ncp3121 ON Semiconductor, ncp3121 Datasheet - Page 17

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

Output Voltage Programming
Thus, if a resistor divider circuit is placed across the
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
Sequencing of Output Voltages
supplies with different voltage levels. These systems often
require voltage sequencing between the core power supply
and the I/O power supply. Without proper sequencing,
latch-up failure or excessive current draw may occur that
could result in damage to the processor's I/O ports or the I/O
ports of a supporting system device such as memory, an
FPGA or a data converter. To ensure that the I/O loads are
not driven until the core voltage is properly biased, tracking
of the core supply and the I/O supply voltage is necessary.
Table 4. Output Voltage Setting
Table 5. Switching Frequency Selection
The controller will maintain 0.8 V at the feedback pin.
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
Some microprocessors and DSP chips need two power
Freq. [kHz]
RT [kW]
REF
V
R
R
OUT
1
2
is the circuit's internal voltage reference, which
[kW]
[kW]
[V]
Figure 29. Feedback divider
open
R
200
2
+ R
180
20
8
1
250
649
V
R1
R2
V
OUT
OUT
V
* V
REF
360
7.5
43
V
300
316
FB
REF
350
205
130
20
6
http://onsemi.com
400
154
NCP3121
68
13
5
17
450
121
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 30 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.
sequencing for signal processing devices like DSPs, FPGAs,
and PLDs may create risks as to reliability or proper
functionality. The risk comes when there are active and
inactive power supply rails on the device for a long time.
During this time, the ESD structures, internal circuits and
components are stressed from interference between
different voltages (from the two separate power supply
rails). When these conditions persist on multi-supply
devices for long time periods (this is a cumulative
phenomenon), the life of the products (DSP, FPGA, and
Designing a system without proper power supply
600
500
400
300
200
100
0
300
200 250 300 350 400 450 500 550 600 650 700 750
75
4
Figure 30. Switching Frequency Selection
500
100
84.5
550
3.3
47
15
OUT
, the controller will regulate the output
73.2
600
freq [kHz]
2.5
51
24
64.9
650
1.8
20
16
57.6
700
1.2
10
20
52.3
750