Applications Information: continued
2ª
microseconds to return to a steady-state. V
control uses
Selecting the Output Capacitor
the ripple voltage from the output capacitor and a ÒfastÓ
control loop to respond to load transients, with the result
that the transient response of the CS5127 is very close to
the theoretical limit. Response times are defined below.
Output capacitors are chosen primarily on the value of
equivalent series resistance, because this is what deter-
mines how much output ripple voltage will be present.
Most polarized capacitors appear resistive at the typical
oscillator frequencies of the CS5127. As a rule of thumb,
physically larger capacitors have lower ESR. The capaci-
torÕs value in µF is not of great importance, and values
from a few tens of µF to several hundreds of µF will work
well. Tantalum capacitors serve very well as output capaci-
tors, despite their bad reputation for spectacular failure
due to excessive inrush current. This is not usually an issue
for output capacitors, because the failure is not associated
with discharge surges. Ripple current in the output capaci-
tor is usually small enough that the ripple current rating is
not an issue. The ripple current waveform is triangular,
and the formula to calculate the ripple current value is:
L(ÆIOUT
(VIN - VOUT) ´ 0.85
)
tRESPONSE(INCREASING)
=
L(ÆIOUT
VOUT
)
tRESPONSE(DECREASING)
=
Note that the response time to a load decrease is limited
only by the inductor value.
Other Inductor Selection Concerns
(VIN - VOUT)VOUT
IRIPPLE
=
Inductor current rating is an important consideration. If
the regulated output is subject to short circuit or overcur-
rent conditions, the inductor must be sized to handle the
fault without damage. Sizing the inductor to handle fault
conditions within the maximum DC current rating helps to
ensure the coil doesnÕt overheat. Not only does this pre-
vent damage to the inductor, but it reduces unwanted heat
generated by the system and makes thermal management
easier.
Selecting an open core inductor will minimize cost, but
EMI/EMC performance may be degraded. This is a tough
choice, since there are no guidelines to ensure these com-
ponents will not prove troublesome.
f ´ L ´ VIN
and output ripple voltage due to inductor ripple current is
given by:
(VIN - VOUT) ´ VOUT ´ ESR
VRIPPLE(ESR)
=
f ´ L ´ VIN
A load step will produce an instantaneous change in
output voltage defined by the magnitude of the load step,
capacitor ESR and ESL.
Core materials influence the saturation current and satura-
tion characteristics of the inductor. For example, a slightly
undersized inductor with a powdered iron core may pro-
vide satisfactory operation because powdered iron cores
have a ÒsoftÓ saturation curve compared to other core
materials.
D
I
DVO = (DIO ´ ESD) +
ESL
DT
A good practice is to first choose the output capacitor to
accommodate voltage transient requirements and then to
choose the inductor value to provide an adequate ripple
voltage.
Small physical size, low core losses and high temperature
operation will also increase cost. Finally, consider whether
an alternate supplier is an important consideration. All of
these factors can increase the cost of the inductor.
Increasing a capacitorÕs value typically reduces its ESR, but
there is a limit to how much improvement can be had. In
most applications, placing several smaller capacitors in
parallel will result in acceptable ESR while maintaining a
Operating in Discontinuous Current Mode
small PC board footprint. A warning is necessary at this
2ª
For light load designs, the CS5127 will operate in discon-
tinuous current mode (DCM). In this regime, external
components can be smaller, since high power dissipation is
not an issue. In discontinuous mode, maximum output
current is defined as:
point. The V
topology relies on the presence of some
amount of output ripple voltage being present to provide
the input signal for the ÒfastÓ control loop, and it is impor-
tant that some ripple voltage be present at the lightest load
condition in normal operation to avoid subharmonic oscil-
lation. Externally generated slope compensation can be
added to ensure proper operation.
(IPK)2 f ´ L(VIN
2VOUT ´ (VIN(MAX) - VOUT
)
IOUT(MAX)
=
)
Selecting the VFFB Lead Components
where IPK is the maximum current allowed in the switch
FET.
The VFFB lead is tied to the PWM comparatorÕs non-invert-
ing input, and provides the connection for the
externally-generated artificial ramp signal that is required
whenever duty cycle is greater than 50%.
8
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
