Application Notes: continued
Step 3: Increase the ESR of the capacitor from zero using
I
IN
the decade box and vary the load current until oscillations
appear. Record the values of load current and ESR that
cause the greatest oscillation. This represents the worst
case load conditions for the regulator at low temperature.
I
OUT
V
IN
Smart
Regulator
V
OUT
Control
Features
}
Step 4: Maintain the worst case load conditions set in step
3 and vary the input voltage until the oscillations increase.
This point represents the worst case input voltage condi-
tions.
I
Q
Step 5: If the capacitor is adequate, repeat steps 3 and 4
with the next smaller valued capacitor. A smaller capacitor
will usually cost less and occupy less board space. If the
output oscillates within the range of expected operating
conditions, repeat steps 3 and 4 with the next larger stan-
dard capacitor value.
Figure 9: Single output regulator with key performance parameters
labeled.
Once the value of PD(max) is known, the maximum permis-
sible value of RQJA can be calculated:
150¡C - TA
(2)
RQJA
=
Step 6: Test the load transient response by switching in
various loads at several frequencies to simulate its real
working environment. Vary the ESR to reduce ringing.
PD
The value of RQJA can then be compared with those in
the package section of the data sheet. Those packages
with RQJA's less than the calculated value in equation 2 will
keep the die temperature below 150¡C.
Step 7: Remove the unit from the environmental chamber
and heat the IC with a heat gun. Vary the load current as
instructed in step 5 to test for any oscillations.
In some cases, none of the packages will be sufficient to
dissipate the heat generated by the IC, and an external
heatsink will be required.
Once the minimum capacitor value with the maximum
ESR is found, a safety factor should be added to allow for
the tolerance of the capacitor and any variations in regula-
tor performance. Most good quality aluminum electrolytic
capacitors have a tolerance of +/- 20% so the minimum
value found should be increased by at least 50% to allow
for this tolerance plus the variation which will occur at
low temperatures. The ESR of the capacitor should be less
than 50% of the maximum allowable ESR found in step 3
above.
Heatsinks
A heatsink effectively increases the surface area of the
package to improve the flow of heat away from the IC and
into the surrounding air.
Each material in the heat flow path between the IC and the
outside environment will have a thermal resistance. Like
series electrical resistances, these resistances are summed
Calculating Power Dissipation
in a Single Output Linear Regulator
to determine the value of RQJA
:
R
QJA = RQJC + RQCS + RQSA
(3)
The maximum power dissipation for a single output regu-
lator (Figure 9) is:
where:
R
R
R
QJC = the junctionÐtoÐcase thermal resistance,
QCS = the caseÐtoÐheatsink thermal resistance, and
QSA = the heatsinkÐtoÐambient thermal resistance.
PD(max) = VIN(max) - VOUT(min)
I
OUT(max) + VIN(max) Q
I
(1)
{
}
where:
VIN(max) is the maximum input voltage,
VOUT(min) is the minimum output voltage,
R
R
QJC appears in the package section of the data sheet. Like
QJA, it too is a function of package type. RQCS and RQSA
IOUT(max) is the maximum output current for the applica-
tion, and
are functions of the package type, heatsink and the inter-
face between them. These values appear in heatsink data
sheets of heatsink manufacturers.
IQ is the quiescent current the regulator consumes at
IOUT(max)
.
11
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
