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NCP1521 Datasheet(PDF) 10 Page - ON Semiconductor

Part No. NCP1521
Description  1.5 MHz, 600 mA, High−Efficiency, Low Quiescent Current, Adjustable Output Voltage Step−Down Converter
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

NCP1521 Datasheet(HTML) 10 Page - ON Semiconductor

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NCP1521
http://onsemi.com
10
APPLICATION INFORMATION
Output Voltage Selection
The output voltage is programmed through an external
resistor divider connected from VOUT to FB then to GND.
For low power consumption and noise immunity, the
resistor from FB to GND (R2) should be in the
[100 k−600 k] range. If R2 is 200 k given the VFB is 0.6 V,
the current through the divider will be 3.0
mA.
The formula below gives the value of VOUT, given the
desired R1 and the R1 value:
VOUT + VFB
(1
) R1
R2
)
(eq. 2)
VOUT: Output Voltage (Volts)
VFB: Feedback Voltage = 0.6 V
R1: Feedback Resistor from VOUT to FB
R2: Feedback Resistor from FB to GND
Input Capacitor Selection
In PWM operating mode, the input current is pulsating
with large switching noise. Using an input bypass capacitor
can reduce the peak current transients drawn from the
input supply source, thereby reducing switching noise
significantly. The capacitance needed for the input bypass
capacitor depends on the source impedance of the input
supply.
The maximum RMS current occurs at 50% duty cycle
with maximum output current, which is IO, max/2.
For NCP1521, a low profile, low ESR ceramic capacitor
of 4.7
mF should be used for most of the cases. For effective
bypass results, the input capacitor should be placed as close
as possible to the VIN pin.
Table 1. List of Input Capacitor
Murata
GRM188R60J475KE
GRM21BR71C475KA
Taiyo Yuden
JMK212BY475MG
TDK
C2012X5ROJ475KB
C1632X5ROJ475KT
Output L−C Filter Design Considerations
The NCP1521 is built in 1.5 MHz frequency and uses
current mode architecture. The correct selection of the
output filter ensures good stability and fast transient
response.
Due to the nature of the buck converter, the output L−C
filter must be selected to work with internal compensation.
For NCP1521, the internal compensation is internally fixed
and it is optimized for an output filter of L = 2.2
mH and
COUT = 10
mF.
The corner frequency is given by:
fc +
1
2
p L
COUT
(eq. 3)
+
1
2
p 2.2 mH
10
mF
+ 34 kHz
The device is intended to operate with inductance values
between 1.0
mH and maximum of 4.7 mH.
If the corner frequency is moved, it is recommended to
check the loop stability depending on the output ripple
voltage accepted and output current required. For lower
frequency, the stability will be increased; a larger output
capacitor value could be chosen without critical effect on
the system. On the other hand, a smaller capacitor value
increases the corner frequency and it should be critical for
the system stability. Take care to check the loop stability.
The phase margin is usually higher than 45
°.
Table 2. L−C Filter Example
Inductance (L)
Output Capacitor (Cout)
1.0
mH
22
mF
2.2
mH
10
mF
4.7
mH
4.7
mF
Inductor Selection
The inductor parameters directly related to device
performances are saturation current and DC resistance and
inductance value. The inductor ripple current (ÄIL)
decreases with higher inductance:
DIL +
VOUT
L
fSW
1−
VOUT
VIN
(eq. 4)
DIL peak to peak inductor ripple current
L inductor value
fSW switching frequency
The saturation current of the inductor should be rated
higher than the maximum load current plus half the ripple
current:
IL(MAX) + IO(MAX) )
DIL
2
(eq. 5)
DIL(MAX) Maximum inductor current
DIO(MAX) Maximum Output current
The inductor’s resistance will factor into the overall
efficiency of the converter. For best performances, the DC
resistance should be less than 0.3
W for good efficiency.


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