CS51313 View Datasheet(PDF) - ON Semiconductor
Part Name
Description
MFG CO.
CS51313
ON Semiconductor
'CS51313' PDF : 23 Pages View PDF
CS51313
where:
ΔtTR = the output voltage transient response time
(assigned by the designer);
ΔVCAP = output voltage deviation due to output capacitor
discharge;
ΔI = Load step.
The total change in output voltage as a result of a load
current transient can be verified by the following formula:
DVOUT + DVESR ) DVESL ) DVCAP
Step 3: Selection of the Duty Cycle,
Switching Frequency, Switch On−Time (TON)
and Switch Off−Time (TOFF)
The duty cycle of a buck converter (including parasitic
losses) is given by the formula:
Duty
Cycle
+
D
+
VOUT ) (VHFET
VIN ) VLFET
)
*
VL ) VDROOP)
VHFET * VL
where:
VOUT = buck regulator output voltage;
VHFET = high side FET voltage drop due to RDS(ON);
VL = output inductor voltage drop due to inductor wire DC
resistance;
VDROOP = droop (current sense) resistor voltage drop;
VIN = buck regulator input voltage;
VLFET = low side FET voltage drop due to RDS(ON).
Step3a: Calculation of Switch On−Time
The Switch On−Time (time during which the switching
MOSFET in a synchronous buck topology is conducting) is
determined by:
TON
+
Duty Cycle
FSW
where FSW = regulator switching frequency selected by the
designer.
Higher operating frequencies allow the use of smaller
inductor and capacitor values. Nevertheless, it is common to
select lower frequency operation because a higher frequency
results in lower efficiency due to MOSFET gate charge
losses. Additionally, the use of smaller inductors at higher
frequencies results in higher ripple current, higher output
voltage ripple, and lower efficiency at light load currents.
Step 3b: Calculation of Switch Off−Time
The Switch Off−Time (time during which the switching
MOSFET is not conducting) can be determined by:
TOFF
+
1.0
FSW
*
TON
The COFF capacitor value has to be selected in order to set
the Off−Time, TOFF, above:
COFF
+
Period
(1.0
3980
*
D)
where:
3980 is a characteristic factor of the CS51313;
D = Duty Cycle.
Step 4: Selection of the Output Inductor
The inductor should be selected based on its inductance,
current capability, and DC resistance. Increasing the
inductor value will decrease output voltage ripple, but
degrade transient response. There are many factors to
consider in selecting the inductor including cost, efficiency,
EMI and ease of manufacture. The inductor must be able to
handle the peak current at the switching frequency without
saturating, and the copper resistance in the winding should
be kept as low as possible to minimize resistive power loss.
There are a variety of materials and types of magnetic cores
that could be used for this application. Among them are
ferrites, molypermalloy cores (MPP), amorphous and
powdered iron cores. Powdered iron cores are very
commonly used. Powdered iron cores are very suitable due
to their high saturation flux density and have low loss at high
frequencies, a distributed gap and exhibit very low EMI.
The inductor value can be determined by:
L
+
(VIN
*
VOUT)
DI
tTR
where:
VIN = input voltage;
VOUT = output voltage;
tTR = output voltage transient response time (assigned by
the designer);
ΔI = load transient.
The inductor ripple current can then be determined:
DIL + VOUT L TOFF
where:
ΔIL = inductor ripple current;
VOUT = output voltage;
TOFF = switch Off−Time;
L = inductor value.
The designer can now verify if the number of output
capacitors from Step 2 will provide an acceptable output
voltage ripple (1.0% of output voltage is common). The
formula below is used:
DIL
+
DVOUT
ESRMAX
Rearranging we have:
ESRMAX
+
DVOUT
DIL
where
ESRMAX = maximum allowable ESR;
ΔVOUT = 1.0% × VOUT = maximum allowable output
voltage ripple ( budgeted by the designer );
ΔIL = inductor ripple current;
VOUT = output voltage.
The number of output capacitors is determined by:
Number
of
capacitors
+
ESRCAP
ESRMAX
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