Datasheet
1.8A 380KHZ 20V PWM Buck DC/DC Converter
TD1410C
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 * ΔI
Where VOUT is the output voltage, VIN is the
input voltage, f is the switching frequency, and
ΔI 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 also
suffice.
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 TD1410C.
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 is
calculated as:
VRIPPLE ~= ΔI * RESR
Where VRIPPLE is the output voltage ripple, ΔI is
the inductor ripple current, and RESR is the
equivalent series resistance of the output
capacitors.
Output Rectifier Diode
The output rectifier diode supplies the current
to the inductor when the high-side switch is off.
To reduce losses due to the diode forward
voltage and recovery times, use a Schottky
rectifier.
Table 1 provides the Schottky rectifier part
numbers based on the maximum input voltage
and current rating.
Choose a rectifier who’s maximum reverse
voltage rating is greater than the maximum
input voltage, and who’s current rating is
greater than the maximum load current.
Feedforward Capacitor (CFF)
For output voltages greater than approximately
8V, an additional capacitor is required. The
compensation capacitor is typically between 100
pF and 33 nF, and is wired in parallel
with the output voltage setting resistor, R1. It
provides additional stability for high output
voltages, low input-output voltages, and/or very
low ESR output capacitors, such as solid
tantalum capacitors.
February, 2007 Rev 2.0
Techcode Semiconductor Limited
10