LTC1705_15 Datasheet, PDF (17/28 Pages) Linear Technology – Dual 550kHz Synchronous Switching Regulator Controller with 5-Bit VID and 150mA LDO

LTC1705
APPLICATIO S I FOR ATIO
out separately at BOOST. An external 1µF capacitor (CCP)
connected between SW and BOOST (Figure 2) supplies
power to BOOST when SW is high and recharges itself
through DCP when SW is low. This simple charge pump
keeps the TG driver alive even as it swings well above
PV_CC. The value of the bootstrap capacitor CCP needs to
be at least 100 times that of the total “effective” gate
capacitance of the topside MOSFET(s). For very large
external MOSFETs (or multiple MOSFETs in parallel), CCP
may need to be increased beyond the 1µF value.
Input Supply
The BiCMOS process that allows the LTC1705 to include
large MOSFET drivers on-chip also limits the maximum
input voltage to 6V. This limits the practical maximum
input supply to a loosely regulated 5V rail. The LTC1705
operates properly with input supplies down to about 3.3V,
so a typical 3.3V supply can also be used if the external
MOSFETs are appropriately chosen (see the Power
MOSFETs section).
At the same time, the input supply needs to supply several
amps of current without excessive voltage drop. The input
supply must have regulation adequate to prevent sudden
load changes from causing the LTC1705 input voltage to
dip. In typical applications where the LTC1705 is generat-
ing a secondary low voltage logic supply, all of these input
conditions are met by the main system logic supply when
fortified with an input bypass capacitor.
Input Bypass Capacitor
A typical LTC1705 circuit running from a 5V logic supply
might provide 1.6V at 15A at its core output. 5V to 1.6V
implies a duty cycle of 32%, which means QTC is on 32%
of each switching cycle. During QTC’s on-time, the current
drawn from the input equals the load current and during the
rest of the cycle, the current drawn from the input is near
zero. This 0A to 15A, 32% duty cycle pulse train adds up to
7A_RMS at the input. At 550kHz, switching cycles last about
1.8µs—most system logic supplies have no hope of regu-
lating output current with that kind of speed. A local input
bypass capacitor is required to make up the difference and
prevent the input supply from dropping drastically when
QTC kicks on. This capacitor is usually chosen for RMS
ripple current capability and ESR as well as value. The
LTC1705 I/O channel typically operates at a much smaller
output current, hence the input bypass capacitor in an
LTC1705 circuit should be chosen primarily to meet the
core output requirement.
Consider our 15A example. The input bypass capacitor
gets exercised in three ways: its ESR must be low enough
to keep the initial drop as QT turns on within a reasonable
value (100mV or so); its RMS current capability must be
adequate to withstand the 7A_RMS ripple current at the
input and the capacitance must be large enough to main-
tain 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 0.006Ω ESR to keep the input
drop under 100mV with a 15A current step and 7A_RMS
ripple current capacity to avoid overheating the capacitor.
These requirements can be met with multiple low ESR
tantalum or electrolytic capacitors in parallel or with a
large monolithic ceramic capacitor.
Tantalum capacitors are a popular choice as input capaci-
tors for LTC1705 applications, but they deserve a special
caution here. Generic tantalum capacitors have a destruc-
tive failure mechanism if they are subjected to large RMS
currents (like those seen at the input of a LTC1705). At
some random time after they are turned on, they can blow
up for no apparent reason. The capacitor manufacturers
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
LTC1705 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
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 LTC1705 is much lower
than at the input, due to the fact that the inductor current
is constantly flowing at the output. The primary concern at
the output is capacitor ESR. Fast load current transitions
at the output appear as voltage across the ESR of the
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