LTC3415 Datasheet, PDF (13/28 Pages) Linear Technology – 7A, PolyPhase Synchronous Step-Down Regulator

LTC3415
OPERATION
In multiphase operation where all the I_TH pins of each
LTC3415 are tied together to achieve accurate load sharing,
internal compensation is not allowed. External compensa-
tion components need to be properly selected for optimal
transient response and stable operation.
Master/Slave Operation
In multiphase single-output operation, the user has the
option to run in multi-master mode where all the V_FB, I_TH,
and output pins of the stages are tied to each other. All
the error amplifiers are effectively operating in parallel and
the total g_m of the system is increased by the number of
stages. The I_TH value, which dictates how much current
is delivered to the load from each stage, is averaged and
smoothed out by the external I_TH compensation compo-
nents. However, in certain applications, the resulting higher
g_m from multiple LTC3415s can make the system loop
harder to compensate. In this case, the user can choose
an alternative mode of operation.
The second mode of operation is single-master operation
where only the error amplifier of the master stage is used
while the error amplifiers of the other stages (slaves) are
disabled. The slave’s error amplifier is disabled by tying
its V_FB pin to SV_IN, which also disables the internal over-
voltage comparator and power-good indicator. The master’s
error amplifier senses the output through its V_FB pin and
drives the I_TH pins of all the stages. To account for ground
voltage differences among the stages, the user should
tie all I_THM pins together and then tie it to the master’s
signal ground. As a result, not only is it easier to do loop
compensation, this single-master operation should also
provide for more accurate current sharing among stages
because it prevents the error amplifier’s output (I_TH) of
each stage from interfering with that of another stage.
Spread Spectrum Operation
Switching Regulators can be particularly troublesome
where electromagnetic interference (EMI) is concerned.
Switching regulators operate on a cycle-by-cycle basis to
transfer power to an output. In most cases, the frequency
of operation is fixed or is a constant based on the output
load. This method of conversion creates large components
of noise at the frequency of operation (fundamental) and
multiples of the operating frequency (harmonics).
To reduce this noise, the LTC3415 can run in spread
spectrum operation by tying the CLKIN pin to SV_IN. In
spread spectrum operation, the LTC3415’s internal oscil-
lator is designed to produce a clock pulse whose period
is random on a cycle-by-cycle basis but fixed between
70% and 130% of the nominal frequency. This has the
benefit of spreading the switching noise over a range of
frequencies, thus significantly reducing the peak noise.
Figures 7 and 8 show how the spread spectrum feature
of the LTC3415 significantly reduces the peak harmonic
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
V_IN = 5V
V_OUT = 1.8V
RBW = 100Hz
–14.1dBm
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
FREQUENCY (MHz)
3415 F07
Figure 7. LTC3415’s Output Noise Spectrum Analysis in
Free-Running Constant Frequency Operation
–10
–20
V_IN = 5V
V_OUT = 1.8V
–30 ^{RBW = 100Hz}
–40
–50
–60
–70
–80
–90
–100
–37.3dBm
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
FREQUENCY (MHz)
3415 F08
Figure 8. LTC3415’s Output Noise Spectrum Analysis in Spread
Spectrum Operation
3415fa
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