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LTC3851-1_15 Datasheet, PDF (15/28 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller | |||
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LTC3851-1
APPLICATIONS INFORMATION
0V to 0.8V on the TK/SS pin. The total soft-start time can
be calculated as:
tSOFT-START
=
0.8
â¢
CSS
1.0μA
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator will always start in pulse skipping mode up
to TK/SS = 0.64V. Between TK/SS = 0.64V and 0.72V, it
will operate in forced continuous mode and revert to the
selected mode once TK/SS > 0.72V. The output ripple
is minimized during the 80mV forced continuous mode
window.
When the regulator is conï¬gured to track another supply,
the feedback voltage of the other supply is duplicated by a
resistor divider and applied to the TK/SS pin. Therefore, the
voltage ramp rate on this pin is determined by the ramp rate
of the other supplyâs voltage. Note that the small soft-start
capacitor charging current is always ï¬owing, producing a
small offset error. To minimize this error, one can select
the tracking resistive divider value to be small enough to
make this error negligible.
In order to track down another supply after the soft-start
phase expires, the LTC3851-1 must be conï¬gured for
forced continuous operation by connecting MODE/PLLIN
to INTVCC.
Output Voltage Tracking
The LTC3851-1 allows the user to program how its output
ramps up and down by means of the TK/SS pins. Through
this pin, the output can be set up to either coincidentally or
ratiometrically track with another supplyâs output, as shown
in Figure 3. In the following discussions, VMASTER refers to
a master supply and VOUT refers to the LTC3851-1âs output
as a slave supply. To implement the coincident tracking in
Figure 3a, connect a resistor divider to VMASTER and connect
its midpoint to the TK/SS pin of the LTC3851-1. The ratio
of this divider should be selected the same as that of the
LTC3851-1âs feedback divider as shown in Figure 4a. In
this tracking mode, VMASTER must be higher than VOUT.
To implement ratiometric tracking, the ratio of the resistor
divider connected to VMASTER is determined by:
VOUT
VMASTER
=
R2 âR3 + R4â
R4 ââR1+ R2â â
So which mode should be programmed? While either
mode in Figure 4 satisï¬es most practical applications,
the coincident mode offers better output regulation.
This concept can be better understood with the help
of Figure 5. At the input stage of the LTC3851-1âs error
ampliï¬er, two common anode diodes are used to clamp
the equivalent reference voltage and an additional diode
VMASTER
VMASTER
VOUT
VOUT
TIME
(3a) Coincident Tracking
TIME
38511 F03
(3b) Ratiometric Tracking
Figure 3. Two Different Modes of Output Voltage Tracking
38511fa
15
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