CS51313 View Datasheet(PDF) - ON Semiconductor
Part Name
Description
MFG CO.
CS51313
ON Semiconductor
'CS51313' PDF : 23 Pages View PDF
CS51313
where ESRCAP = maximum ESR per capacitor (specified in
manufacturer’s data sheet).
The designer must also verify that the inductor value
yields reasonable inductor peak and valley currents (the
inductor current is a triangular waveform):
IL(PEAK)
+
IOUT
)
DIL
2.0
where:
IL(PEAK) = inductor peak current;
IOUT = load current;
ΔIL = inductor ripple current.
IL(VALLEY)
+
IOUT
*
DIL
2.0
where IL(VALLEY) = inductor valley current.
Given the requirements of an application such as a buck
converter, it is found that a toroid powdered iron core is quite
suitable due to its low cost, low core losses at the switching
frequency, and low EMI.
Step 5: Selection of the Input Capacitors
These components must be selected and placed carefully
to yield optimal results. Capacitors should be chosen to
provide acceptable ripple on the input supply lines. A key
specification for input capacitors is their ripple current
rating. The input capacitor should also be able to handle the
input RMS current IIN(RMS).
The combination of the input capacitors CIN discharges
during the on−time.
The input capacitor discharge current is given by:
ICINDIS(RMS) +
IL(PEAK)2
ȡ ȣ ) (IL(PEAK) IL(VALLEY))
ǸȧȢ ȧȤ ) IL(VALLEY)2
D
3.0
where:
ICINDIS(RMS) = input capacitor discharge current;
IL(PEAK) = inductor peak current;
IL(VALLEY) = inductor valley current.
CIN charges during the off−time, the average current
through the capacitor over one switching cycle is zero:
ICIN(CH) + ICIN(DIS)
D
1.0 * D
where:
ICIN(CH) = input capacitor charge current;
ICIN(DIS) = input capacitor discharge current;
D = Duty Cycle.
The total Input RMS current is:
Ǹ ICIN(RMS) +
(ICIN(DIS)2 D)
) (ICIN(CH)2 (1.0 * D))
The number of input capacitors required is then
determined by:
NCIN
+
ICIN(RMS)
IRIPPLE
where:
NCIN = number of input capacitors;
ICIN(RMS) = total input RMS current;
IRIPPLE = input capacitor ripple current rating (specified
in manufacturer’s data sheets).
The total input capacitor ESR needs to be determined in
order to calculate the power dissipation of the input
capacitors:
ESRCIN
+
ESRCAP
NCIN
where:
ESRCIN = total input capacitor ESR;
ESRCAP = maximum ESR per capacitor (specified in
manufacturer’s data sheets);
NCIN = number of input capacitors.
Once the total ESR of the input capacitors is known, the
input capacitor ripple voltage can be determined using the
formula:
VCIN(RMS) + ICIN(RMS) ESRCIN
where:
VCIN(RMS) = input capacitor RMS voltage;
ICIN(RMS) = total input RMS current;
ESRCIN = total input capacitor ESR.
The designer must determine the input capacitor power
loss in order to ensure there isn’t excessive power
dissipation through these components. The following
formula is used:
PCIN(RMS) + ICIN(RMS)2 ESRCIN
where:
PCIN(RMS) = input capacitor RMS power dissipation;
ICIN(RMS) = total input RMS current;
ESRCIN = total input capacitor ESR.
Step 6: Selection of the Input Inductor
A CPU switching regulator, such as the one in a buck
topology, must not disturb the primary +5.0 V supply. One
method of achieving this is by using an input inductor and
a bypass capacitor. The input inductor isolates the +5.0 V
supply from the noise generated in the switching portion of
the microprocessor 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 CPU 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.
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