CS8391
Application Notes
General
I
OUT2
(max) is the maximum output current for the appli-
cation, and
I
Q
is the quiescent current the regulator consumes at both
I
OUT1
(max) and I
OUT2
(max).
Once the value of PD(max) is known, the maximum per-
missible value of R
QJA
can be calculated:
150¡C - T
A
(2)
P
D
The value of R
QJA
can then be compared with those in
the package section of the data sheet. Those packages with
R
QJA
's less than the calculated value in equation 2 will keep
the die temperature below 150¡C.
R
QJA
=
In some cases, none of the packages will be sufficient to
dissipate the heat generated by the IC, and an external heat
sink will be required.
The CS8391 is a micropower dual 5V regulator. All bias
required to operate the internal circuitry is derived from
the standby output, V
OUT2
. If this output experiences an
over current situation and collapses, then V
OUT1
will also
collapse (see timing diagrams).
If there is critical circuitry that must remain active under
most conditions it should be connected to V
OUT2
. Any cir-
cuitry that is likely to be subjected to a short circuit, e.g.,
circuitry outside the module, should be connected to V
OUT1
.
External Capacitors
Output capacitors are required for stability with the CS8391.
Without them, the regulator outputs will oscillate. Actual
size and type may vary depending upon the application
load and temperature range. Capacitor effective series
resistance (ESR) is also a factor in the IC stability. Worst-
case is determined at the minimum ambient temperature
and maximum load expected.
Output capacitors can be increased in size to any desired
value above the minimum. One possible purpose of this
would be to maintain the output voltages during brief con-
ditions of negative input transients that might be character-
istic of a particular system.
Capacitors must also be rated at all ambient temperatures
expected in the system. To maintain regulator stability
down to -40ûC, capacitors rated at that temperature must be
used.
More information on capacitor selection for Smart
Regulatorsª is available in the Smart Regulator applica-
tion note,
Compensation for Linear Regulators.
I
IN
V
IN
Smart
Regulator
I
OUT
1
V
OUT
1
I
OUT
2
V
OUT
2
}
Control
Features
I
Q
Figure 1: Dual output regulator with key performance parameters
labeled.
Heat Sinks
ENABLE
The ENABLE function controls V
OUT1
. When ENABLE is
high, V
OUT1
is on. When ENABLE is low, V
OUT1
is off.
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 R
QJA
:
R
QJA
= R
QJC
+ R
QCS
+ R
QSA
where:
R
QJC
= the junctionÐtoÐcase thermal resistance,
R
QCS
= the caseÐtoÐheat sink thermal resistance, and
R
QSA
= the heat sinkÐtoÐambient thermal resistance.
R
QJC
appears in the package section of the data sheet. Like
R
QJA
, it too is a function of package type. R
QCS
and R
QSA
are
functions of the package type, heat sink and the interface
between them. These values appear in heat sink data sheets
of heat sink manufacturers.
(3)
Calculating Power Dissipation
in a Dual Output Linear Regulator
The maximum power dissipation for a dual output regula-
tor (Figure 1) is:
PD(max) = {V
IN
(max)ÐV
OUT1
(min)}I
OUT1
(max)+
{V
IN
(max)ÐV
OUT2
(min)}I
OUT2
(max)+V
IN
(max)I
Q
(1)
where:
V
IN
(max) is the maximum input voltage,
V
OUT1
(min) is the minimum output voltage from V
OUT1
,
V
OUT2
(min) is the minimum output voltage from V
OUT2
,
I
OUT1
(max) is the maximum output current for the appli-
cation,
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
