LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 31

• Manufacturer Part Number: LTC3855
• Description: Dual, Fast, Accurate Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
V
ratiometric tracking, the master channel’s feedback divider
can be also used to provide TRACK/SS voltage for the slave
channel, since the additional divider, if used, should be of
the same ratio as the master channel’s feedback divider.
So which mode should be programmed? While either
mode satisfies most practical applications, some trade-
offs exist. The ratiometric mode saves a pair of resistors,
but the coincident mode offers better output regulation.
When the master channel’s output experiences dynamic
excursion (under load transient, for example), the slave
channel output will be affected as well. For better output
regulation, use the coincident tracking mode instead of
ratiometric.
Phase and Frequency Synchronization
For applications that require better control of EMI and
switching noise or have special synchronization needs, the
LTC3838 can synchronize the turn-on of the top MOSFET
to an external clock signal applied to the MODE/PLLIN pin.
The applied clock signal needs to be within ±30% of the
RT programmed frequency to ensure proper frequency
and phase lock. The clock signal levels should generally
comply to V
PLLIN pin has an internal 600k pull-down resistor to ensure
discontinuous current mode operation if the pin is left open.
The LTC3838 uses the voltages on V
as R
phase and frequency lock for wide ranges of V
and R
As the on-time is a function of the switching regulator’s
output voltage, this output is measured by the SENSE
to set the required on-time. The SENSE
regulator’s local output point to the IC for most applica-
tions, as the remotely regulated output point could be
significantly different from the local output point due to
line losses, and local output versus local ground is typically
the V
OUT1
t
ON
T
OUT
T
to adjust the top gate on-time in order to maintain
must be set higher than V
-programmed switching frequency f:
≈
V
required for the calculation of t
V
IN
OUT
PLLIN(H)
• f
> 2V and V
PLLIN(L)
OUT2
IN
. To implement the
< 0.5V. The MODE/
–
and V
ON
pin is tied to the
.
OUT
IN
as well
, V
–
OUT
pin
However, there could be circumstances where this V
programmed on-time differs significantly different from
the on-time required in order to maintain frequency
and phase lock. For example, lower efficiencies in the
switching regulator can cause the required on-time to be
substantially higher than the internally set on-time (see
Efficiency Considerations). If a regulated V
low, proportionally there could be significant error caused
by the difference between the local ground and remote
ground, due to other currents flowing through the shared
ground plane.
If necessary, the R
or even the common mode voltage of the SENSE pins may
be programmed externally to correct for such systematic
errors. The goal is to set the on-time programmed by V
V
system will have sufficient range to correct for component
and operating condition variations, or to synchronize to the
external clock. Note that there is an internal 500k resistor
on each SENSE
During dynamic transient conditions either in the line
voltage or load current (e.g., load step or release), the top
switch will turn on more or less frequently in response
to achieve faster transient response. This is the benefit
of the LTC3838’s controlled on-time, valley current mode
architecture. However, this process may understandably
lose phase and even frequency lock momentarily. For
relatively slow changes, phase and frequency lock can
still be maintained. For large load current steps with fast
slew rates, phase lock will be lost until the system returns
back to a steady-state condition (see Figure 10). It may
take up to several hundred microseconds to fully resume
the phase lock, but the frequency lock generally recovers
quickly, long before phase lock does.
For light load conditions, the phase and frequency syn-
chronization depends on the MODE/PLLIN pin setting. If
the external clock is applied, synchronization will be active
and switching in continuous mode. If MODE/PLLIN is tied
to INTV
the R
tied to SGND, the LTC3838 will operate in discontinuous
OUT
T
and R
-programmed frequency. If the MODE/PLLIN pin is
CC
, it will operate in forced continuous mode at
T
close to the stready-state on-time so that the
–
pin to SGND, but not on the SENSE
T
resistor value, voltage on the V
LTC3838
OUT
is relatively
31
IN
+
3838fa
pin,
pin.
OUT
IN
,