LTC1704 Linear Technology, LTC1704 Datasheet - Page 18

• Manufacturer Part Number: LTC1704
• Description: 550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1704/LTC1704B
to keep the initial drop as QT turns on within reason
(100mV or so); its RMS current capability must be ad-
equate to withstand the 4.66A capacitor ripple current is
not the same as input RMS current at the input and the
capacitance must be large enough to maintain the input
voltage until the input supply can make up the difference.
Generally, a capacitor that meets the first two parameters
will have far more capacitance than is required to keep
capacitance-based droop under control. In our example,
we need 0.01 ESR to keep the input drop under 100mV
with a 10A current step and 5.65A
capacity to avoid overheating the capacitor. These re-
quirements can be met with multiple low ESR tantalum or
electrolytic capacitors in parallel, or with a large mono-
lithic ceramic capacitor.
Tantalum capacitors are a popular choice as input capaci-
tors for LTC1704 applications, but they deserve a special
caution here. Generic tantalum capacitors have a destruc-
tive failure mechanism when they are subjected to large
RMS currents (like those seen at the input of an LTC1704).
At some random time after they are turned on, they can
blow up for no apparent reason. The capacitor manufac-
turers are aware of this and sell special “surge tested”
tantalum capacitors specifically designed for use with
switching regulators. When choosing a tantalum input
capacitor, make sure that it is rated to carry the RMS
current that the LTC1704 will draw. If the data sheet
doesn’t give an RMS current rating, chances are the
capacitor isn’t surge tested. Don’t use it!
Output Bypass Capacitor Selection
The output bypass capacitor has quite different require-
ments from the input capacitor. The ripple current at the
output of a buck regulator, like the LTC1704’s switcher
controller, is much lower than at the input because the
inductor current is constantly flowing at the output when-
ever the LTC1704 is operating in Continuous mode. The
primary concern at the output is capacitor ESR. Fast load
current transitions at the output will appear as voltage
18
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across the ESR of the output bypass capacitor until the
feedback loop in the LTC1704 can change the inductor
current to match the new load current value. This ESR step
at the output is often the single largest budget item in the
load regulation calculation. As an example, our hypotheti-
cal 1.6V, 10A switcher with a 0.01 ESR output capacitor
would experience a 100mV step at the output with a 0A to
10A load step—a 6.3% output change!
Usually the solution is to parallel several capacitors at the
output. For example, to keep the transient response inside
of 3% with the previous design, we’d need an output ESR
better than 0.0048 . This can be met with three 0.014 ,
470 F tantalum capacitors in parallel.
Inductor Selection
The inductor in a typical LTC1704 circuit is chosen prima-
rily for value and saturation current. The inductor value
sets the ripple current, which is commonly chosen at
around 40% of the anticipated full load current. Ripple
current is set by:
In our hypothetical 1.6V, 10A example, we’d set the ripple
to 40% of 10A or 4A, and the inductor value would be:
The inductor must not saturate at the expected peak
current. In this case, if the current limit was set to 15A, the
inductor should be rated to withstand 15A + 1/2I
17A without saturating.
FEEDBACK LOOP/COMPENSATION
Feedback Loop Types
In a typical LTC1704 switcher circuit, the feedback loop
consists of the modulator, the external inductor and
output capacitor, and the feedback amplifier and its com-
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