LTC3857IUH#TRPBF Linear Technology, LTC3857IUH#TRPBF Datasheet - Page 20

• Manufacturer Part Number: LTC3857IUH#TRPBF
• Description: IC CTRLR STP-DN SYNC DUAL 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3857IUH#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 24 V
• Frequency - Switching: 50kHz ~ 900kHz
• Voltage - Input: 4 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

LTC3857
Tracking and Soft-Start (TRACK/SS Pins)
The start-up of each V
the respective TRACK/SS pin. When the voltage on the
TRACK/SS pin is less than the internal 0.8V reference,
the LTC3857 regulates the V
on the TRACK/SS pin instead of 0.8V. The TRACK/SS pin
can be used to program an external soft-start function or
to allow V
Soft-start is enabled by simply connecting a capacitor
from the TRACK/SS pin to ground, as shown in Figure 6.
An internal 1μA current source charges the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3857 will regulate the V
according to the voltage on the TRACK/SS pin, allowing
V
The total soft-start time will be approximately:
Alternatively, the TRACK/SS pin can be used to track two
(or more) supplies during start-up, as shown qualita-
tively in Figures 7a and 7b. To do this, a resistor divider
should be connected from the master supply (V
TRACK/SS pin of the slave supply (V
Figure 8. During start-up V
to the ratio set by the resistor divider:
For coincident tracking (V
APPLICATIONS INFORMATION
20
OUT
R
R
t
V
SS
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
A
B
V
OUT
to rise smoothly from 0V to its final regulated value.
X
= R
= R
=C
=
SS
TRACKA
TRACKB
OUT
R
•
TRACKA
0.8V
1μA
R
to track another supply during start-up.
A
•
R
OUT
C
TRACKA
SS
OUT
is controlled by the voltage on
TRACK/SS
SGND
1/2 LTC3857
R
OUT
FB
A
= V
pin voltage to the voltage
+R
+R
FB
will track V
3857 F06
X
TRACKB
B
pin (and hence V
during start-up):
OUT
), as shown in
X
according
X
) to the
OUT
)
Figure 7. Two Different Modes of Output Voltage Tracking
Figure 8. Using the TRACK/SS Pin for Tracking
R
R
TRACKB
TRACKA
(7b) Ratiometric Tracking
(7a) Coincident Tracking
V
x
V
TIME
TIME
OUT
R
R
B
A
V
TRACK/SS
1/2 LTC3857
FB
3857 F07a
3857 F07b
3857 F08
V
V
V
V
X(MASTER)
OUT(SLAVE)
X(MASTER)
OUT(SLAVE)
3857fc