LTC3828
14
3828fc
APPLICATIONS INFORMATION
By selecting different resistors, the LTC3828 can achieve
different modes of tracking including the two in Figure 4.
Figure 6 helps to explaining the tracking function. At the
input stage of an error amplifi er, two diodes are used to
clamp the equivalent reference voltage and an additional
diode is used to match the shifted common mode voltage.
The top two current sources are of the same value. When
the TRCKSS voltage is low, switch S1 is on and V
OSENSE
follows the TRCKSS voltage. When the TRCKSS voltage
is close to 0.8V, the reference voltage, switch S1, is off
and V
OSENSE
follows the reference voltage. The regulation
for both channels’ outputs are not affected by the tracking
mode. In the ratiometric tracking mode, the two channels
do not exhibit cross talk.
The number of resistors in Figure 5a can be further reduced
with the scheme in Figure 7.
In a system that requires more than two tracked supplies,
multiple LTC3828s can be daisy-chained through the
TRCKSS1 pin. TRCKSS1 clamps channel 1’s reference in
the same manner TRCKSS2 clamps channel 2. To eliminate
the possibility of multiple LTC3828s coming on at different
times, only the master LTC3828’s TRCKSS1 pin should
be connected to a soft-start capacitor. Figure 8 shows the
circuit with four outputs. Three of them are programmed
in the coincident mode while the fourth one tracks ratio-
metrically. If output tracking is not needed, the TRCKSS pins
are used as soft start-up pins. The capacitors connected
to those pins set the soft-start ramping up speed.
Figure 15 is a basic LTC3828 application circuit. External
component selection is driven by the load requirement,
and begins with the selection of R
SENSE
and the inductor
value. Next, the power MOSFETs and D1 are selected.
Finally, C
IN
and C
OUT
are selected. The circuit shown in
Figure 15 is confi gured for operation up to an input voltage
of 28V (limited by the external MOSFETs).
R
SENSE
Selection For Output Current
R
SENSE
is chosen based on the required output current.
The current comparator has a maximum threshold of
75mV/R
SENSE
and an input common mode range of SGND
R3 R1
R4 R2
R3
V
OUT2
R4
(5a) Coincident Tracking Setup
TO
V
OSENSE1
PIN
TO
TRCKSS2
PIN
TO
V
OSENSE2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3828 F05
(5b) Ratiometric Tracking Setup
TO
V
OSENSE1
PIN
TO
TRCKSS2
PIN
TO
V
OSENSE2
PIN
V
OUT1
Figure 5. Setup for Coincident and Ratiometric Tracking
R
R
V
R
R
V
T
OUT OUT
1
208
1
3
408
1
12
==
⎛
⎝
⎜
⎞
⎠
⎟
.
–,
.
–
–
+
II
D1
TRCKSS
0.8V
V
OSENSE
D2
D3
3828 F06
EA
S1
R1 R4
V
OUT1
V
OUT2
TO TRCKSS2 PIN
TO V
OSENSE1
PIN
TO V
OSENSE2
PIN
R2 R5
R3
3828 F07
Figure 7. Alternative Setup for Coincident Tracking
RR
R
V
R
RR
R
R
V
OUT OUT
12
308
1
1
23
4
508
1
12
+
=
+
==
⎛
.
–,
.
–
⎝⎝
⎜
⎞
⎠
⎟
Figure 6. Equivalent Input Circuit of
Error Amplifi er of Channel 2
