Applications Information; External Component Selection For Buck Regulators - Analog Devices ADP50460008 Owner's Manual

ADI Confidential

APPLICATIONS INFORMATION

EXTERNAL COMPONENT SELECTION FOR BUCK
REGULATORS
Trade-offs between performance parameters such as efficiency
and transient response can be made by varying the choice of
external components in the applications circuit, as shown in
Figure 62.
Inductor
The high switching frequency of Channel 3 and Channel 6 allows
for the selection of small chip inductors. For best performance,
use inductor values between 0.7 μH and 3 μH for 2.5 MHz
switching. Recommended inductors are shown in Figure 62.
The peak-to-peak inductor current ripple is calculated using
Equation 2.
× −
V ( V
= OUT IN
I
RIPPLE
×
V f
IN SW
where:
f is the switching frequency.
SW
L is the inductor value.
The minimum dc current rating of the inductor must be greater
than the inductor peak current. The inductor peak current is
calculated using Equation 3.
I
RIPPLE
= +
I I
PEAK LOAD ( MAX )
Inductor conduction loss is caused by the flow of current through
the inductor, which has an associated internal dc resistance (DCR).
Larger sized inductors have smaller DCR, which may decrease
inductor conduction loss. Inductor core loss is related to the
magnetic permeability of the core material.
Output Capacitor
Higher output capacitor values reduce the output voltage ripple
and improve load transient response. When choosing an output
capacitor value, it is also important to account for the loss of
capacitance due to output voltage dc bias.
Ceramic capacitors are manufactured with a variety of dielectrics,
each with different behavior over temperature and applied voltage.
Capacitors must have a dielectric that is adequate to ensure the
minimum capacitance over the necessary temperature range and
dc bias conditions. The X5R or X7R dielectrics with a voltage rating
of 6.3 V or 10 V are recommended for best performance. The Y5V
V )
OUT
×
L
2
and Z5U dielectrics are not recommended for use with any dc-
to-dc converters because of their poor temperature and dc bias
characteristics.
The peak-to-peak output voltage ripple for the selected output
capacitor and inductor values is calculated using Equation 4.
=
V
RIPPLE
Capacitors with lower equivalent series resistance (ESR) are
preferred to guarantee low output voltage ripple, as shown in
Equation 5.
≤
ESR
COUT
The minimum capacitance needed for stability, which includes
temperature and dc bias effects, is 10 µF for Channel 3 and 10 µF
(2)
for Channel 6 when the switching frequency is 2.5 MHz.
Channel 4 and Channel 6 Adjustable Output Voltage Setting
Figure 59. Channel 4 and Channel 6 Output Voltage (V
In the case of the external VID setting ([VID4_1, VID4_0] = 00b for
(3)
Channel 4 at Address 9 or [VID6_2, VID6_1, VID6_0] = 000b for
Channel 6 at Address 10), the output voltages of these channels are
programmed externally through resistor dividers to a value that is
between 0.5 V and V
that are too low (for example, 0.5 V) may go out of regulation at
high V conditions due to the minimum on time limitation of
IN
Channel 6.
It is recommended that a current level of at least 300 µA lead to
the FB node for Channel 6. In contrast, a current level in the 10s
of microamps (µA) should be used for Channel 4. The output
voltage is regulated around the reference voltage of the internal
sense amplifier, which is set to 0.5 V (V
between the output voltage and the resistor divider network is
provided by Equation 6.
=
V V
VOUTx
Note that it could be required to change the output capacitor
value when the output voltage level is extremely out of range
from the VID option table. If the adjustable mode is used in the
applications, it is recommended to contact an Analog Devices
representative.
Rev. Sp0 | Page 47 of 60
ADP50460008
V
IN =
( )
π × × × ×
2 f 2 L C 8
SW OUT
V
RIPPLE
I
RIPPLE
VOUTx
R1
TO FBx
R2
× 0.95. Note that output voltage settings
IN
, V
FB4
R1
 + 
×
1
 
FBx
 
R2
I
RIPPLE
(4)
× ×
f C
SW OUT
(5)
) Setting
OUTx
). The relationship
FB6
(6)