RT6908
VCORE Buck External MOSFET Selection
Buck Output Capacitor Selection
The VCORE buck controller drives two externalN-MOSFETs
as the switch. There are some considerations to choose
the external MOSFET. It includes MOSFET drain to source
voltage stress, on-resistance, total gate charge
characteristics and power dissipation for thermal
performance.
The selection of COUT is determined by the required ESR
to minimize voltage ripple. Moreover, the amount of bulk
capacitance is also a key for COUT selection to ensure
that the control loop is stable. Loop stability can be
checked by viewing the load transient response as
described in a later section. The output ripple, VOUT, is
determined by :
Buck Inductor Selection
1
⎛
⎝
⎞
⎟
⎠
ΔVOUT = ΔIL × ESR +
⎜
8×fOSC ×COUT
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current, ΔIL, will increase with higher
VIN and decrease with higher inductance, as shown in
below equation :
The output ripple will be highest at the maximum input
voltage since IL increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirement. Suitable
candidates such as dry tantalum, special polymer,
aluminum electrolytic and ceramic capacitors are all
available in surface mount packages. Special polymer
capacitors offer very low ESR value. However, it provides
lower capacitance density than other types. Although
tantalum capacitors have the highest capacitance density,
it is important to only use types that pass the surge test
for use in switching power supplies.Aluminum electrolytic
capacitors have significantly higher ESR. However, it can
be used in cost-sensitive applications requiring high ripple
current rating and long term reliability. Ceramic capacitors
have excellent low ESR characteristics but can have a
high voltage coefficient and audible piezoelectric effects.
The high Q of ceramic capacitors with trace inductance
can also lead to significant ringing. Nevertheless, higher
value, lower cost ceramic capacitors are now becoming
available in smaller case sizes. Their high ripple current,
high voltage rating and low ESR make them ideal for
switching regulator applications. However, care must be
taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input,
VIN, and the power is supplied by a wall adapter through
long wires, a load step at the output can induce ringing at
the input. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at VINlarge enough to damage the
part.
VOUT
fOSC ×L
VOUT
V
IN
⎛
⎜
⎝
⎞ ⎛
⎠ ⎝
⎞
⎟
⎠
ΔIL =
× 1−
⎟ ⎜
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
the highest efficiency operation. However, it requires a
large inductor to achieve this goal. For the ripple current
selection, the value of IL(MAX) = 0.4 is a reasonable starting
point. The largest ripple current occurs at the highest VIN.
To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation :
⎛
⎞ ⎛
⎞
V
×ΔI
V
OUT
V
IN(MAX)
OUT
L =
× 1−
⎜
⎝
⎟ ⎜
⎟
⎠
f
OSC
L(MAX)
⎠ ⎝
Buck Input Capacitor Selection
The input capacitance, CIN, is needed to filter the
trapezoidal current at the source of the high-side MOSFET.
To prevent large ripple current, a low ESR input capacitor
sized for the maximum RMS current should be used. The
RMS current is given by :
V
V
V
IN
OUT
I
= I
×
×
−1
RMS
OUT(MAX)
V
OUT
IN
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT / 2. This simple worst-case condition is commonly
used for design because even significant deviations do
not offer much relief. Choose a capacitor rated at a higher
temperature than required. Several capacitors may also
be paralleled to meet size or height requirements in the
design. For the input capacitor, a 10μF low ESR ceramic
capacitor is recommended.
Copyright 2013 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6908-01 March 2013
www.richtek.com
19
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