LTC3828_15 Datasheet, PDF (14/32 Pages) Linear Technology – Dual, 2-Phase Step-Down Controller with Tracking

LTC3828
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 amplifier, 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 configured 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
V_OUT1
TO
TRCKSS2
PIN
R3 R1
TO
TO
V_OSENSE1 V_OSENSE2
PIN
PIN
R4 R2
V_OUT2
R3
R4
(5a) Coincident Tracking Setup
V_OUT1
TO
TRCKSS2
PIN
R1
TO
V_OSENSE1
PIN
TO
V_OSENSE2
PIN
R2
V_OUT2
R3
R4
3828 F05
(5b) Ratiometric Tracking Setup
Figure 5. Setup for Coincident and Ratiometric Tracking
⎛
⎝⎜
R1
R2
=
V_OUT1
0.8
– 1,R3
R4
=
V_OUT2
0.8
– 1^{⎞⎠⎟}
T
TRCKSS
0.8V
V_OSENSE
I
I
S1
+
D1
D2
EA
–
D3
3828 F06
Figure 6. Equivalent Input Circuit of
Error Amplifier of Channel 2
14
V_OUT1
TO TRCKSS2 PIN
TO V_OSENSE1 PIN
V_OUT2
R1
R4
TO V_OSENSE2 PIN
R2
R5
R3
3828 F07
Figure 7. Alternative Setup for Coincident Tracking
⎛
⎝⎜
R1+ R2
R3
=
V_OUT1
0.8
–
1, ^R1
R2 +R3
=
R4
R5
=
V_OUT2
0.8
– 1^{⎞⎠⎟}
3828fc