Theory Of Operation - Analog Devices DC3018A Manual

THEORY OF OPERATION

Introduction to the DC3018A
The DC3018A demonstration circuit features the
LTC3311S, a low voltage synchronous step-down Silent
Switcher. The LTC3311S is a monolithic, constant fre-
quency, current mode step-down DC/DC converter. An
oscillator, with frequency set using a resistor on the RT
pin, turns on the internal top power switch at the begin-
ning of each clock cycle. Current in the inductor then
increases until the top switch comparator trips and turns
off the top power switch. The peak inductor current at
which the top switch turns off is controlled by the volt-
age on the internal ITH node. The error amplifier servos
the ITH node by comparing the voltage on the VFB pin
with an internal 500mV reference. When the load current
increases, it causes a reduction in the feedback voltage
relative to the reference leading the error amplifier to
raise the ITH voltage until the average inductor current
matches the new load current. When the top switch turns
off, the synchronous power switch turns on until the next
clock cycle begins or the inductor current falls to zero.
If overload conditions result in excessive current flowing
through the bottom switch, the next clock cycle will be
delayed until the switch current returns to a safe level.
If the EN pin is low, the LT3311S is in shutdown and in a
low quiescent current state. When the EN pin is above its
threshold, the switching regulator will be enabled.
The MOD/SYNC pin synchronizes the switching frequency
to an external clock, is a clock output or sets the PWM
mode. The PWM modes of operation are either pulse skip
or forced continuous. See the LTC3311S data sheet for
more detailed information.
The maximum allowable operating frequency is influ-
enced by the minimum on time of the top switch, the
ratio of V to V and the available inductor values. The
OUT IN
DEMO MANUAL DC3018A
maximum allowable operating frequency may be calcu-
lated using a minimum t
ON
V
OUT
f =
SW(MAX)
V • t
IN(MAX)
Select an operating switching frequency below f
Typically, it is desired to obtain an inductor current of 30%
of the maximum LTC3311S operating load, 12.5A. Use the
formulas below to calculate the inductor value to obtain a
30% (3.75A) inductor ripple for the operating frequency.
⎛
V
OUT
L ≥ • 1−
⎜
3.75A • f
⎝
SW
0.25• V
IN(MAX)
L ≥
for
3.75A • f
SW
When determining the compensation components, C4,
C10, C11 and R12, controlling the loop stability and
transient response are the two main considerations. The
LTC3311S has been designed to operate at a high band-
width for fast transient response capabilities. This reduces
output capacitance required to meet the desired tran-
sient response. The mid-band gain of the loop increases
with R12 and the bandwidth of the loop increases with
decreasing C11. C4 along with R4 provides a phase lead
which will improve the phase margin. C10 along with
R12 provide a high frequency pole to reduce the high
frequency gain.
Loop stability is generally measured using the Bode plot
method of plotting loop gain in dB and phase in degrees.
The 0dB crossover frequency should be less than 1/6 of
the operating frequency to reduce the effects of added
phase shift of the modulator. The control loop phase mar-
gin goal should be 45° or greater and a gain margin goal
of 8dB or greater.
of 35ns in the formula below.
ON(MIN)
SW(MAX)
⎞
V V
OUT OUT
≤ 0.5
⎟ for
V V
⎠
IN(MAX) IN(MAX)
V
OUT
> 0.5
V
IN(MAX)
.
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