Datasheet

LTC3839
29
3839fa
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
Figure 10. Phase and Frequency Locking Behavior During Transient Conditions
PHASE AND
FREQUENCY
LOCK LOST
DUE TO FAST
LOAD STEP
FREQUENCY
RESTORED
QUICKLY
PHASE AND
FREQUENCY
LOCK LOST
DUE TO FAST
LOAD STEP
FREQUENCY
RESTORED
QUICKLY
PHASE LOCK
RESUMED
3839 F10
PHASE AND
FREQUENCY
LOCKED
I
LOAD
CLOCK
INPUT
SW
V
OUT