LTC3876_15 Datasheet, PDF (30/48 Pages) Linear Technology – Dual DC/DC Controller for DDR Power with Differential VDDQ Sensing and 50mA VTT Reference

LTC3876
APPLICATIONS INFORMATION
As the on-time is a function of the switching regulator’s
output voltage, this output is measured by the V_OUT pin
to set the required on-time. Simply connecting V_OUT to
the regulator’s local output point is preferable for most
applications, 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_OUT required for the calculation of t_ON.
However, there could be circumstances where this V_OUT
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_OUT is relatively
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.
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 LTC3876’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 9). 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_CC, it will operate in forced continuous mode at
the R_T-programmed frequency. If the MODE/PLLIN pin is
tied to SGND, the LTC3876 will operate in discontinuous
mode at light load and switch into continuous conduction
at the R_T programmed frequency as load increases. The TG
on-time during discontinuous conduction is intentionally
slightly extended (approximately 1.2 times the continuous
conduction on-time as calculated from V_IN, V_OUT and f) to
create hysteresis at the load-current boundary of continu-
ous/discontinuous conduction.
I_LOAD
CLOCK
INPUT
PHASE AND
FREQUENCY
LOCKED
SW
PHASE AND
FREQUENCY
LOCK LOST
DUE TO FAST
LOAD STEP
FREQUENCY
RESTORED
QUICKLY
V_OUT
PHASE LOCK
RESUMED
PHASE AND
FREQUENCY
LOCK LOST
DUE TO FAST
LOAD STEP
FREQUENCY
RESTORED
QUICKLY
3876 F09
Figure 9. Phase and Frequency Locking Behavior During Transient Conditions
3876f
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