CS403
Circuit Description
The CS403 RESET function is very precise, has hysteresis
on both the RESET and Delay comparators, a latching
Delay capacitor discharge circuit, and operates down to
1V.
The RESET circuit output is an open collector type with
ON and OFF parameters as specified. The RESET output
NPN transistor is controlled by the two circuits described
(see Block Diagram).
Low Voltage Inhibit Circuit
This circuit monitors output voltage, and when output
voltage is below the specified minimum, causes the
RESET output transistor to be in the ON (saturation) state.
When the output voltage is above the specified level, this
circuit permits the RESET output transistor to go into the
OFF state if allowed by the reset Delay circuit.
Reset Delay Circuit
This circuit provides a programmable (external capacitor)
delay on the RESET output lead. The Delay lead provides
source current to the external delay capacitor only when
the Low Voltage Inhibit circuit indicates that output volt-
age is above V
RT(ON)
. Otherwise, the Delay lead sinks cur-
rent to ground (used to discharge the Delay capacitor).
The discharge current is latched ON when the output volt-
age is below V
RT(OFF)
, or when the voltage on the Delay
capacitor is above V
DIS
. In other words, the Delay capaci-
tor is fully discharged any time the output voltage falls
out of regulation, even for a short period of time. This fea-
ture ensures a controlled RESET pulse is generated follow-
ing detection of an error condition. The circuit allows
the RESET output transistor to go to the OFF (open) state
only when the voltage on the Delay lead is higher than
V
DIS
.
Test Circuit
V
IN
C
1
*
100nF
V
OUT
C
2
**
C
OUT
=10mF to
100mF
CS403
Delay
Gnd
RESET
100nF
C
d
C
1
* is required if the regulator is far from the power source filter.
C
2
** is required for stability
Application Notes
Stability Considerations
The output or compensation capacitor 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
expensive solution, but, if the circuit operates at low tem-
peratures (-25¡C to -40¡C), both the value and ESR of the
capacitor will vary considerably. The capacitor manufac-
turers data sheet usually provides this information.
4
The value for the output capacitor C
OUT
shown in the test
and applications circuit should work for most applica-
tions, however it is not necessarily the optimized solution.
To determine an acceptable value for C
OUT
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 capac-
itor of the recommended value in an environmental cham-
ber 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
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
