NCP1573D ONSEMI [ON Semiconductor], NCP1573D Datasheet - Page 12

• Manufacturer Part Number: NCP1573D
• Description: Low Voltage Synchronous Buck Controller
• Manufacturer: ONSEMI [ON Semiconductor]

half of the peak current. Peak current must be less than the
maximum rated FET switch current, and must also be less
than the inductor saturation current. Thus, the maximum
output current can be defined as:
maximum switch current, the minimum inductance required
can be determined.
provide the full rated switch current as inductor ripple current,
and will usually result in inefficient system operation. The
system will sink current away from the load during some
portion of the duty cycle unless load current is greater than
half of the rated switch current. Some value larger than the
minimum inductance must be used to ensure the converter
does not sink current. Choosing larger values of inductor will
reduce the ripple current, and inductor value can be designed
to accommodate a particular value of ripple current by
replacing I
response times to increase. The response times for both
increasing and decreasing current steps are shown below.
ripple voltage the system can tolerate. Output ripple voltage
is defined as the product of the output ripple current and the
output filter capacitor ESR.
V RIPPLE + ESR C I RIPPLE +
output inductors. Power dissipation is proportional to the
square of inductor current:
surrounding it is defined as the product of power dissipation
and thermal resistance to ambient:
I OUT(MAX) + I SWITCH(MAX) *
Peak inductor current is defined as the load current plus
Since the maximum output current must be less than the
This equation identifies the value of inductor that will
However, reducing the ripple current will cause transient
Inductor value selection also depends on how much output
Thus, output ripple voltage can be calculated as:
Finally, we should consider power dissipation in the
The temperature rise of the inductor relative to the air
T RESPONSE(INCREASING) +
L (MIN) +
T RESPONSE(DECREASING) +
L (RIPPLE) +
SWITCH(MAX)
I RIPPLE +
( f OSC )( I SWITCH(MAX) )( V IN(MIN) )
DT(inductor) + (Ra)(P D )
P D + (I
( V IN(MIN) * V OUT ) V OUT
( f OSC )( I RIPPLE )( V IN(MIN) )
( V IN(MIN) * V OUT ) V OUT
with a desired value of I
( V IN * V OUT ) V OUT
( f OSC )( L )( V IN )
2
L
) ( ESR L )
ESR C V IN * V OUT V OUT
V IN(MAX) * V OUT V OUT
2 f OSC L V IN(MAX)
( V IN * V OUT )
f OSC L V IN
( L )( DI OUT )
( L )( DI OUT )
( V OUT )
RIPPLE
http://onsemi.com
:
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approximately 45°C/W. The inductor temperature is given as:
V
and the V
capacitor should be sufficient to ensure the controller IC does
not operate erratically due to injected noise, and will also
supply reserve charge for the onboard gate drivers.
Input Filter Capacitors
minimizes supply voltage variations due to changes in current
flowing through the switch FETs. These capacitors must be
chosen primarily for ripple current rating.
current flowing in the input inductor L
output current is:
the switch FETs are off, and negative out of the capacitor
when the switch FETs are on. When the switches are off,
I
capacitor current is equal to the per−phase output current
minus I
to the output ripple current, we can approximate the input
capacitor current waveform as a square wave. We can then
calculate the RMS input capacitor ripple current:
I RMS(CIN) +
worst case input ripple current. This will require several
capacitors in parallel. In addition to the worst case current,
attention must be paid to the capacitor manufacturer’s
derating for operation over temperature.
5 V to 3.3 V conversion at 10 A at an ambient temperature
of 60°C. Efficiency of 80% is assumed. Average input
current in the input filter inductor is:
IN(AVE)
CC
Ra for an inductor designed to conduct 20 A to 30 A is
A small RC filter should be added between module V
The input filter capacitors provide a charge reservoir that
Consider the schematic shown in Figure 22. The average
Input capacitor current is positive into the capacitor when
The input capacitance must be designed to conduct the
As an example, let us define the input capacitance for a
V
IN
Bypass Filtering
IN(AVE)
flows into the capacitor. When the switches are on,
T(inductor) + DT(inductor) ) Tambient
CC
I
IN(AVE)
I
RMS(CIN)
L
IN
input to the IC. A 10 Ω resistor and a 0.47 μF
. If we ignore the small current variation due
I IN(AVE) + I OUT
I
2
IN(AVE)
I OUT per phase * I IN(AVE) 2 * I
Figure 22.
)
C
CONTROL
IN
INPUT
V OUT
V IN
V OUT
L
V IN
OUT
IN
for any given
C
OUT
V
2
IN(AVE)
OUT
CC