Diodes AP6508 - Manual (Page 11)

AP6508
500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER
AP6508
Document number: DS33437 Rev. 5 - 2
11 of 14
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October 2011
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Application Information (cont.)
Inductor (cont.)
Peak current determines the required saturation current
rating, which influences the size of the inductor.
Saturating the inductor decreases the converter
efficiency while increasing the temperatures of the
inductor and the internal MOSFETs. Hence choosing an
inductor with appropriate saturation current rating is
important.
A 1µH to 10µH inductor with a DC current rating of at
least 25% percent higher than the maximum load current
is recommended for most applications.
For highest efficiency, the inductor’s DC resistance
should be less than 200m
Ω. Use a larger inductance for
improved efficiency under light load conditions.
Input Capacitor
The input capacitor reduces the surge current drawn from
the input supply and the switching noise from the device.
The input capacitor has to sustain the ripple current
produced during the on time on the upper MOSFET. It
must hence have a low ESR to minimize the losses.
The RMS current rating of the input capacitor is a critical
parameter that must be higher than the RMS input
current. As a rule of thumb, select an input capacitor
which has an RMS rating that is greater than half of the
maximum load current.
Due to large dI/dt through the input capacitors,
electrolytic or ceramics should be used. If a tantalum
must be used, it must be surge protected. Otherwise,
capacitor failure could occur. For most applications, a
4.7µF ceramic capacitor is sufficient.
Output Capacitor
The output capacitor keeps the output voltage ripple
small, ensures feedback loop stability and reduces the
overshoot of the output voltage. The output capacitor is a
basic component for the fast response of the power
supply. In fact, during load transient, for the first few
microseconds it supplies the current to the load. The
converter recognizes the load transient and sets the duty
cycle to maximum, but the current slope is limited by the
inductor value.
Maximum capacitance required can be calculated from
the following equation:
2
out
2
out
2 inductor
out
o
V ) V V ( Δ
)
2
ΔI
L(I
C
− +
+
=
Where
ΔV
is the maximum output voltage overshoot.
ESR of the output capacitor dominates the output voltage
ripple. The amount of ripple can be calculated from the
equation below:
ESR * ΔI Vout
inductor capacitor
=
An output capacitor with ample capacitance and low ESR
is the best option. For most applications, a 22µF ceramic
capacitor will be sufficient.
PC Board Layout
This is a high switching frequency converter. Hence
attention must be paid to the switching currents
interference in the layout. Switching current from one
power device to another can generate voltage transients
across the impedances of the interconnecting bond wires
and circuit traces. These interconnecting impedances
should be minimized by using wide, short printed circuit
traces.
The input capacitor C1
must be placed as close
as possible to the IC and
the inductor L1 The external feedback
resistor divider must be
placed as close as possible
to the FB pin of the IC
52mm
34
mm
AP6508 is exposed at the bottom of the package and
must be soldered directly to a well designed thermal pad
on the PCB. This will help to increase the power
dissipation.
External Bootstrap Diode
It is recommended that an external bootstrap diode be
added when the input voltage is lower than or equal to 5V
and the duty cycle is greater than 65%. This external
diode can be connected to the input or a 5V rail that is
available in the system. This helps improve the efficiency
of the converter. The bootstrap diode can be a low cost
one such as BAT54 or a Schottky that has a low Vf.
Figure 3. External Bootstrap Diode