Temperature Considerations
3-2
Circuit Design
3.1
Temperature Considerations
To protect the device and assure the specifications, the maximum junction
temperature should not exceed 125
°
C. If the temperature exceeds 150
°
C,
thermal shutdown will turn off the device. This restriction limits the power
dissipation the regulator can handle in any given application. To ensure the
junction temperature is within acceptable limits, calculate the maximum
allowable dissipation, P
D(max)
, and the actual dissipation, P
D
, which must be
less than or equal to P
D(max)
. The maximum power dissipation limit is
determined using the following equation:
P
D(max)
+
T
J,max
*
T
A
R
q
JA
Where:
-
T
J,max
is the maximum allowed junction temperature [
°
C], i.e., 125
°
C for
the TPS701xx families
-
R
θ
JA
is the thermal resistance junction-to-ambient for the package, i.e.,
32.6
°
C/W for the 20-terminal TSSOP package
-
T
A
is the ambient temperature
The regulator dissipation is calculated using:
P
D
+
ǒ
V
IN
*
V
OUT
Ǔ
I
OUT
3.2
ESR and Transient Response
LDOs typically require an external output capacitor for stability. In fast transient
response applications, capacitors are used to support the load current while
the LDO amplifier is responding. In most applications, one capacitor is used
to support both functions.
Besides its capacitance, every capacitor also contains parasitic impedances.
These impedances are resistive as well as inductive. The resistive impedance
is called equivalent series resistance (ESR), and the inductive impedance is
called equivalent series inductance (ESL). The equivalent schematic diagram
of any capacitor can therefore be drawn as shown in Figure 3–1.
Figure 3–1. ESR and ESL
R
ESR
L
ESL
C
In most cases one can neglect the effect of inductive impedance ESL. There-
fore, the following application focuses mainly on the parasitic resistance ESR.