NCV8506 Series
http://onsemi.com
9
CIRCUIT DESCRIPTION
REGULATOR CONTROL FUNCTIONS
The NCV8506 contains the microprocessor compatible
control function RESET
(Figure 13).
Figure 13. Reset and Delay Circuit Wave Forms
V
IN
V
OUT
RESET
DELAY
(V
DT
)
Threshold
DELAY
Threshold
RESET
T
d
T
d
RESET Function
A RESET signal (low voltage) is generated as the IC
powers up until V
OUT
is within 1.5% of the regulated output
voltage, or when V
OUT
drops out of regulation,and is lower
than 4.0% below the regulated output voltage. Hysteresis is
included in the function to minimize oscillations.
The RESET
output is an open collector NPN transistor,
controlled by a low voltage detection circuit. The circuit is
functionally independent of the rest of the IC thereby
guaranteeing that the RESET
signal is valid for V
OUT
as low
as 1.0 V.
DELAY Function
The reset delay circuit provides a programmable (by
external capacitor) delay on the RESET
output lead.
The DELAY lead provides source current (typically 4.0 mA)
to the external DELAY capacitor during the following
proceedings:
1. During Power Up (once the regulation threshold
has been verified).
2. After a reset event has occurred and the device is
back in regulation. The DELAY capacitor is
discharged when the regulation (RESET
threshold)
has been violated. This is a latched incident. The
capacitor will fully discharge and wait for the
device to regulate before going through the delay
time event again.
Voltage Adjust
Figure 14 shows the device setup for a user configurable
output voltage. The feedback to the V
ADJ
pin is taken from
a voltage divider referenced to the output voltage. The loop
is balanced around the Unity Gain threshold (1.30 V
typical).
Figure 14. Adjustable Output
Voltage
V
OUT
V
ADJ
NCV8506
15 k
5.1 k
C
OUT
≈5.0 V
1.28 V
APPLICATION NOTES
SETTING THE DELAY TIME
The delay time is controlled by the Reset Delay Low
Voltage, Delay Switching Threshold, and the Delay Charge
Current. The delay follows the equation:
t
DELAY
+
ƪ
C
DELAY
(V
dt
* Reset Delay Low Voltage)
ƫ
Delay Charge Current
Example:
Using C
DELAY
= 33 nF.
Assume reset Delay Low Voltage = 0.
Use the typical value for V
dt
= 1.8 V (2.5 V, 3.3 V, and
5.0 V options).
Use the typical value for Delay Charge Current = 4.2 mA.
t
DELAY
+
ƪ
33 nF(1.8 * 0)
ƫ
4.2 mA
+ 14 ms
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
instability. The aluminum electrolytic capacitor is the least
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
manufacturers data sheet usually provides this information.
