Applications Information
variables the designer must consider. Inductance values
Selection of Feedback Lead Divider Resistor Values
between 1µH and 50µH are suitable for use with the CS5127.
Low values within this range minimize the component size
and improve transient response, but larger values reduce
ripple current. Choosing the inductor value requires the
designer to make some choices early in the design. Output
current, output voltage and the input voltage range should
be known in order to make a good choice.
The feedback (VFB) leads are connected to external resistor
dividers to set the output voltage. The on-chip error ampli-
fier is referenced to 1.275V, and the resistor divider values
are determined by selecting the desired output voltage
and the value of the divider resistor connected between
the VFB lead and ground.
The input voltage range is bracketed by the maximum and
minimum expected values of VIN. Most computer applica-
tions use a fairly well-regulated supply with a typical
output voltage tolerance on the order of ±5%. The values
of VIN(MAX) and VIN(MIN) are used to calculate peak current
and minimum inductance value, respectively. However, if
the supply is well-regulated, these calculations may both
be made using the typical input voltage value with very
little error.
Resistor R1 is chosen first based on a design trade-off of
system efficiency vs. output voltage accuracy. Low values
of divider resistance consume more current which decreas-
es system efficiency. However, the VFB lead has a 1µA
maximum bias current which can introduce errors in the
output voltage if large resistance values are used. The
approximate value of current sinking through the resistor
divider is given by
1.275V
IV(FB)
=
Current in the inductor while operating in the continuous
current mode (CCM) is defined as the load current plus
the inductor ripple current:
R1
The output voltage error that can be expected due to the
bias current is given by
IL = IOUT + IRIPPLE
(1E - 6) ´ R1
Error Percentage =
´ 100%
1.275
The ripple current waveform is triangular, and the current
is a function of the voltage across the inductor, the switch
on-time and the inductor value. Switch on-time is the duty
cycle divided by the operating frequency, and duty cycle
can be defined as the ratio of VOUT to VIN, such that
where R1 is given in ohms. For example, setting R1 = 5K
yields an output voltage error of 0.39% while setting the
feedback divider current at 255µA. Larger currents will
result in reduced error.
(VIN - VOUT)VOUT
IRIPPLE
=
f ´ L ´ VIN
Output
Driver
The peak current can be described as the load current plus
half of the ripple current. Peak current must be less than
the maximum rated switch current. This limits the maxi-
mum load current that can be provided. It is also
important that the inductor can deliver the peak current
without saturating.
VOUT
1.275V
+
R2
-
VFB
(VIN(MAX) - VOUT)VOUT
I
OUT(MAX) = ISWITCH(MAX) -
R1
2f ´ L ´ VIN(MAX)
GATE
COMP
Since the peak inductor current must be less than or equal
to the peak switch current, the minimum value of induc-
tance can be calculated:
Figure 3: Feedback resistor divider.
R2 can be sized according to the following formula once
the desired output voltage and the value of R1 have been
determined:
(VIN(MIN) - VOUT)VOUT
LMIN
=
f ´ VIN(MIN) ´ ISWITCH(MAX)
VOUT
1.275
R2 = R1
-1
(
)
Load Current Transient Response
The theoretical limit on load current transient response is a
function of the inductor value, the load transient and the
voltage across the inductor. In conventionally-controlled
regulators, the actual limit is the time required by the con-
trol loop. Conventional current-mode and voltage-mode
control loops adjust the switch duty cycle over many oscil-
lator periods, often requiring tens or even hundreds of
Selecting the Inductor
There are many factors to consider when choosing the
inductor. Maximum load current, core losses, winding
losses, output voltage ripple, short circuit current, satura-
tion, component height, EMI/EMC and cost are all
7
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