ML4866 データシートの表示（PDF） - Micro Linear Corporation
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ML4866 Datasheet PDF : 8 Pages
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DESIGN CONSIDERATIONS
INDUCTOR SELECTION
Figure 1 shows the inductor current in a step-down
converter operating in CCM. Note that the inductor
current does not reach zero during each switching cycle.
This is unlike discontinuous conduction mode (DCM)
where the inductor current is allowed to reach zero. CCM
operation generally results in lower peak to peak output
ripple voltage and higher circuit efficiencies because of
lower peak and RMS currents in the switching FETs and
buck inductor. The minimum value of inductance required
for CCM operation with a 6.5V input and a load range of
100mA to 500mA is:
L > VOUT (VIN (MAX) - VOUT )
2 VIN (MAX) IOUT (MIN) fSW
(1)
L > 3.3V (6.5V - 3.3V) > 68mH
2 6.5V 100mA 120kHz
To guarantee reliable operation, the peak inductor current
must be between 80% and 85% of its maximum rated
value. This value is the sum of the inductor peak to peak
current and the maximum output current:
IL(P–P)
=
2
VOUT (MAX) (VIN (MIN) - VOUT
VIN (MIN) fSW (MIN) L
(MAX) )
(2)
IL(P–P)
=
2 3.465V (4.0V - 3.465V)
4.0V 90kHz 100mH
= 103mA
IL(PEAK)
= IOUT
(MAX)
+
VOUT
(MAX) (VIN (MIN) - VOUT
VIN (MIN) fSW L
(MAX) )
(3)
IL(PEAK)
= IOUT (MAX)
+
3.465V (4.0V - 3.465V)
4.0V 120kHz 100mH
= 550mA
For the highest efficiency, inductor core and copper losses
must be minimized. Good high frequency core material
such as Kool-Mu, ferrite or Molyperm are popular choices
for this converter. Disregarding physical size
requirements, the lowest loss inductor will generally be
the one with the highest peak current rating.
Figure 2 displays the efficiency of the ML4866 under
various input voltage and output current conditions. These
results were obtained using a Coiltronics CTX100-4
inductor having the following specifications:
Nominal Inductance - 100µH
Peak Current Rating - 950mA
DC Resistance - 175mW
ML4866
100
98
96
94
92 IOUT = 10mA
IOUT = 100mA
90
88
IOUT = 500mA
86
3.5 4.0 4.5 5.0 5.5 6.0 6.5
INPUT VOLTAGE (V)
Figure 2. Efficiency vs. Input Voltage
A partial listing of inductor manufacturers with standard
parts which meet the criteria for use with the ML4866 is
given below.
Coiltronics
Dale
Coilcraft
XFMRS, Inc
Sumida
(561) 241-7876
(605) 665-9301
(847) 639-6400
(317) 834-1066
(847) 956-0666
CAPACITOR SELECTION
A typical digital system requires a peak to peak output
ripple voltage of no greater than 1% to 3% of the nominal
output voltage. In a step-down converter, the largest
contributor to ripple voltage is almost always the product
of the inductor peak-to-peak current times the output
capacitor’s equivalent series resistance. To select the
correct capacitor, first calculate the minimum
capacitance value required:
COUT
>
8
VOUT (VIN (MAX) - VOUT )
VP-P (MAX) VIN (MAX) L fSW 2
(4)
COUT
>
8
3.3 (6.5V - 3.3V)
33mV 6.5V 100mH 120kHz2
>
4.27mF
Next, calculate the maximum permissible ESR of the
output capacitor:
ESR < (0.033) < 0.33W
(0.1)
(5)
When limited space is available, tantalum capacitors are
the best choice. Electrolytic capacitors can be used and
will be less expensive, but the ESR for low capacitance
values as needed here will be much higher than for the
same value tantalum. Table 2 lists the ESR values for a
number of general purpose tantalum capacitors which are
widely available from a number of sources. A 47µF
capacitor was chosen for the design example.
5
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