Circuit Description
VTRK Output Voltage
ENABLE Function
This output uses the same type of output device as VSTBY
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
,
The
function switches the output transistor for
ENABLE
ENABLE
VTRK on and off. When the
lead voltage exceeds
1.4V(typ), VTRK turns off. This input has several hundred
millivolts of hysteresis to prevent spurious output activity
during power-up or power-down.
done with a simple resistor divider (Figure 2), and follow-
ing the formula:
RESET Function
VTRK = VSTBY ´ (1 + R1/R2) + IAdj ´ R1
RESET
The
trolled by a low voltage detection circuit sensing the VSTBY
RESET
is an open collector NPN transistor, con-
If another 5V output is needed, simply connect the Adj
lead to the VTRK output lead.
(5V) output voltage. This circuit guarantees the
output stays below 1V (0.1V typ) when VSTBY is as low as
1V to ensure reliable operation of microprocessor-based
systems.
Application Notes
IOUT2(max) is the maximum output current, for the
application
External Capacitors
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.
IQ is the quiescent current the regulator consumes at
IOUT(max).
Once the value of PD(max) is known, the maximum per-
missible value of RQJA can be calculated:
150¡C - TA
RQJA
=
(2)
PD
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.
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.
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.
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
I
I
IN
OUT1
Smart
Regulator
V
IN
V
V
OUT1
I
OUT2
Calculating Power Dissipation
OUT2
in a Dual Output Linear Regulator
Control
Features
}
The maximum power dissipation for a dual output regula-
tor (Figure 1) is:
I
Q
PD(max) = {VIN(max)ÐVOUT1(min)}IOUT1(max)+
{VIN(max)ÐVOUT2(min)}IOUT2(max)+VIN(max)IQ
(1)
Where
VIN(max) is the maximum input voltage,
Figure 1: Dual output regulator with key performance parameters
labeled.
VOUT1(min) is the minimum output voltage from VOUT1
,
,
VOUT2(min) is the minimum output voltage from VOUT2
IOUT1(max) is the maximum output current, for the
application
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
