Datasheet

LTC3415
13
3415fa
FREQUENCY (MHz)
1.0
AMPLITUDE (dBm)
–50
–30
–10
1.8
3415 F08
–70
–90
–60
–40
–20
–80
–100
1.21.1
1.41.3
1.6 1.7 1.9
1.5
2.0
V
IN
= 5V
V
OUT
= 1.8V
RBW = 100Hz
–37.3dBm
FREQUENCY (MHz)
1.0
AMPLITUDE (dBm)
–50
–30
–10
1.8
3415 F07
–70
–90
–60
–40
–20
–80
–100
1.21.1
1.41.3
1.6 1.7 1.9
1.5
2.0
V
IN
= 5V
V
OUT
= 1.8V
RBW = 100Hz
–14.1dBm
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 amplifi ers 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 amplifi er of the master stage is used
while the error amplifi ers of the other stages (slaves) are
disabled. The slave’s error amplifi er 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 amplifi er 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 amplifi er’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 fi xed 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 fi xed between
70% and 130% of the nominal frequency. This has the
benefi t of spreading the switching noise over a range of
frequencies, thus signifi cantly reducing the peak noise.
Figures 7 and 8 show how the spread spectrum feature
of the LTC3415 signifi cantly reduces the peak harmonic
Figure 8. LTC3415’s Output Noise Spectrum Analysis in Spread
Spectrum Operation
Figure 7. LTC3415’s Output Noise Spectrum Analysis in
Free-Running Constant Frequency Operation
OPERATION