CS8391 Cherry Semiconductor Corporation, CS8391 Datasheet - Page 4

• Manufacturer Part Number: CS8391
• Description: 5V/250mA/ 5V/100mA Micropower Low Dropout Regulator with ENABLE
• Manufacturer: Cherry Semiconductor Corporation

The CS8391 is a micropower dual 5V regulator. All bias
required to operate the internal circuitry is derived from
the standby output, V
over current situation and collapses, then V
collapse (see timing diagrams).
If there is critical circuitry that must remain active under
most conditions it should be connected to V
cuitry that is likely to be subjected to a short circuit, e.g.,
circuitry outside the module, should be connected to V
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.
The ENABLE function controls V
high, V
The maximum power dissipation for a dual output regula-
tor (Figure 1) is:
PD(max) = {V
where:
V
V
V
I
cation,
OUT1
IN
OUT1
OUT2
(max) is the maximum input voltage,
(max) is the maximum output current for the appli-
OUT1
(min) is the minimum output voltage from V
(min) is the minimum output voltage from V
{V
is on. When ENABLE is low, V
IN
IN
in a Dual Output Linear Regulator
(max)ÐV
(max)ÐV
Calculating Power Dissipation
External Capacitors
OUT2
OUT2
OUT1
ENABLE
General
. If this output experiences an
(min)}I
(min)}I
OUT2
OUT1
OUT1
. When ENABLE is
(max)+V
(max)+
OUT1
OUT1
OUT2
IN
is off.
(max)I
. Any cir-
will also
Application Notes
OUT1
OUT2
Q
OUT1
(1)
,
,
.
4
Once the value of PD(max) is known, the maximum per-
missible value of R
The value of R
the package section of the data sheet. Those packages with
R
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 heat
sink will be required.
Figure 1: Dual output regulator with key performance parameters
labeled.
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
where:
R
R
functions of the package type, heat sink and the interface
between them. These values appear in heat sink data sheets
of heat sink manufacturers.
QJA
QJC
QJA
V
I
cation, and
I
I
R
R
R
OUT2
Q
OUT1
IN
QJC
QCS
QSA
's less than the calculated value in equation 2 will keep
, it too is a function of package type. R
is the quiescent current the regulator consumes at both
appears in the package section of the data sheet. Like
= the junctionÐtoÐcase thermal resistance,
(max) is the maximum output current for the appli-
(max) and I
= the caseÐtoÐheat sink thermal resistance, and
= the heat sinkÐtoÐambient thermal resistance.
I
IN
QJA
R
QJA
can then be compared with those in
OUT2
QJA
= R
R
}
QJA
Control
Features
can be calculated:
(max).
Regulator
QJC
Heat Sinks
Smart
QJA
=
+ R
150¡C - T
:
QCS
I
Q
P
D
+ R
A
QSA
I
I
OUT
OUT
QCS
1
2
and R
V
V
QSA
OUT
OUT
(2)
(3)
1
2
are