LTC3839 Datasheet, PDF (27/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
APPLICATIONS INFORMATION
As voltage on the RUN pin increases, typically beyond
3V, its bias current will start to reverse direction and flow
into the RUN pin. Keep in mind that the RUN pin can sink
up to 50μA; Even if a RUN pin may slightly exceed 6V
when sinking 50μA, the RUN pin should never be forced
to higher than 6V by a low impedance voltage source to
prevent faulty conditions.
Soft-Start and Tracking
The LTC3839 has the ability to either soft-start by itself
with a capacitor or track an external supply. Note that the
soft-start and tracking features are achieved not by limit-
ing the maximum output current of the controller, but by
controlling the output ramp voltage according to the ramp
rate on the TRACK/SS pin.
When the LTC3839 is configured to soft-start by itself, a
capacitor should be connected to its TRACK/SS pin. TRACK/
SS is pulled low until the RUN pin voltage exceeds 1.2V
and UVLO is released, at which point an internal current
of 1μA charges the soft-start capacitor, C_SS, connected
to the TRACK/SS pin. Current-limit foldback is disabled
during this phase to ensure smooth soft-start or track-
ing. The soft-start or tracking range is defined to be the
voltage range from 0V to 0.6V on the TRACK/SS pin. The
total soft-start time can be calculated as:
t_SS(SEC)
=
0.6(V)
•
C_SS(µF)
1(µA)
EXTERNAL SUPPLY
EXTERNAL SUPPLY
V_OUT
V_OUT
TIME
Coincident Tracking
TIME
3839 F09a
Ratiometric Tracking
Figure 9a. Two Different Modes of Output Tracking
EXTERNAL SUPPLY
TO
TRACK/SS
PIN
R_FB2(2)
R_FB1(2)
V_OUT
R_FB2(1)
TO
V_OUTSENSE+
R_FB1(1)
PIN
TO
V_{OUTSENSE–}
PIN _{3839 F09b}
Figure 9b. Setup for External Supply Tracking
3839fa
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