ncp3121 ON Semiconductor, ncp3121 Datasheet - Page 16

• Manufacturer Part Number: ncp3121
• Description: Dual 3.0 A, Step-down Dc/dc Switching Regulator
• Manufacturer: ON Semiconductor

maximum load current and maximum input voltage. The
rectifying diodes should be placed close to the SW pin to
avoid the possibility of ringing due to trace inductance.
Input Capacitor
discontinuous. The input capacitor has to maintain the DC
input voltage and to sustain the ripple current produced by
internal MOSFET switching. For stable operation of the
switch mode converter, a low ESR capacitor is needed to
prevent large voltage transients from appearing at the input.
Therefore, ceramic capacitors are preferred, but the circuit
works in a stable manner also with electrolytic capacitors. It
must be located near the regulator and use short leads. Also,
paralleling ceramic capacitors will increase the regulator
stability.
where:
MOSFET.
= 0.5, where:
following equation:
where:
capacitance.
capacitor value. Therefore, the input capacitor can be
estimated by:
Output Capacitor
and provides low impedance for load current changes. The
MBRA340T3G
MBRS340T3G
MBRS330T3G
Table 3. Schottky Diode Example
The worst case of the diode average current occurs during
The input current to the step down converter is
The RMS value of the input capacitor current ripple is:
The duty cycle is:
V
V
The equation reaches its maximum value with duty cycle
Losses in the input capacitor can be calculated using the
ESR
The input capacitor voltage ripple depends on the C
The output capacitor filters output inductor ripple current
Part Number
D
DSAT
is the voltage drop across the rectifying diode and
CIN
C
is the switch saturation voltage on the power
is the effective series resistance of the input
IN
+
f
I
SW
D +
P
RMS
I
CIN
LOAD
@ DV
+ I
+ I
V
I
3 A, 40 V Schottky Rectifier
3 A, 40 V Schottky Rectifier
3 A, 30 V Schottky Rectifier
RMS
IN
IN
LOAD
V
) V
RMS
@
+
OUT
V
2 @ ESR
Description
V
I
OUT
D
LOAD
IN
D ( 1 * D )
) V
* V
2
@ 1 *
D
DSAT
CIN
V
V
OUT
IN
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V
NCP3121
RRM
IN
[V]
40
40
30
16
min
principle consideration for the output capacitor is the ripple
current induced by the switches through the inductor. It
supplies the current to the load in DCM or during load
transient and filters the output voltage ripple. For low output
ripple voltage and good stability, low ESR output capacitors
are recommended. The inductor ripple current acting against
the ESR of the output capacitor is the major contributor to
the output ripple voltage.
output and provides regulator loop stability.
value of the inductor ripple current are the main factors
contributing to the output ripple voltage value.
equation:
where:
capacitor.
to yield optimal results. Key specifications for output
capacitors are their ESR (equivalent series resistance) and
ESL (equivalent series inductance) values. For best transient
response, a combination of low value/high frequency and
bulk capacitors placed close to the load will be required.
be sufficient. X5R or X7R dielectrics ceramic capacitors are
recommended.
Soft-Start Capacitor Selection
capacitor connected from the SS pin to AGND, which can
be calculated by:
where:
to use this capacitor.
DV
An output capacitor has two main functions: it filters the
The ESR of the output capacitor and the peak-to-peak
The output voltage ripple is given by the following
ESR is the equivalent series resistance of the output
The output capacitor value can by expressed by:
These components must be selected and placed carefully
For most applications, a 22 mF ceramic capacitor should
The soft-start time is programmed by an external
- t
Note: See the “Sequencing and Tracking” section on how
OUT
V
SS
F
0.45
0.5
0.5
[V]
max
+
is the soft-start/stop interval.
f
C
SW
V
OUT
OUT
@ L
I
+
O(rec)
@ 1 *
8 @ f
[A]
C
3
3
3
SS
max
SW
[
V
@ DV
V
t
OUT
SS
IN
Package
@ 8.75 mA
0.8 V
SMC
SMC
SMA
OUT
@ ESR )
DI
L
* DI
L
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@ ESR
8 @ f
Web
SW
1
@ C
OUT