NCV8665
http://onsemi.com
7
Circuit Description
The NCV8665 is an integrated low dropout regulator that
provides 5.0 V, 150 mA protected output and a signal for
power on reset. The regulation is provided by a PNP pass
transistor controlled by an error amplifier with a bandgap
reference, which gives it the lowest possible drop out
voltage and best possible temperature stability. The output
current capability is 150 mA, and the base drive quiescent
current is controlled to prevent over saturation when the
input voltage is low or when the output is overloaded. The
regulator is protected by both current limit and thermal
shutdown. Thermal shutdown occurs above 150°C to
protect the IC during overloads and extreme ambient
temperatures. The delay time for the reset output is
adjustable by selection of the timing capacitor. See Figure 2,
Application Circuit, for circuit element nomenclature
illustration.
Regulator
The error amplifier compares the reference voltage to a
sample of the output voltage (V
OUT
) and drives the base of
a PNP series pass transistor by a buffer. The reference is a
bandgap design to give it a temperature−stable output.
Saturation control of the PNP is a function of the load current
and input voltage. Oversaturation of the output power
device is prevented, and quiescent current in the ground pin
is minimized.
Regulator Stability Considerations
The input capacitor (C
IN
) is necessary to stabilize the
input impedance to avoid voltage line influences. The output
capacitor helps determine three main characteristics of a
linear regulator: startup delay, load transient response and
loop stability. The capacitor value and type should be based
on cost, availability, size and temperature constraints.
Tantalum, aluminum electrolytic, film, or ceramic
capacitors are all acceptable solutions, however attention
must be paid to ESR constraints. The aluminum electrolytic
capacitor is the least expensive solution, but, if the circuit
operates at low temperatures (−25°C to −40°C), both the
capacitance and ESR of the capacitor will vary considerably.
The capacitor manufacturer’s data sheet usually provides
this information. The value for the output capacitor C
OUT
shown in Figure 2, Application Circuit, should work for
most applications; however, it is not necessarily the
optimized solution.
Reset Output
The reset output is used as the power on indicator to the
microcontroller. This signal indicates when the output
voltage is suitable for reliable operation of the controller. It
pulls low when the output is not considered to be ready. RO
is pulled up to V
OUT
by an external resistor, typically 5.0 kW
in value. The input and output conditions that control the
Reset Output and the relative timing are illustrated in
Figure 11, Reset Timing. Output voltage regulation must be
maintained for the delay time before the reset output signals
a valid condition. The delay for the reset output is defined as
the amount of time it takes the timing capacitor on the delay
pin to charge from a residual voltage of 0 V to the upper
timing threshold voltage V
DU
of 1.8 V. The charging current
for this is I
D
of 5.5 mA. By using typical IC parameters with
a 47 nF capacitor on the D Pin, the following time delay is
derived:
t
RD
= C
D
* V
DU
/ I
D
t
RD
= 47 nF * (1.8 V) / 5.5 mA = 15.4 ms
Other time delays can be obtained by changing the C
D
capacitor value.
