TD1583P 데이터 시트보기 (PDF) - Unspecified
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TD1583P Datasheet PDF : 16 Pages
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Techcode®
3A 380KHz 28V PWM Buck DC/DC Converter
Function Description(Cont.)
DATASHEET
TD1583
output voltage down by the ratio:
VFB = VOUT * R2 / (R1 + R2)
Thus the output voltage is:
VOUT = 1.222 * (R1 + R2) / R2
R2 can be as high as 100KΩ, but a typicalvalue is 10KΩ.
Using that value, R1 is determined by:
R1 ~= 8.18 * (VOUT – 1.222) (KΩ)
For example, for a 3.3V output voltage, R2 is10KΩ, and
R1 is 17KΩ.
Inductor
The inductor is required to supply constant current to the
output load while being driven by the switched input
voltage. A larger value inductor results in less ripple
current that in turn results in lower output ripple voltage.
However, the larger value inductor has a larger physical
size, higher series resistance, and/or lower saturation
current. Choose an inductor that does not saturate under
the worst-case load conditions. A good rule for
determining the inductance is to allow the peak-to-peak
ripple current in the inductor to be approximately 30% of
the maximum load current. Also, make sure that the peak
inductor current (the load current plus half the
peak-to-peak inductor ripple current) is below the TBDA
minimum current limit. The inductance value can be
calculated by the equation:
L = (VOUT) * (VIN-VOUT) / VIN * f * ∆I
Where VOUT is the output voltage, VIN is the input
voltage, f is the switching frequency, and ∆I is the
peak-to-peak inductor ripple current.
Input Capacitor
The input current to the step-down converter is
discontinuous, and so a capacitor is required to supply
the AC current to the step-down converter while
maintaining the DC input voltage. A low ESR capacitor is
required to keep the noise at the IC to a minimum.
Ceramic capacitors are preferred, but tantalum or
low-ESR electrolytic capacitors may alsosuffice.
The input capacitor value should be greater than 10μF.
The capacitor can be electrolytic, tantalum or ceramic.
However since it absorbs the input switching current it
requires an adequate ripple current rating. Its RMS
current rating should be greater than approximately 1/2
of the DC load current.For insuring stable operation
should be placed as close to the IC as possible.
Alternately a smaller high quality ceramic 0.1μF
capacitor may be placed closer to the IC and a larger
capacitor placed further away. If using this technique, it is
recommended that the larger capacitor be a tantalum or
electrolytic type. All ceramic capacitors should be places
close to the TD1583.
Output Capacitor
The output capacitor is required to maintain the DC
output voltage. Low ESR capacitors are preferred to
keep the output voltage ripple low. The characteristics of
the output capacitor also affect the stability of the
regulation control system. Ceramic, tantalum, or low
ESR electrolytic capacitors are recommended. In the
case of ceramic capacitors, the impedance at the
switching frequency is dominated by the capacitance,
and so the output voltage ripple is mostly independent of
the ESR. The output voltage ripple is estimated to be:
VRIPPLE ~= 1.4 * VIN * (fLC/fSW)^2
Where VRIPPLE is the output ripple voltage, VIN is the
input voltage, fLC is the resonant frequency of the LC
filter, fSW is the switching frequency. In the case of
tanatalum or low-ESR electrolytic capacitors, the ESR
dominates the impedance at the switching frequency,
and so the output ripple iscalculated as:
VRIPPLE ~= ∆I * RESR
Where VRIPPLE is the output voltage ripple, ∆I is
December, 23, 2009.
Techcode Semiconductor Limited
12
www.techcodesemi.com
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