ncp3122 ON Semiconductor, ncp3122 Datasheet - Page 17

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

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:
the output voltage is shown in the following equation:
where:
equals 0.8 V.
between efficiency and output voltage accuracy. For high
values of R1, there is less current consumption in the
feedback network. However, the trade-off is output voltage
accuracy due to the bias current in the error amplifier. Once
R1 has been determined, R2 can be calculated.
Selecting the Switching Frequency
component size and power losses. Operation at higher
switching frequencies allows the use of smaller inductor and
capacitor values. Nevertheless, it is common to select lower
Table 4. Output Voltage Setting
Table 5. Switching Frequency Selection
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
The relationship between the resistor divider network and
V
Resistor R1 is selected based on a design trade-off
Selecting the switching frequency is a trade-off between
RT [kW]
[kHz]
Freq
REF
V
R
R
OUT
1
2
is the circuit's internal voltage reference, which
[kW]
[kW]
[V]
open
200
Figure 31. Feedback divider
R
2
+ R
300
316
180
20
8
1
V
400
154
R1
R2
V
OUT
OUT
V
* V
REF
360
500
100
7.5
43
V
FB
REF
73.2
600
130
20
6
57.6
700
http://onsemi.com
47.5
800
NCP3122
68
13
5
17
40.2
900
where:
to use this capacitor.
Output Voltage Programming
Thus, if a resistor divider circuit is placed across the
feedback pin to V
voltage in proportion to the resistor divider network in order
to maintain 0.8 V at the FB pin.
frequency operation because a higher frequency results in
lower efficiency due to MOSFET gate charge losses.
Additionally, the use of smaller inductors at higher
frequencies results in higher ripple current, higher output
voltage ripple, and lower efficiency at light load currents.
The value of the oscillator resistor is designed to be linearly
related to the switching period. There are two ways to
determine the RT resistor value: by using the standard curve
shown in Figure 32 or by using Table 5. The frequency on
the RT pin will set the master oscillator. The actual operating
frequency on each channel will be one-half the master
oscillator.
200
180
160
140
120
100
- t
Note: See the “Sequencing and Tracking” section on how
The controller will maintain 0.8 V at the feedback pin.
80
60
40
20
0
200
SS
300
75
4
1000
34.8
Figure 32. Switching Frequency Selection
is the soft-start/stop interval.
1200
26.7
600
3.3
47
15
OUT
C
, the controller will regulate the output
SS
1400
21.5
[
1000
freq [kHz]
t
SS
2.5
51
24
1600
18.2
@ 8.75 mA
0.8 V
1400
1800
15.8
1.8
20
16
2000
1800
13.3
1.2
10
20
2200
11.8
2200