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LTC3807_15 Datasheet, PDF (20/32 Pages) Linear Technology – Low IQ, Synchronous Step-Down Controller with 24V Output Voltage Capability
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LTC3807
APPLICATIONS INFORMATION
The rising and falling UVLO thresholds are calculated using
the RUN pin thresholds:
VUVLO(RISING)
=
1.21V


1+
RB
RA


–
7µA
•
RB
VUVLO(FALLING)
=
1.16V


1+
RB
RA


–
7µA
• RB
The resistor values should be carefully chosen such that
the absolute maximum ratings of the RUN pin do not get
violated over the entire VIN voltage range.
pin of the slave supply (VOUT), as shown in Figure 7.
During start-up VOUT will track VX according to the ratio
set by the resistor divider:
VX = RA • RTRACKA +RTRACKB
VOUT RTRACKA
RA +RB
For coincident tracking (VOUT = VX during start-up):
RA = RTRACKA
RB = RTRACKB
Tracking and Soft-Start (TRACK/SS Pin)
The start-up of VOUT is controlled by the voltage on the
TRACK/SS pin. When the voltage on the TRACK/SS pin is
less than the internal 0.8V reference, the LTC3807 regulates
the VFB pin voltage to the voltage 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 VOUT to track
another supply during start-up.
Soft-start is enabled by simply connecting a capacitor
from the TRACK/SS pin to ground, as shown in Figure 5.
An internal 10μA current source charges the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3807 will regulate the VFB pin (and hence VOUT)
according to the voltage on the TRACK/SS pin, allowing
VOUT to rise smoothly from 0V to its final regulated value.
The total soft-start time will be approximately:
tSS
=
CSS
•
0.8V
10µA
LTC3807
TRACK/SS
CSS
SGND
3807 F05
Figure 5. Using the TRACK/SS Pin to Program Soft-Start
Alternatively, the TRACK/SS pin can be used to track
another supply during start-up, as shown qualitatively in
Figures 6a and 6b. To do this, a resistor divider should be
connected from the master supply (VX) to the TRACK/SS
VX(MASTER)
VOUT(SLAVE)
TIME
3807 F06a
(6a) Coincident Tracking
VX(MASTER)
VOUT(SLAVE)
TIME
3807 F06b
(6b) Ratiometric Tracking
Figure 6. Two Different Modes of Output Voltage Tracking
Vx VOUT
RTRACKB
RTRACKA
RB
LTC3807
VFB
RA
TRACK/SS
3807 F07
Figure 7. Using the TRACK/SS Pin for Tracking
3807f
20
For more information www.linear.com/LTC3807
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