LTC3829 Datasheet, PDF (27/40 Pages) Linear Technology – 3-Phase, Single Output Synchronous Step-Down DC/DC Controller with Diffamp

LTC3829
Applications Information
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
–
0.5
– V_IFAST
5
•
1.2
The top gate of all channels will turn off if:
V_FB
=
V_REF
+
0.5
–
V_IFAST
5
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 actively pulled
to ground in this shutdown state. If the RUN pin voltage
is above 1.22V, the controller powers up. A soft-start cur-
rent 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 control-
ler 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
•
C_SS
1.25µA
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 operate in
forced continuous mode and revert to the selected mode
once TK/SS > 0.54V. The output ripple is minimized dur-
ing 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 when there
is no load. After TK/SS drops below 0.1V, the controller
operates in discontinuous mode.
3829f
27