LTC3855IFE#PBF Linear Technology, LTC3855IFE#PBF Datasheet - Page 23

• Manufacturer Part Number: LTC3855IFE#PBF
• Description: IC CTLR DC/DC MULTIPHASE 38SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3855IFE#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 3.3 V, 0.6 ~ 12.5 V
• Current - Output: 25A
• Frequency - Switching: 250kHz ~ 770kHz
• Voltage - Input: 4.5 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 38-TSSOP Exposed Pad, 38-eTSSOP, 38-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

applicaTions inForMaTion
producing a small offset error. To minimize this error, select
the tracking resistive divider value to be small enough to
make this error negligible.
In order to track down another channel or supply after
the soft-start phase expires, the LTC3855 is forced into
continuous mode of operation as soon as V
undervoltage threshold of 0.54V regardless of the setting
on the MODE/PLLIN pin. However, the LTC3855 should
always be set in force continuous mode tracking down
when there is no load. After TK/SS drops below 0.1V, its
channel will operate in discontinuous mode.
Output Voltage Tracking
The LTC3855 allows the user to program how its output
ramps up and down by means of the TK/SS pins. Through
these pins, the output can be set up to either coincidentally
or ratiometrically track another supply’s output, as shown
in Figure 8. In the following discussions, V
the LTC3855’s output 1 as a master channel and V
refers to the LTC3855’s output 2 as a slave channel. In
practice, though, either phase can be used as the master.
TK/SS2
V
OUT1
PIN
(8a) Coincident Tracking
TO
(9a) Coincident Tracking Setup
TIME
R3
R4
R1
R2
Figure 8. Two Different Modes of Output Voltage Tracking
Figure 9. Setup for Coincident and Ratiometric Tracking
TO
V
PIN
FB1
3855
FB
OUT1
F08a
V
V
V
PIN
OUT1
OUT2
FB2
TO
is below the
refers to
R3
R4
OUT2
V
OUT2
To implement the coincident tracking in Figure 8a, con-
nect an additional resistive divider to V
its midpoint to the TK/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 9a. In this
tracking mode, V
implement the ratiometric tracking in Figure 9b, the ratio of
the V
channel’s feedback divider shown in Figure 9b. By select-
ing different resistors, the LTC3855 can achieve different
modes of tracking including the two in Figure 8.
So which mode should be programmed? While either
mode in Figure 8 satisfies most practical applications,
some tradeoffs exist. The ratiometric mode saves a pair
of resistors, but the coincident mode offers better output
regulation.
When the master channel’s output experiences dynamic
excursion (under load transient, for example), the slave
channel output will be affected as well. For better output
regulation, use the coincident tracking mode instead of
ratiometric.
OUT2
V
TK/SS2
OUT1
PIN
(9b) Ratiometric Tracking Setup
TO
divider should be exactly the same as the master
R1
R2
OUT1
(8b) Ratiometric Tracking
TO
V
PIN
FB1
must be set higher than V
V
TIME
PIN
FB2
TO
R3
R4
3855
F09
V
OUT2
OUT1
LTC3855
3855
F08b
V
V
OUT1
OUT2
and connect
OUT2

. To
3855f