NCP1575 ON Semiconductor, NCP1575 Datasheet - Page 12

• Manufacturer Part Number: NCP1575
• Description: Low Voltage Synchronous Buck Controller
• Manufacturer: ON Semiconductor

Series Resistance (ESR), and Equivalent Series Inductance
(ESL). For best transient response, a combination of low
value/high frequency and bulk capacitors placed close to the
load will be required.
maximum voltage transient allowed during load transitions
has to be specified. The output capacitors must hold the
output voltage within these limits since the inductor current
can not change with the required slew rate. The output
capacitors must therefore have a very low ESL and ESR.
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:
be verified and compared to the value assigned by the
designer:
the following formula:
In order to determine the number of output capacitors the
The voltage change during the load current transient is:
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
Similarly, the maximum allowable ESL is calculated from
DI
DI
Dt = load transient duration time;
ESL = Maximum allowable ESL including capacitors,
ESR = Maximum allowable ESR including capacitors
t
DV
ESR
ESR
TR
DV OUT + DI OUT
OUT
OUT
ESR
= output voltage transient response time.
CAP
MAX
/ Dt = 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 +
ESL
DV ESL
Dt
DV ESR
DI OUT
) ESR )
DI
ESR MAX
ESR MAX
ESR CAP
Dt
C OUT
t TR
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NCP1575
12
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:
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.
A common requirement is that the buck controller must
The minimum inductance value for the input inductor is
L
DV = 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
= input inductor value;
swing;
MAX
= maximum allowable input current slew rate.
L IN +
f C +
(dI dt) MAX
2p
DV
1
LC