NCP1573D ONSEMI [ON Semiconductor], NCP1573D Datasheet - Page 11

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

where:
change in output voltage due to ESR, ESL, and output
capacitor discharging or charging. Empirical data indicates
that most of the output voltage change (droop or spike
depending on the load current transition) results from the
total output capacitor ESR.
according to the formula:
where:
number of output capacitors can be found by using the
formula:
where:
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
The designer has to independently assign values for the
The maximum allowable ESR can then be determined
Once the maximum allowable ESR is determined, the
The actual output voltage deviation due to ESR can then be
Similarly, the maximum allowable ESL is calculated from
A common requirement is that the buck controller must
ΔI
ΔI
Δt = load transient duration time;
ESL = Maximum allowable ESL including capacitors,
ESR = Maximum allowable ESR including capacitors
t
ΔV
ESR
ESR
TR
OUT
OUT
ESR
= output voltage transient response time.
CAP
MAX
/ Δt = load current slew rate;
= load transient;
circuit traces, and vias;
and circuit traces;
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 ESR.
manufacturer’s data sheet).
ESL MAX +
ESR MAX +
DV ESL
DV ESR
DI OUT
DI
ESR MAX
ESR MAX
ESR CAP
Dt
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11
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:
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
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
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
IN
= input inductor value;
swing;
MAX
IN(MAX)
= maximum allowable input current slew rate.
I L + I LOAD ) I RIPPLE
or V
IN
L IN +
f C +
in these calculations with little error.
IN(MIN)
(dI dt) MAX
2p
DV
1
, it is possible to use the
LC
OUT
to V
IN
. Thus,