
T_{A_MAX} = the maximum ambient
temperature
R_JA = the thermal resistance from
junction to air
Given the following example:
Find:
V_IN =
V_OUT =
I_LOAD =
T_A =
3.0V to 4.1V
2.85V ±2.5%
120 mA (output current)
+85°C (max. desired ambient)
1. Internal power dissipation:
P_DMAX = V_{IN_MAX} – V_{OUT_MIN}   I_LOAD
= 4.1V – 2.85  0.975  120mA
= 158.5mW
2. Maximum allowable ambient temperature:
T_{A_MAX} = T_{J_MAX} – P_DMAX  R_JA
= 125C – 158.5mW  255C/W
= 125C – 40.5C
= 84.5C
3. Maximum allowable power dissipation at
desired ambient:
P_D = ^T----J--^_--^M-R---^A-_--^X--_J--_A-^–----T----^A-
= -1---2---25--5-^---5C---_--^–-C----8/--W-5---^---C--
= 157mW
In this example, the TC1017 dissipates approximately
158.5 mW and the junction temperature is raised
40.5°C over the ambient. The absolute maximum
power dissipation is 157 mW when given a maximum
ambient temperature of +85°C.
Input voltage, output voltage or load current limits can
also be determined by substituting known values in the
power dissipation equations.
Figure 5-3 and Figure 5-4 depict typical maximum
power dissipation versus ambient temperature, as well
as typical maximum current versus ambient tempera-
ture with a 1V input voltage to output voltage
differential, respectively.
 2005-2013 Microchip Technology Inc.
DS21813F-page 13