LTC3810-5 Datasheet, PDF (28/36 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3810-5
APPLICATIONS INFORMATION
disabled and PGOOD signal is forced low during this phase.
The total soft-start time can be calculated as:
t_SOFTSTART = 0.8 • C_SS/1.4μA
When the device is configured to track another supply,
the feedback voltage of the other supply is duplicated
by a resistor divider and applied to the TRACK/SS pin.
Therefore, the voltage ramp rate on this pin is determined
by the ramp rate of the other supply output voltage.
Output Voltage Tracking
The LTC3810-5 allows the user to program how its out-
put ramps up by means of the TRACK/SS pin. Through
this pin, the output can be set up to either coincidentally
or ratiometrically track with another supply’s output, as
shown in Figure 15. In the following discussions, V_OUT1
refers to the master LTC3810-5’s output and V_OUT2 refers
to the slave LTC3810-5’s output.
To implement the coincident tracking in Figure 15a, con-
nect an additional resistive divider to V_OUT1 and connect its
midpoint to the TRACK/SS pin of the slave IC. The ratio of
this divider should be selected the same as that of the slave
IC’s feedback divider shown in Figure 16. In this tracking
mode, V_OUT1 must be set higher than V_OUT2. To implement
the ratiometric tracking, the ratio of the divider should be
exactly the same as the master IC’s feedback divider. Note
that the internal soft-start current will introduce a small
error on the tracking voltage depending on the absolute
values of the tracking resistive divider.
By selecting different resistors, the LTC3810-5 can achieve
different modes of tracking including the two in Figure 15.
So which mode should be programmed? While either
mode in Figure 15 satisfies most practical applications,
there do exist some tradeoffs. The ratiometric mode saves
a pair of resistors, but the coincident mode offers better
output regulation. This can be better understood with the
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
(15a) Coincident Tracking
TIME
38105 F15
(15b) Ratiometric Tracking
Figure 15. Two Different Modes of Output Voltage Tracking
V_OUT1
TO
TRACK/SS2
PIN
R3 R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R4 R2
V_OUT2
R3
R4
(16a) Coincident Tracking Setup
V_OUT1
TO
TRACK/SS2
PIN
R1
TO
V_FB1
PIN
TO
V_FB2
PIN
R2
V_OUT2
R3
R4
38105 F16
(16b) Ratiometric Tracking Setup
Figure 16. Setup for Coincident and Ratiometric Tracking
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