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NCV47721 Datasheet(PDF) 11 Page - ON Semiconductor

No. de Pieza. NCV47721
Descripción  Adjustable LDO with Adjustable Current Limit
Descarga  16 Pages
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Fabricante  ONSEMI [ON Semiconductor]
Página de inicio  http://www.onsemi.com
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NCV47721 Datasheet(HTML) 11 Page - ON Semiconductor

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NCV47721
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11
APPLICATIONS INFORMATION
Circuit Description
The NCV47721 is an integrated low dropout regulator
that provides a regulated voltage at 200 mA to output. It is
enabled with an input to the enable pin. The regulator
voltage is provided by a PNP pass transistor controlled by an
error amplifier with a bandgap reference, which gives it the
lowest possible dropout voltage. The output current
capability of the LDO is 200 mA and the base drive
quiescent current is controlled to prevent oversaturation
when the input voltage is low or when the output is
overloaded. The integrated current sense feature provides
diagnosis and system protection functionality. The current
limit of the device is adjustable by resistor connected to CSO
pin. Voltage on CSO pin is proportional to output current.
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.
Regulator
The error amplifier compares the reference voltage to a
sample of the output voltage (Vout) and drives the base of a
PNP series pass transistor via 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 (Cin) is necessary to stabilize the input
impedance to avoid voltage line influences. The output
capacitor (Cout) 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.
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 manufacturer’s data
sheet usually provides this information. The value for the
output capacitor Cout, shown in Figure 1 should work for
most applications; see also Figure 16 for output stability at
various load and Output Capacitor ESR conditions. Stable
region of ESR in Figure 16 shows ESR values at which the
LDO output voltage does not have any permanent
oscillations at any dynamic changes of output load current.
Marginal ESR is the value at which the output voltage
waving is fully damped during four periods after the load
change and no oscillation is further observable.
ESR characteristics were measured with ceramic
capacitors and additional series resistors to emulate ESR.
Low duty cycle pulse load current technique has been used
to maintain junction temperature close to ambient
temperature.
Calculating Bypass Capacitor
If improved stability (reducing output voltage ringing
during transients) is demanded, connect the bypass
capacitor Cb between Adjustable Input pin and Vout pin
according to Applications circuit at Figure 1. Parallel
combination of bypass capacitor Cb with the feedback
resistor R1 contributes in the device transfer function as an
additional zero and affects the device loop stability,
therefore its value must be optimized. Attention to the
Output Capacitor value and its ESR must be paid. See also
Stability in High Speed Linear LDO Regulators Application
Note, AND8037/D for more information. Optimal value of
bypass capacitor is given by following expression
Cn +
1
2
p
fz
R
1
(F)
(eq. 1)
where
R1
the upper feedback resistor
fz
the frequency of the zero added into the device
transfer function by R1 and Cb external
components.
Set the R1 resistor according to output voltage requirement.
Choose the fz with regard on the output capacitance Cout,
refer to the table below.
Cout (mF)
10
22
47
100
fZ range (kHz)
max 24
max 37
N/A*
N/A*
NOTE:
* For Cout = 47 mF and higher, Cb capacitors are not
needed for stability improvement. Cb capacitors are
useful for noise reduction. See electrical characteristic
table.
Ceramic capacitors and their part numbers listed below
have been used as low ESR output capacitors Cout from the
table above to define the frequency ranges of additional zero
required for stability:
GRM31CR71C106KAC7 (10
mF, 16 V, X7R, 1206)
GRM32ER71C226KE18 (22
mF, 16 V, X7R, 1210)
GRM32ER61C476ME15 (47
mF, 16 V, X5R, 1210)
GRM32ER60J107ME20 (100
mF, 6.3 V, X5R, 1210)
Enable Input
The enable pin is used to turn the regulator on or off. By
holding the pin down to a voltage less than 0.99 V, the output
of the regulator will be turned off. When the voltage on the
enable pin is greater than 2.31 V, the output of the regulator
will be enabled to power its output to the regulated output
voltage. The enable pin may be connected directly to the
input pin to give constant enable to the output regulator.
Setting the Output Voltage
The output voltage range can be set between 3.3 V and
20 V. This is accomplished with an external resistor divider
feeding back the voltage to the IC back to the error amplifier
by the voltage adjust pin ADJ. The internal reference voltage


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