CS8361
Circuit Description
ENABLE Function
V
TRK
Output Voltage
The ENABLE function switches the output transistor for
V
TRK
on and off. When the ENABLE lead voltage exceeds
1.4V(typ), V
TRK
turns off. This input has several hundred
millivolts of hysteresis to prevent spurious output activity
during power-up or power-down.
RESET Function
This output uses the same type of output device as V
STBY
,
but is rated for 250mA. The output is configured as a
tracking regulator of the standby output. By using the
standby output as a voltage reference, giving the user an
external programming lead (Adj lead), output voltages
from 5V to 20V are easily realized. The programming is
done with a simple resistor divider (Figure 2), and follow-
ing the formula:
V
TRK
= V
STBY
´
(1 + R1/R2) + I
Adj
´
R1
If another 5V output is needed, simply connect the Adj
lead to the V
TRK
output lead.
The RESET is an open collector NPN transistor, con-
trolled by a low voltage detection circuit sensing the V
STBY
(5V) output voltage. This circuit guarantees the RESET
output stays below 1V (0.1V typ) when V
STBY
is as low as
1V to ensure reliable operation of microprocessor-based
systems.
Application Notes
External Capacitors
I
OUT2
(max) is the maximum output current, for the
application
I
Q
is the quiescent current the regulator consumes at
I
OUT
(max).
Once the value of PD(max) is known, the maximum per-
missible value of R
QJA
can be calculated:
150¡C - T
A
R
QJA
=
(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.
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.
Output capacitors for the CS8361 are required for stability.
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
conditions of negative input transients that might be char-
acteristic 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, ÒCompensation for Linear Regulators.Ó
Calculating Power Dissipation
in a Dual Output Linear Regulator
I
IN
V
IN
Smart
Regulator
I
OUT
1
V
OUT
1
I
OUT
2
V
OUT
2
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)IQ
}
Control
Features
I
Q
(1)
Figure 1: Dual output regulator with key performance parameters
labeled.
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
application
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
