NCV8141
http://onsemi.com
6
DEFINITION OF TERMS
Dropout Voltage:
The input−output voltage differential
at which the circuit ceases to regulate against further
reduction in input voltage. Measured when the output
voltage has dropped 100 mV from the nominal value
obtained at 14 V input, dropout voltage is dependent upon
load current and junction temperature.
Input Voltage:
The DC voltage applied to the input
terminals with respect to ground.
Line Regulation:
The change in output voltage for a
change in the input voltage. The measurement is made under
conditions of low dissipation or by using pulse techniques
such that the average chip temperature is not significantly
affected.
Load Regulation:
The change in output voltage for a
change in load current at constant chip temperature.
Quiescent Current:
The part of the positive input current
that does not contribute to the positive load current. The
regulator ground lead current.
Ripple Rejection: The ratio of the peak−to−peak input
ripple voltage to the peak−to−peak output ripple voltage.
Current Limit:
Peak current that can be delivered to the
output.
CIRCUIT DESCRIPTION
The NCV8141 is a 5.0 V Watchdog Regulator with
protection circuitry and three logic control functions that
allow a microprocessor to control its own power supply. The
NCV8141 is designed for use in automotive, switch mode
power supply post regulator, and battery powered systems.
Basic regulator performance characteristics include a low
noise, low drift, 5.0 V ±3.0% precision output voltage with
low dropout voltage (1.25 V @ I
OUT
= 500 mA) and low
quiescent current (7.0 mA @ I
OUT
= 500 mA). On board
short circuit, thermal, and overvoltage protection make it
possible to use this regulator in particularly harsh operating
environments.
The Watchdog logic function monitors an input signal
(WDI) from the microprocessor or other signal source.
When the signal frequency goes below the externally
programmable limit, a RESET
signal is generated (RESET).
Proper operation has been verified at a frequency up to 100
kHz. No abnormal RESET
signals will occur with
frequencies lower than 100 kHz and the maximum
Threshold Frequency (96 Hz). An external capacitor
(C
DELAY
) programs the watchdog frequency limit as well as
the power on reset (POR) and RESET
delay.
The RESET
function is activated by any of three
conditions: the watchdog signal moves outside of its preset
limits; the output voltage drops out of regulation by more
than 4.5%; or the IC is in its power up sequence. The RESET
signal is independent of V
IN
and reliable down to V
OUT
=
1.0 V.
In conjunction with the Watchdog, the ENABLE
function controls the regulator’s power consumption. The
NCV8141’s output stage and its attendant circuitry are
enabled by setting the ENABLE lead high. The regulator
goes into sleep mode when the ENABLE lead goes low and
the watchdog signal moves outside its preset limit. This
unique combination of control functions in the NCV8141
gives the microprocessor control over its own power down
sequence: i.e. it gives the microprocessor the flexibility to
perform housekeeping functions before it powers down.
VOLTAGE REFERENCE AND OUTPUT CIRCUITRY
Precision Voltage Reference
The regulated output voltage depends on the precision band
gap voltage reference in the IC. By adding an error amplifier
into the feedback loop, the output voltage is maintained within
±3.0% over temperature and supply variation.
Output Stage
The composite PNP−NPN output structure (Figure 7)
provides 500 mA (min) of output current while maintaining
a low drop out voltage (1.25 V) and drawing little quiescent
current (7.0 mA).
Figure 7. Composite Output Stage of the NCV8141
V
OUT
V
IN
The NPN pass device prevents deep saturation of the
output stage which in turn improves the IC’s efficiency by
preventing excess current from being used and dissipated by
the IC.
Output Stage Protection
The output stage is protected against overvoltage, short
circuit and thermal runaway conditions (Figure 8).
If the input voltage rises above 30 V (e.g. load dump), the
output shuts down. This response protects the internal
circuitry and enables the IC to survive unexpected voltage
transients.
