CS8161 Cherry Semiconductor Corporation, CS8161 Datasheet - Page 7

• Manufacturer Part Number: CS8161
• Description: 12V/ 5V Low Dropout Dual Regulator with ENABLE
• Manufacturer: Cherry Semiconductor Corporation

Step 1: Place the completed circuit with a tantalum capaci-
tor of the recommended value in an environmental cham-
ber at the lowest specified operating temperature and
monitor the outputs on the oscilloscope. A decade box
connected in series with the capacitor C
higher ESR of an aluminum capacitor. (Leave the decade
box outside the chamber, the small resistance added by
the longer leads is negligible)
Step 2: With the input voltage at its maximum value,
increase the load current slowly from zero to full load
while observing the output for any oscillations. If no oscil-
lations are observed, the capacitor is large enough to
ensure a stable design under steady state conditions.
Step 3: Increase the ESR of the capacitor from zero using
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.
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.
Step 5: If the capacitor C
with the next smaller valued capacitor. (A smaller capaci-
tor will usually cost less and occupy less board space.) If
the capacitor oscillates within the range of expected oper-
ating conditions, repeat steps 3 and 4 with the next larger
standard capacitor value.
Step 6: Test the load transient response by switching in
various loads at several frequencies to simulate its real
work environment. Vary the ESR to reduce ringing.
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.
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. Once the value for C2 is determined, repeat the
steps to determine the appropriate value for C3.
The maximum power dissipation for a dual output regula-
tor (Figure 1) is
P
Where
D(max)
V
V
IN(max)
OUT 1 (min)
= {V
is the maximum input voltage,
{V
IN(max)
is the minimum output voltage from V
IN(max)
in a Dual Output Linear Regulator
Calculating Power Dissipation
ÐV
ÐV
OUT1(min)
OUT 2 (min)
2
is adequate, repeat steps 3 and 4
}I
OUT 1 (max)
}I
OUT 2 (max)
+
+V
2
will simulate the
IN(max)
Application Notes: continued
IQ
OUT 1
,
(1)
7
Once the value of P
sible value of RQ
The value of RQ
the package section of the data sheet. Those packages
with RQ
will keep the die temperature below 150¡C.
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.
A heat sink 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
to determine the value of RQ
where
RQ
RQ
are functions of the package type, heatsink and the inter-
face between them. These values appear in heat sink data
sheets of heat sink manufacturers.
V
Figure 1: Dual output regulator with key performance parameters
labeled.
IN
V
I
cation
I
cation
I
I
RQ
RQ
RQ
OUT 1 (max)
OUT 2 (max)
Q
OUT(max)
JC
JA
OUT 2 (min)
is the quiescent current the regulator consumes at
, it too is a function of package type. RQ
JC
CS
SA
appears in the package section of the data sheet. Like
= the junctionÐtoÐcase thermal resistance,
= the caseÐtoÐheatsink thermal resistance, and
= the heatsinkÐtoÐambient thermal resistance.
I
JA
IN
's less than the calculated value in equation 2
.
is the maximum output current, for the appli-
is the maximum output current, for the appli-
is the minimum output voltage from V
RQ
JA
JA
JA
can then be compared with those in
RQ
D(max)
can be calculated:
= RQ
}
Control
Features
JA
Regulator
=
Smart
Heat Sinks
JC
is known, the maximum permis-
+ RQ
150¡C - T
JA
I
Q
P
.
CS
D
+ RQ
A
SA
I
I
OUT 1
OUT 2
CS
and RQ
OUT 2
V
V
OUT 1
OUT 2
(2)
SA
(3)
,