20
LTC1702
1702fa
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 LTC1702 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 LTC1702 is much lower
than at the input, due to the fact that the inductor current
is constantly flowing at the output whenever the LTC1702
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 across the ESR of the
output bypass capacitor until the feedback loop in the
LTC1702 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 hypothetical 1.6V, 10A
switcher with a 0.01Ω ESR output capacitor would expe-
rience a 100mV step at the output with a 0 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 0.0048Ω. This can be met with three 0.014Ω,
470µF low ESR tantalum capacitors in parallel.
INDUCTOR
The inductor in a typical LTC1702 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:
I
tV
L
RIPPLE
ON Q OUT
=
()
()2
In our hypothetical 1.6V, 10A example, we'd set the ripple
current to 40% of 10A or 4A, and the inductor value would
be:
APPLICATIONS INFORMATION
WUU
U
L
tV
I
sV
A
H
with t
V
V
kHz s
ON Q OUT
RIPPLE
ON Q
=
()
=
µ
()()
=µ
= −
⎛
⎝
⎜
⎞
⎠
⎟
=µ
()
()
..
.
.
/.
2
2
12 16
4
05
1
16
5
550 1 2
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 + 1/2 I
RIPPLE
,
or 17A without saturating.
FEEDBACK LOOP/COMPENSATION
1
Feedback Loop Types
In a typical LTC1702 circuit, the feedback loop consists of
the modulator, the external inductor and output capacitor,
and the feedback amplifier and its compensation network.
All of these components affect loop behavior and need to
be accounted for in the loop compensation. The modulator
consists of the internal PWM generator, the output MOSFET
drivers and the external MOSFETs themselves. From a
feedback loop point of view, it looks like a linear voltage
transfer function from COMP to SW and has a gain roughly
equal to the input voltage. It has fairly benign AC behavior
at typical loop compensation frequencies with significant
phase shift appearing at half the switching frequency.
The external inductor/output capacitor combination makes
a more significant contribution to loop behavior. These
components cause a second order LC roll-off at the
output, with the attendant 180° phase shift. This roll-off is
what filters the PWM waveform, resulting in the desired
DC output voltage, but the phase shift complicates the
loop compensation if the gain is still higher than unity at
the pole frequency. Eventually (usually well above the LC
pole frequency), the reactance of the output capacitor will
approach its ESR, and the roll-off due to the capacitor will
stop, leaving 6dB/octave and 90° of phase shift (Figure 8).
So far, the AC response of the loop is pretty well out of the
user’s control. The modulator is a fundamental piece of the
LTC1702 design, and the external L and C are usually
chosen based on the regulation and load current require-
ments without considering the AC loop response. The
1
The information in this section is based on the paper “The K Factor: A New Mathematical Tool for
Stability Analysis and Synthesis” by H. Dean Venable, Venable Industries, Inc. For complete paper,
see “Reference Reading #4” at www.linear-tech.com.
