NCV4275C
http://onsemi.com
9
APPLICATION INFORMATION
V
I
C
I1
1000 mF
C
I2
100 nF
C
D
47 nF
I
I
I
D
I
D
1
4
5
2
3
GND
C
Q
22 mF
I
RO
I
Q
Q
RO
R
ext
5.0 k
V
Q
V
RO
Figure 21. Test Circuit
NCV4275C
I
q
Circuit Description
The NCV4275C is an integrated low dropout regulator
that provides 5.0 V or 3.3 V, 450 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 450 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 21, Test Circuit, for circuit element nomenclature
illustration.
Regulator
The error amplifier compares the reference voltage to a
sample of the output voltage (V
Q
) 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. Over saturation of the output
power device is prevented, and quiescent current in the
ground pin is minimized.
Regulator Stability Considerations
The input capacitors (C
I1
and C
I2
) are necessary to
stabilize the input impedance to avoid voltage line
influences. Using a resistor of approximately 1.0 W in
series with C
I2
can stop potential oscillations caused by
stray inductance and capacitance.
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. A tantalum, aluminum or ceramic
capacitors can be used. The range of stability versus
capacitance, load current and capacitive ESR is illustrated
in Figures 2 to 5. Minimum ESR for C
Q
= 22 mF is native
ESR of ceramic capacitors. 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
Q
shown in
Figure 21, Test Circuit, should work for most applications;
however, it is not necessarily the optimized solution.
Stability is guaranteed for C
Q
≥ 22 mF and an ESR ≤ 4.5 W
(5.0 V Version), 3.5 W (3.3 V Version). ESR characteristics
were measured with ceramic capacitors and additional
resistors to emulate ESR. Murata ceramic capacitors were
used, GRM32ER71A226ME20 (22 mF, 10 V, X7R, 1210),
GRM31MR71E105KA01 (1 mF, 25 V, X7R, 1206).
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
Q
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 22, 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 0.0 V to the upper timing
threshold voltage V
DU
. The charging current for this is I
D,C
and D pin voltage in steady state is typically 2.4 V. By using
typical IC parameters with a 47 nF capacitor on the D pin,
the following time delay for 5.0 V regulator is derived:
t
RD
= C
D
V
DU
/ I
D,C
t
RD
= 47 nF (1.8 V) / 5.5 mA = 15.4 ms
Other time delays can be obtained by changing the
capacitor value.
