Diodes AUR9716 - Manual (Page 10)

Data Sheet
1.0MHz to 1.4MHz, 2A, STEP DOWN DC-DC CONVERTER AUR9716
Apr. 2012 Rev. 1. 0 BCD Semiconductor Manufacturing Limited
10
Application Information
The AUR9716 is a synchronous buck converter which
can support switching frequency range from 1.0MHz to
1.4MHz and the output current can be up to 2A.
The basic AUR9716 application circuits are shown as
Figure 27, external components selection is determined
by the load current and is critical with the selection of
inductor and capacitor values.
1. Inductor Selection
For most applications, the value of inductor is chosen
based on the required ripple current with the range of
1.5µH to 4.7µH.
) 1 (
1
IN
OUT
OUT L V
V
V
L f
I −
×
= ∆
The largest ripple current occurs at the highest input
voltage. Having a small ripple current reduces the ESR
loss in the output capacitor and improves the efficiency.
The highest efficiency is realized at low operating
frequency with small ripple current. However, the larger
value inductors will be required. A reasonable starting
point for ripple current setting is
MAX L
I % I 40 = ∆ . For a
maximum ripple current stays below a specified
value, the inductor should be chosen according to the
following equation:
]
) (
1 ][
) (
[
MAX V
V
MAX I f
V
L
IN
OUT
L
OUT −
∆ ×
=
The DC current rating of the inductor should be at
least equal to the maximum output current plus half
of the highest ripple current to prevent inductor core
saturation. For better efficiency, the lower
DC-resistance inductor should be selected.
2. Capacitor Selection
The input capacitance, C
IN
, is needed to filer the
trapezoidal current at the source of the top MOSFET.
To prevent the large ripple voltage, a low ESR input
capacitor sized for the maximum RMS current must
be used. The maximum RMS capacitor current is
given by:
IN
OUT IN OUT
OMAX RMS V
V V V
I I
2
1
)] ( [ −
× =
It indicates a maximum value at
OUT IN
V 2 V = ,
where
2
OUT
RMS
I
I = . This simple worse-case condi-
tion is commonly used for design because even
significant deviations do not much relief. The
selection of C
OUT
is determined by the equivalent
series resistance (ESR) that is required to minimize
output voltage ripple and load step transients, as well
as the amount of bulk capacitor that is necessary to
ensure the control loop is stable. Loop stability can be
also checked by viewing the load step transient
response as described in a latter section. The output
ripple,
OUT
V ∆ , is determined by:
]
8
1
[
OUT
L OUT C f
ESR I V
Ч Ч
+ ∆ ≤ ∆
The output ripple is the highest at the maximum input
voltage since
L
I ∆ increases with input voltage.
3. Load Transient
A switching regulator typically takes several cycles to
respond to the load current step. When a load step
occurs, V
OUT
immediately shifts by an amount equal
to ) ESR I (
LOAD
× ∆ , where ESR is the equivalent
series resistance of output capacitor.
LOAD
I ∆ also
begins to charge or discharge C
OUT
generating a
feedback error signal used by the regulator to return
V
OUT
to its steady-state value. During the recovery
time, V
OUT
can be monitored for overshoot or ringing
that would indicate a stability problem.
4. Output Voltage Setting
The output voltage of AUR9716 can be adjusted by a
resistive divider according to the following formula:
) 1 ( 8 . 0 ) 1 (
2
1
2
1
R
R
V
R
R
V V
FB OUT
+ Ч = + Ч =
When V
FB
is the 0.8V feedback reference voltage, the
resistive divider senses the fraction of the output
voltage as shown in Figure 23.
Figure 23. Setting the Output Voltage