NCP1571 Datasheet, PDF (12/16 Pages) ON Semiconductor – Low Voltage Synchronous Buck Controller

NCP1571
Current in the inductor while operating in the continuous
current mode is defined as the load current plus ripple
current.
IL + ILOAD ) IRIPPLE
The ripple current waveform is triangular, and the 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_OUT to V_IN. Thus,
IRIPPLE
+
(VIN * VOUT)VOUT
(fOSC)(L)(VIN)
Peak inductor current is defined as the load current plus
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:
IOUT(MAX)
+
ISWITCH(MAX)
*
ǒ_{VIN(MAX) * VOUT}Ǔ_VOUT
ǒ₂Ǔǒ_fOSCǓǒ_LǓǒ_VIN(MAX)Ǔ
Since the maximum output current must be less than the
maximum switch current, the minimum inductance required
can be determined.
L(MIN)
+
(VIN(MIN) * VOUT)VOUT
(fOSC)(ISWITCH(MAX))(VIN(MIN))
This equation identifies the value of inductor that will
provide the full rated switch current as inductor ripple
current, and will usually result in inefficient system
operation. The system will sink current away from the load
during some portion of the duty cycle unless load current is
greater than half of the rated switch current. Some value
larger than the minimum inductance must be used to ensure
the converter does not sink current. Choosing larger values
of inductor will reduce the ripple current, and inductor value
can be designed to accommodate a particular value of ripple
current by replacing I_SWITCH(MAX) with a desired value of
I_RIPPLE:
L(RIPPLE)
+
(VIN(MIN) * VOUT)VOUT
(fOSC)(IRIPPLE)(VIN(MIN))
However, reducing the ripple current will cause transient
response times to increase. The response times for both
increasing and decreasing current steps are shown below.
TRESPONSE(INCREASING)
+
(L)(DIOUT)
(VIN * VOUT)
TRESPONSE(DECREASING)
+
(L)(DIOUT)
(VOUT)
Inductor value selection also depends on how much output
ripple voltage the system can tolerate. Output ripple voltage
is defined as the product of the output ripple current and the
output filter capacitor ESR.
Thus, output ripple voltage can be calculated as:
VRIPPLE
+
ǒ_ESRCǓǒ_IRIPPLEǓ
+
ǒ_ESRCǓǒ_{VIN * VOUT}Ǔ_VOUT
ǒ_fOSCǓǒ_LǓǒ_VINǓ
Finally, we should consider power dissipation in the
output inductors. Power dissipation is proportional to the
square of inductor current:
PD + (I^2L)(ESRL)
The temperature rise of the inductor relative to the air
surrounding it is defined as the product of power dissipation
and thermal resistance to ambient:
DT(inductor) + (Ra)(PD)
Ra for an inductor designed to conduct 20 A to 30 A is
approximately 45°C/W. The inductor temperature is given as:
T(inductor) + DT(inductor) ) Tambient
V_CC Bypass Filtering
A small RC filter should be added between module V_CC
and the V_CC input to the IC. A 10 W resistor and a 0.47 mF
capacitor should be sufficient to ensure the controller IC does
not operate erratically due to injected noise, and will also
supply reserve charge for the onboard gate drivers.
Input Filter Capacitors
The input filter capacitors provide a charge reservoir that
minimizes supply voltage variations due to changes in current
flowing through the switch FETs. These capacitors must be
chosen primarily for ripple current rating.
L_IN
V_IN
I_IN(AVE)
I_RMS(CIN)
L_OUT
C_IN
CONTROL
INPUT
V_OUT
C_OUT
Figure 24.
Consider the schematic shown in Figure 24. The average
current flowing in the input inductor L_IN for any given
output current is:
IIN(AVE) + IOUT
VOUT
VIN
Input capacitor current is positive into the capacitor when
the switch FETs are off, and negative out of the capacitor
when the switch FETs are on. When the switches are off,
I_IN(AVE) flows into the capacitor. When the switches are on,
capacitor current is equal to the per−phase output current
minus I_IN(AVE). If we ignore the small current variation due
to the output ripple current, we can approximate the input
capacitor current waveform as a square wave. We can then
calculate the RMS input capacitor ripple current:
Ǹ _{ƪ ƫ} ^{IRMS(CIN) +}
I^2IN(AVE)
)
VOUT
VIN
ǒ_{IOUT per phase * IIN(AVE)}Ǔ2 _{* I}^2IN(AVE)
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