CS8121
Applications Notes
V
BAT
C
1
0.1mF
V
IN
V
OUT
C
2
10mF
V
CC
500kW
CS–8121
ENABLE
RESET
R
RST
mP
SWITCH
Gnd
RESET
C
RST
I/O Port
Q
1
100kW
100kW
500kW
Figure 5. Microprocessor control of CS8121 using external switching transistor Q
1
.
The circuit depicted in Figure 5 lets the microprocessor
control its power source, the CS8121 regulator. An I/O
port on the µP and the SWITCH port are used to drive the
base of Q1. When Q1 is driven into saturation, the voltage
on the ENABLE lead falls below its lower threshold and
the regulatorÕs output is switched on. When the drive cur-
rent is removed, the voltage on the ENABLE lead rises,
the output is switched off and the IC moves into Sleep
mode where it typically draws 250µA.
By coupling these two controls with ENABLE , the system
has added flexibility. Once the system is running, the
state of the SWITCH is irrelevant as long as the I/O port
continues to drive Q1. The µP can turn off its own power
by withdrawing drive current, once the SWITCH is open.
This software control at the I/O port allows the µP to fin-
ish key housekeeping functions before power is removed.
The logic options are summarized in Table 1 below
Table 1: Logic Control of CS8121 Output
µP I/O drive
SWITCH ENABLE
Output
ON
OFF
Closed
Open
Closed
Open
LOW
LOW
LOW
HIGH
ON
ON
ON
OFF
The I/O port of the µP typically provides 50 µA to Q1. In
automotive applications the SWITCH is connected to the
ignition switch.
Stability Considerations
The output or compensation capacitor C
2
helps determine
three main characteristics of a linear regulator: start-up
delay, load transient response and loop stability.
The capacitor value and type should be based on cost,
availability, size and temperature constraints. A tantalum
or aluminum electrolytic capacitor is best, since a film or
ceramic capacitor with almost zero ESR can cause insta-
bility. The aluminum electrolytic capacitor is the least
6
expensive solution, but, if the circuit operates at low
temperatures (-25¡C to -40¡C), both the value and ESR of
the capacitor will vary considerably. The capacitor manu-
facturers data sheet usually provides this information.
The value for the output capacitor C
2
shown in the test
and applications circuit should work for most applica-
tions, however it is not necessarily the optimized solu-
tion.
To determine an acceptable value for C
2
for a particular
application, start with a tantalum capacitor of the recom-
mended value and work towards a less expensive alterna-
tive part.
Step 1:
Place the completed circuit with a tantalum
capacitor of the recommended value in an environmental
chamber at the lowest specified operating temperature
and monitor the outputs with an oscilloscope. A decade
box connected in series with the capacitor will simulate
the 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
oscillations 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 volt-
age conditions.
Step 5:
If the capacitor is adequate, repeat steps 3 and 4
with the next smaller valued capacitor. A smaller capaci-
tor will usually cost less and occupy less board space. If
the output oscillates within the range of expected operat-
ing conditions, repeat steps 3 and 4 with the next larger
standard capacitor value.
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
