ncp1570d ON Semiconductor, ncp1570d Datasheet - Page 11

• Manufacturer Part Number: ncp1570d
• Description: Low Voltage Synchronous Buck Controller
• Manufacturer: ON Semiconductor

where:
number of output capacitors can be found by using the
formula:
where:
be verified and compared to the value assigned by the
designer:
the following formula:
Selection of the Input Inductor
not disturb the input voltage. One method of achieving this
is by using an input inductor and a bypass capacitor. The
input inductor isolates the supply from the noise generated
in the switching portion of the buck regulator and also limits
the inrush current into the input capacitors upon power up.
The inductor’s limiting effect on the input current slew rate
becomes increasingly beneficial during load transients. The
worst case is when the load changes from no load to full load
(load step), a condition under which the highest voltage
change across the input capacitors is also seen by the input
inductor. The inductor successfully blocks the ripple current
while placing the transient current requirements on the input
bypass capacitor bank, which has to initially support the
sudden load change.
therefore:
where:
that at least 40 dB attenuation is obtained at the regulator
switching frequency. The LC filter is a double−pole network
with a slope of −2.0, a roll−off rate of −40 dB/dec, and a
corner frequency:
Once the maximum allowable ESR is determined, the
The actual output voltage deviation due to ESR can then
Similarly, the maximum allowable ESL is calculated from
A common requirement is that the buck controller must
The minimum inductance value for the input inductor is
L
ΔV = voltage seen by the input inductor during a full load
(dI/dt)
The designer must select the LC filter pole frequency so
IN
ΔV
ESR
ESR
= input inductor value;
ESR
swing;
MAX
CAP
MAX
Number of capacitors +
= change in output voltage due to ESR (assigned
by the designer)
= maximum ESR per capacitor (specified in
DV ESR + DI OUT
= maximum allowable input current slew rate.
= maximum allowable ESR.
manufacturer’s data sheet).
ESL MAX +
ESR MAX +
L IN +
(dI dt) MAX
DV ESL
DV
DV ESR
DI OUT
DI
ESR MAX
ESR MAX
ESR CAP
Dt
http://onsemi.com
11
where:
Selection of the Output Inductor
output inductor. Maximum load current, core and winding
losses, ripple current, short circuit current, saturation
characteristics, component height and cost are all variables
that the designer should consider. However, the most
important consideration may be the effect inductor value has
on transient response.
a current transient is defined as the product of the current
step and the output filter capacitor ESR. Choosing the
inductor value appropriately can minimize the amount of
energy that must be transferred from the inductor to the
capacitor or vice−versa. In the subsequent paragraphs, we
will determine the minimum value of inductance required
for our system and consider the trade−off of ripple current
vs. transient response.
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.
I OUT(MAX) + I SWITCH(MAX) *
L = input inductor;
C = input capacitor(s).
There are many factors to consider when choosing the
The amount of overshoot or undershoot exhibited during
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
L (MIN) +
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
f C +
in these calculations with little error.
IN(MIN)
( V IN * V OUT ) V OUT
2p
( f OSC )( L )( V IN )
1
, it is possible to use the
V IN(MAX) * V OUT V OUT
LC
2 f OSC L V IN(MAX)
OUT
to V
IN
. Thus,