LTC3829
26
3829fc
For more information www.linear.com/LTC3829
APPLICATIONS INFORMATION
voltage. When V
IFAST
is set below 0.5V, the difference of
0.5V and V
IFAST
sets the threshold voltage that triggers
nonlinear control. Nonlinear control is only enabled when
V
FB
is within the UV and OV window. It should be enabled
only for forced continuous mode of operation.
Once nonlinear control is enabled, the top gate of all chan
-
nels will turn on if:
V
FB
= V
REF
–
IFAST
•1.
The top gate of all channels will turn off if:
V
FB
= V
REF
+
IFAST
where V
REF
is the reference voltage, normally at 0.6V, and
V
FB
is the feedback voltage.
Soft-Start and Tracking
The LTC3829 has the ability to either soft-start by itself
with a capacitor or track the output of another channel
or external supply. When the controller is configured to
soft-start by itself, a capacitor should be connected to its
TK/SS pin. The controller is in the shutdown state if its
RUN pin voltage is below 1.22V and its TK/SS pin is ac
-
tively pulled
to ground in this shutdown state. If the RUN
pin
voltage is above 1.22V, the controller powers up. A
soft-start current of 1.25µA then starts to charge the TK/
SS soft-start capacitor. Note that soft-start or tracking is
achieved not by limiting the maximum output current of
the controller but by controlling the output ramp voltage
according to the ramp rate on the TK/SS pin. Current
foldback is disabled during this phase to ensure smooth
soft-start or tracking. The soft-start or tracking range is
defined to be the voltage range from 0V to 0.6V on the
TK/SS pin. The total soft-start time can be
calculated as:
t
SOFTSTART
= 0.6•
SS
Regardless of the mode selected by the MODE pin, the
controller always starts in discontinuous mode up to TK/
SS = 0.5V. Between TK/SS = 0.5V and 0.54V, it will oper
-
ate in forced continuous mode and revert to the selected
mode
once TK/SS > 0.54V. The output ripple is minimized
during the 40mV forced continuous mode window ensuring
a clean PGOOD signal. When the channel is configured
to track another supply, the feedback voltage of the other
supply is duplicated by a resistor divider and applied to
the TK/SS pin. Therefore, the voltage ramp rate on this
pin is determined by the ramp rate of the other supply’s
voltage. Note that the small soft-start capacitor charging
current is always flowing, producing a small offset error.
To minimize this error, select the tracking resistive divider
value to be small enough to make this error negligible.
In order to track down another channel or supply after
the soft-start phase expires, the LTC3829 is forced into
continuous mode of operation as soon as V
FB
is below
the undervoltage threshold of 0.54V regardless of the
setting on the MODE pin. However, the LTC3829 should
always be set in forced
continuous mode tracking down
