Evaluation Board Hardware; Introduction To The Eval-Ltc3313Ev-A-Z - Analog Devices EVAL-LTC3313EV-A-Z User Manual

User Guide

EVALUATION BOARD HARDWARE

INTRODUCTION TO THE EVAL-LTC3313EV-A-Z

The EVAL-LTC3313EV-A-Z evaluation board features the LTC3313,
a low voltage, synchronous step-down Silent Switcher. The
LTC3313 is a monolithic, constant frequency, current mode step-
down DC/DC converter. An oscillator turns on the internal top power
switch at the beginning 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 voltage on the ITH node.
The error amplifier servos the ITH pin by comparing the voltage
on the internal VFB pin with an internal 500 mV 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
bottom power switch turns on until the next clock cycle begins. In
pulse-skipping mode, the bottom switch also turns off when 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 LTC3313 is in a shutdown state with a low
quiescent current. When the EN pin is more than its threshold, the
switching regulator enables.
The MODE/SYNC pin synchronizes the switching frequency to an
external clock that can be a clock output for multiphase operation.
The MODE/SYNC pin also sets the regulator operation modes. The
operation modes are either forced continuous or pulse-skipping.
See the LTC3313 data sheet for more detailed information.
The maximum allowable operating frequency (f
enced by the minimum on time (t
of V to V and the inductance of the output inductor. Use the
OUT IN
following equation to calculate the maximum allowable operating
frequency:
analog.com
) is influ-
SW(MAX)
) of the top switch, the ratio
ON(MIN)
Figure 2. LTC3313, 1.2 V/15 A Step-Down Converter Typical Solution
f =
V OUT
SW MAX
V IN MAX × t ON MIN
Select an operating switching frequency below f
sired to obtain an inductor current of 30% of the maximum LTC3313
operating load, 15 A. Use the following equations to calculate
the inductor value to obtain a 30% (4.5 A) inductor ripple for the
L ≥ × 1 −
operating frequency:
V OUT
3.75 A × f SW V IN MAX
for ≤ 0 . 5
V OUT
V IN MAX
L ≥ for
0 . 25 × V IN MAX
3.75 A × f SW
When determining the compensation components, C4, C10, C24,
and R12, controlling the loop stability and transient response are
the two main considerations. The LTC3313 was designed to oper-
ate at a high bandwidth for fast transient response capabilities,
which reduces the required output capacitance to meet the desired
transient voltage range. The midband gain of the loop increases
with R12 and the bandwidth of the loop increases by decreasing
C24. C4 and R9 provide a phase lead that improves the phase
margin. C10 and 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 decibels and phase shift in degrees. The
0 dB crossover frequency must be less than 1/6 of the operating
frequency to reduce the effects of the added phase shift of the
modulator. The control-loop phase margin goal must be 45º or
greater, and the gain margin goal must be 8 dB or greater.
EVAL-LTC3313
. It is de-
SW(MAX)
V OUT
> 0 . 5
V OUT
V IN MAX
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