ncp3120 ON Semiconductor, ncp3120 Datasheet - Page 17

• Manufacturer Part Number: ncp3120
• Description: Dual 2.0 A, Step-down Dc/dc Switching Regulator
• Manufacturer: ON Semiconductor

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.
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
Table 5. Switching Frequency Selection
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
Designing a system without proper power supply
Freq. [kHz]
RT [kW]
REF
is the circuit's internal voltage reference, which
Figure 31. Feedback divider
open
R
200
2
+ R
1
250
649
V
R1
R2
V
OUT
OUT
V
* V
REF
V
300
316
FB
REF
350
205
http://onsemi.com
400
154
NCP3120
17
450
121
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.
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
PLD devices) is drastically reduced. The failure is often a
result of high currents flowing to the pins or the high voltage
difference between pins.
supplies generating different voltage levels for core and I/O
peripherals over time. NCP3120 offers ratiometric
sequencing, sequential sequencing and tracking sections to
manage the output voltages behavior during start-up and
power-down. Basically, the DSP, FPGA, and PLD
manufacturers do not specify the method of power
sequencing, but they do specify restrictions on the time or
voltage differences during power-up and power-down. The
power-up sequence for microprocessors should be finished
approximately within a few seconds to prevent the risks
mentioned above. For more information, see the
microprocessor manufacturers' datasheets.
In that case, the signal processors require multiple power
600
500
400
300
200
100
0
200 250 300 350 400 450 500 550 600 650 700 750
Figure 32. Switching Frequency Selection
500
100
84.5
550
73.2
600
freq [kHz]
64.9
650
57.6
700
52.3
750