NCP1575D ON Semiconductor, NCP1575D Datasheet - Page 13

• Manufacturer Part Number: NCP1575D
• Description: IC CTLR SYNCH BUCK LV 8-SOIC
• Manufacturer: ON Semiconductor
• Type: Step-Down (Buck)r

Specifications of NCP1575D

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: Adj to 0.98V
• Frequency - Switching: 200kHz, programmable
• Voltage - Input: 9 ~ 20 V
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Mounting Style: SMD/SMT
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -
• Power - Output: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

voltage, output voltage and output current must be known.
Most computer applications use reasonably well regulated
bulk power supplies so that, while the equations below
specify V
nominal value of V
current mode is defined as the load current plus ripple
current.
is a function of voltage across the inductor, switch FET
on−time and the inductor value. FET on−time can be defined
as the product of duty cycle and switch frequency, and duty
cycle can be defined as a ratio of V
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
I
response times to increase. The response times for both
increasing and decreasing current steps are shown below.
I OUT(MAX) + I SWITCH(MAX) *
RIPPLE
In order to choose the minimum value of inductance, input
Current in the inductor while operating in the continuous
The ripple current waveform is triangular, and the current
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
T RESPONSE(INCREASING) +
L (MIN) +
T RESPONSE(DECREASING) +
:
L (RIPPLE) +
IN(MAX)
I RIPPLE +
( f OSC )( I SWITCH(MAX) )( V IN(MIN) )
I L + I LOAD ) I RIPPLE
or V
IN
( V IN(MIN) * V OUT ) V OUT
SWITCH(MAX)
in these calculations with little error.
( f OSC )( I RIPPLE )( V IN(MIN) )
( V IN(MIN) * V OUT ) V OUT
IN(MIN)
( V IN * V OUT ) V OUT
( f OSC )( L )( V IN )
, it is possible to use the
V IN(MAX) * V OUT V OUT
OUT
2 f OSC L V IN(MAX)
with a desired value of
( V IN * V OUT )
to V
( L )( DI OUT )
( L )( DI OUT )
( V OUT )
IN
. Thus,
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NCP1575
13
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:
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
IN(AVE)
CC
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
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 23. The average
Input capacitor current is positive into the capacitor when
V
IN
Bypass Filtering
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 W resistor and a 0.47 mF
I IN(AVE) + I OUT
DT(inductor) + (Ra)(P D )
P D + (I
Figure 23.
C
CONTROL
IN
INPUT
2
L
) ( ESR L )
ESR C V IN * V OUT V OUT
V OUT
L
V IN
OUT
f OSC L V IN
IN
for any given
C
OUT
V
OUT
CC