ncp1521 ON Semiconductor, ncp1521 Datasheet - Page 10

• Manufacturer Part Number: ncp1521
• Description: 1.5 Mhz, 600 Ma, Highefficiency, Low Quiescent Current, Adjustable Output Voltage Stepdown Converter
• Manufacturer: ON Semiconductor

Output Voltage Selection
resistor divider connected from V
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 V
the current through the divider will be 3.0 mA.
desired R1 and the R1 value:
•
•
•
•
Input Capacitor Selection
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.
with maximum output current, which is IO, max/2.
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 V
Output L-C Filter Design Considerations
current mode architecture. The correct selection of the
output filter ensures good stability and fast transient
response.
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
C
Table 1. List of Input Capacitor
OUT
The output voltage is programmed through an external
The formula below gives the value of V
In PWM operating mode, the input current is pulsating
The maximum RMS current occurs at 50% duty cycle
For NCP1521, a low profile, low ESR ceramic capacitor
The NCP1521 is built in 1.5 MHz frequency and uses
Due to the nature of the buck converter, the output L-C
V
V
R1: Feedback Resistor from V
R2: Feedback Resistor from FB to GND
OUT
FB
= 10 mF.
Taiyo Yuden
: Feedback Voltage = 0.6 V
: Output Voltage (Volts)
Murata
TDK
V OUT + V FB
IN
pin.
GRM21BR71C475KA
GRM188R60J475KE
C2012X5ROJ475KB
C1632X5ROJ475KT
JMK212BY475MG
(1 ) R1
OUT
OUT
to FB
R2
to FB then to GND.
)
OUT
APPLICATION INFORMATION
FB
, given the
is 0.6 V,
http://onsemi.com
(eq. 2)
NCP1521
10
between 1.0 mH and maximum of 4.7 mH.
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°.
Inductor Selection
performances are saturation current and DC resistance and
inductance value. The inductor ripple current (ÄI
decreases with higher inductance:
DI
L inductor value
f
higher than the maximum load current plus half the ripple
current:
DI
DI
efficiency of the converter. For best performances, the DC
resistance should be less than 0.3 W for good efficiency.
SW
f c +
Table 2. L-C Filter Example
The corner frequency is given by:
The device is intended to operate with inductance values
If the corner frequency is moved, it is recommended to
The inductor parameters directly related to device
The saturation current of the inductor should be rated
The inductor's resistance will factor into the overall
L
L(MAX)
O(MAX)
peak to peak inductor ripple current
switching frequency
Inductance (L)
2p L
1.0
2.2
4.7
Maximum inductor current
Maximum Output current
m
m
m
1
H
H
H
C OUT
DI L +
I L(MAX) + I O(MAX) )
+
L
V OUT
2p 2.2 mH
f SW
Output Capacitor (C
1-
1
V OUT
V IN
4.7
22
10
10 mF
DI L
2
m
m
m
F
F
F
+ 34 kHz
out
)
(eq. 3)
(eq. 4)
(eq. 5)
L
)