LTC3838-1_15 Datasheet, PDF (29/52 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with Dual Differential Output Sensing

LTC3838-1
APPLICATIONS INFORMATION
When one particular channel is configured to track an
external supply, a voltage divider can be used from the
external supply to the TRACK/SS pin to scale the ramp
rate appropriately. Two common implementations are co-
incidental tracking and ratiometric tracking. For coincident
tracking, make the divider ratio from the external supply
the same as the divider ratio for the differential feedback
voltage. Ratiometric tracking could be achieved by using
a different ratio than the differential feedback.
Note that the 1µA soft-start capacitor charging current is
still flowing, producing a small offset error. To minimize
this error, select the tracking resistive divider values to be
small enough to make this offset error negligible.
The LTC3838-1 allows the user to program how its two
channels’ outputs track each other ramping up or down.
In the following discussions, V_OUT1 refers to the LTC3838-
1’s output 1 as a master channel and V_OUT2 refers to the
LTC3838-1’s output 2 as a slave channel. In practice
though, either channel can be used as the master.
By selecting different resistors, the LTC3838-1 can
achieve different modes of tracking including the two in
Figure 9. To implement the coincident tracking, connect
an additional resistive divider to V_OUT1 and connect its
midpoint to the TRACK/SS pin of the slave channel. The
ratio of this divider should be the same as that of the slave
channel’s feedback divider shown in Figure 9b. In this
tracking mode, V_OUT1 must be set higher than V_OUT2. To
implement the ratiometric tracking as shown in Figure 9,
the additional divider should be of the same ratio as the
master channel’s feedback divider.
Under the ratiometric mode, when the master channel’s
output experiences dynamic excursion (under load tran-
sient, for example), the slave channel output will be affected
as well. For better output regulation, use the coincident
tracking mode instead of ratiometric, or use the additional
divider with a ratio of somewhere between coincident and
ratiometric tracking modes.
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
Coincident Tracking
TIME
38381 F09a
Ratiometric Tracking
Figure 9a. Two Different Modes of Output Tracking
V_OUT1+
TO
TRACK/SS2
PIN
R_DFB2
R_DFB1
R_FB2(1)
TO
V_OUTSENSE1+
R_FB1(1)
PIN
TO
V_{OUTSENSE1–}
PIN
TO
V_DFB2+
PIN
TO
V_DFB2–
PIN
V_OUT2+
V_OUT1+
R_DFB2
TO
TRACK/SS2
PIN
R_DFB1 R_DFB1//R_DFB2
R_FB2(1)
R_FB1(1)
R_FB2(1)
TO
TO
V_OUTSENSE1+ V_DFB2+
PIN
R_{FB1(1) TO}
PIN
TO
V_{OUTSENSE1–} V_DFB2–
PIN
PIN
V_OUT2+
R_DFB2
R_DFB1 R_DFB1//R_DFB2
SGND V_OUT1–
V_OUT2– SGND
SGND
V_OUT1–
V_OUT2– SGND
Coincident Tracking Setup
Ratiometric Tracking Setup
Figure 9b. Setup for Coincident and Ratiometric Tracking
38381 F09b
38381f
For more information www.linear.com3838-1
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