LTC3839 Datasheet, PDF (29/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
APPLICATIONS INFORMATION
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 LTC3839 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.
If an application requires very low (approaching minimum)
on-time, the system may not be able to maintain its full
frequency synchronization range. Getting closer to mini-
mum on-time, it may even lose phase/frequency lock at no
load or light load conditions, under which the SW on-time
is effectively longer than TG on-time due to TG/BG dead
times. This is discussed further under Minimum On-Time,
Minimum Off-Time and Dropout Operation.
Minimum On-Time, Minimum Off-Time
and Dropout Operation
The minimum on-time is the smallest duration that
LTC3839’s TG (top gate) pin can be in high or “on” state.
It has dependency on the operating conditions of the
switching regulator, and is a function of voltages on the
V_IN and V_OUT pins, as well as the value of external resistor
R_T. A minimum on-time of 30ns can be achieved when the
V_OUT pin is tied to its minimum value of 0.6V while the V_IN
is tied to its maximum value of 38V. For larger values of
V_OUT and/or smaller values of V_IN, the minimum achievable
on-time will be longer. The valley mode control architecture
allows low on-time, making the LTC3839 suitable for high
step-down ratio applications.
The effective on-time, as determined by the SW node
pulse width, can be different from this TG on-time, as it
also depends on external components, as well as loading
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
3839 F10
Figure 10. Phase and Frequency Locking Behavior During Transient Conditions
3839fa
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