ncp1536 ON Semiconductor, ncp1536 Datasheet - Page 12

• Manufacturer Part Number: ncp1536
• Description: 3.0 A, Step-down Switching Regulator
• Manufacturer: ON Semiconductor

Catch Diode
Locate the Catch Diode Close to the NCP1536
a fast diode to provide a return path for the inductor current
when the switch turns off. This diode must be located close
to the NCP1536 using short leads and short printed circuit
traces to avoid EMI problems.
Use a Schottky or a Soft Switching
Ultra−Fast Recovery Diode
losses within switching power supplies, choosing the
rectifier that best fits into the converter design is an
important process. Schottky diodes provide the best
performance because of their fast switching speed and low
forward voltage drop.
voltage applications (5.0 V and lower). Another choice
could be Fast−Recovery, or Ultra−Fast Recovery diodes. It
has to be noted, that some types of these diodes with an
abrupt turnoff characteristic may cause instability or
EMI troubles.
can better fulfill some quality, low noise design requirements.
Table 1 provides a list of suitable diodes for the NCP1536
regulator. Standard 50/60 Hz rectifier diodes, such as the
1N4001 series or 1N5400 series are NOT suitable.
Inductor
switching power supply designs. The style of the core and
the winding technique used in the magnetic component’s
design has a great influence on the reliability of the overall
power supply.
high voltage spikes generated by the rate of transitions in
current within the switching power supply, and the
possibility of core saturation can arise during an abnormal
operational mode. Voltage spikes can cause the
semiconductors to enter avalanche breakdown and the part
can instantly fail if enough energy is applied. It can also
cause significant RFI (Radio Frequency Interference) and
EMI (Electro−Magnetic Interference) problems.
Continuous and Discontinuous Mode of Operation
the continuous and the discontinuous modes of operation.
The regulator works in the continuous mode when loads are
relatively heavy, the current flows through the inductor
continuously and never falls to zero. Under light load
conditions, the circuit will be forced to the discontinuous
mode when inductor current falls to zero for certain period
of time (see Figure 10 and Figure 11). Each mode has
distinctively different operating characteristics, which can
affect the regulator performance and requirements. In many
cases the preferred mode of operation is the continuous
mode. It offers greater output power, lower peak currents in
the switch, inductor and diode, and can have a lower output
The NCP1536 is a step−down buck converter; it requires
Since the rectifier diodes are very significant sources of
They provide the best efficiency especially in low output
A fast−recovery diode with soft recovery characteristics
The magnetic components are the cornerstone of all
Using an improper or poorly designed inductor can cause
The NCP1536 step−down converter can operate in both
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NCP1536
12
ripple voltage. On the other hand it does require larger
inductor values to keep the inductor current flowing
continuously, especially at low output load currents and/or
high input voltages.
selection guide for the NCP1536 regulator was added to this
data sheet (Figures 5 through 9). This guide assumes that the
regulator is operating in the continuous mode, and selects an
inductor that will allow a peak−to−peak inductor ripple
current to be a certain percentage of the maximum design
load current. This percentage is allowed to change as
different design load currents are selected. For light loads
(less than approximately 300 mA) it may be desirable to
operate the regulator in the discontinuous mode, because the
inductor value and size can be kept relatively low.
Consequently, the percentage of inductor peak−to−peak
current increases. This discontinuous mode of operation is
perfectly acceptable for this type of switching converter.
Any buck regulator will be forced to enter discontinuous
mode if the load current is light enough.
Selecting the Right Inductor Style
are core material, cost, the output power of the power supply,
the physical volume the inductor must fit within, and the
amount of EMI (Electro−Magnetic Interference) shielding
that the core must provide. The inductor selection guide
covers different styles of inductors, such as pot core, E−core,
toroid and bobbin core, as well as different core materials
such as ferrites and powdered iron from different
manufacturers.
be the best choice. Since the magnetic flux is contained
within the core, it generates less EMI, reducing noise
problems in sensitive circuits. The least expensive is the
bobbin core type, which consists of wire wound on a ferrite
rod core. This type of inductor generates more EMI due to
the fact that its core is open, and the magnetic flux is not
contained within the core.
Waveform
Waveform
To simplify the inductor selection process, an inductor
Some important considerations when selecting a core type
For high quality design regulators the toroid core seems to
Inductor
Current
Current
Switch
Power
Figure 10. Continuous Mode Switching Current
2.0 A
2.0 A
0 A
0 A
HORIZONTAL TIME BASE: 5.0 ms/DIV
Waveforms