ncp3122 ON Semiconductor, ncp3122 Datasheet - Page 16

• Manufacturer Part Number: ncp3122
• Description: Dual 2.0 A, Step-down Dc/dc Switching Regulator
• Manufacturer: ON Semiconductor

voltage. The peak conducting current is clamped by the
current limit of the NCP3122. Use of Schottky barrier diodes
reduces diode reverse recovery input current spikes. For
switching regulators operating at low duty cycles, it is
beneficial to use rectifying diodes with somewhat higher
RMS current ratings (thus lower forward voltages). This is
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:
MBRS2040LT3G
MBRS230LT3G
MBRS240LT3G
SS24T3G
MBRA340T3G
MBRS330T3G
Table 3. Schottky Diode Example
The peak reverse voltage is equal to the maximum input
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
Part Number
D
DSAT
is the voltage drop across the rectifying diode and
is the switch saturation voltage on the power
I
D +
P
RMS
CIN
2 A, 40 V Low V
2 A, 30 V Low V
2 A, 40 V Low V
+ I
+ I
V
I
2 A, 40 V Schottky Rectifier
3 A, 40 V Schottky Rectifier
3 A, 30 V Schottky Rectifier
RMS
IN
LOAD
V
) V
RMS
+
OUT
2 @ ESR
Description
I
D
LOAD
D ( 1 * D )
) V
* V
2
f
f
f
Schottky Rectifier
Schottky Rectifier
Schottky Rectifier
D
DSAT
CIN
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V
NCP3122
RRM
[V]
40
30
40
40
40
30
16
min
because the diode conduction interval is much longer than
that of the transistor. Converter efficiency will be improved
if the voltage drop across the diode is lower. The average
current can be calculated from:
capacitance.
capacitor value. Therefore, the input capacitor can be
estimated by:
Output Capacitor
and provides low impedance for load current changes. The
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.
DV
ESR
The input capacitor voltage ripple depends on the C
The output capacitor filters output inductor ripple current
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:
OUT
V
F
0.43
0.49
0.43
0.45
0.5
0.5
[V]
CIN
max
+
C
f
C
SW
V
is the effective series resistance of the input
IN
OUT
OUT
@ L
+
I
+
I
O(rec)
D(AVG)
f
@ 1 *
SW
8 @ f
[A]
I
2
2
2
2
3
3
LOAD
@ DV
max
+
SW
V
I
IN
@ DV
V
LOAD
OUT
@
IN
Package
V
V
OUT
V
SMC
SMB
SMB
SMB
SMB
SMA
OUT
IN
@ ESR )
DI
IN
V
IN
L
* V
@ 1 *
* DI
OUT
L
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@ ESR
V
8 @ f
V
OUT
IN
Web
SW
1
@ C
OUT
IN