ncv8925 ON Semiconductor, ncv8925 Datasheet - Page 11

• Manufacturer Part Number: ncv8925
• Description: 3 Mhz, 600 Ma Step-down Dc-dc 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 a large switching noise. Using an input bypass
capacitor can reduce the peak current transients drawn
from
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 I
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
voltage 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 NCV8925, 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 AUTOMOTIVE CAPACITORS
Murata
TDK
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 NCV8925, a low profile ceramic capacitor of 4.7 mF
The NCV8925 operates at 3 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
OUT
: Feedback Voltage = 0.6 V
: Output Voltage (Volts)
= 4.7 mF.
V OUT + V FB
IN
GCM31CR71C475
C3216X7R1E475MT
pin.
(1 ) R1
OUT
OUT
to FB
R2
to FB then to GND.
)
out_max
OUT
APPLICATION INFORMATION
FB
, given the
/2.
4.7 mF
4.7 mF
is 0.6 V,
http://onsemi.com
(eq. 2)
the
NCV8925
11
of 2.2 mH.
check the loop stability depending on the accepted output
ripple voltage and the required output curret. 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 (DI
decreases with higher inductance:
•
•
•
higher than the maximum load current plus half the ripple
current:
•
•
efficiency of the converter. For best performance, the DC
resistance should be less than 0.3 W for good efficiency.
f c +
Table 2. L-C Filter Example
Table 3. LIST OF AUTOMOTIVE INDUCTORS
Coilcraft
TDK
The corner frequency is given by:
The device is intended to operate with inductance value
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
DI
L: Inductor Value
f
DI
DI
SW:
L:
L(MAX):
O(MAX):
Inductance (L)
2p L
Peak to Peak Inductor Ripple Current
Switching Frequency
1.0
2.2
m
m
1
Maximum Inductor Current
Maximum Output Current
H
H
C OUT
DI L +
I L(MAX) + I O(MAX) )
DO3316T-102ML
DO3316T-222ML
NLCV32T-1R0M-PF
NLCV32T-2R2M-PF
+
L
V OUT
2p 2.2 mH
f SW
Output Capacitor (C
1-
1
V OUT
V IN
4.7
10
4.7 mF
DI L
2
m
m
1.0 mH
2.2 mH
1.0 mH
2.2 mH
F
F
+ 49 kHz
OUT
0.77 A
7.8 A
1.0 A
(eq. 3)
)
(eq. 4)
(eq. 5)
11 A
L
)