LTC3827 Datasheet, PDF (19/36 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3827
APPLICATIONS INFORMATION
where f is the operating frequency, C_OUT is the output
capacitance and I_RIPPLE is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since I_RIPPLE increases with input voltage.
Setting Output Voltage
The LTC3827 output voltages are each set by an external
feedback resistor divider carefully placed across the out-
put, as shown in Figure 3. The regulated output voltage
is determined by:
V_OUT
=
0.8V
•



1+
R_B
R_A


To improve the frequency response, a feed-forward ca-
pacitor, C_FF, may be used. Great care should be taken to
route the V_FB line away from noise sources, such as the
inductor or the SW line.
SENSE⁺ and SENSE^– Pins
The common mode input range of the current comparator
is from 0V to 10V. Continuous linear operation is provided
throughout this range allowing output voltages from 0.8V
to 10V. The input stage of the current comparator requires
that current either be sourced or sunk from the SENSE pins
depending on the output voltage, as shown in the curve in
Figure 4. If the output voltage is below 1.5V, current will
flow out of both SENSE pins to the main output. In these
cases, the output can be easily pre-loaded by the V_OUT
V_OUT
1/2 LTC3827
V_FB
R_B
C_FF
R_A
3827 F03
Figure 3. Setting Output Voltage
200
100
0
–100
–200
–300
–400
–500
–600
–700
0
1 2 3 4 5 6 7 8 9 10
V_SENSE COMMON MODE VOLTAGE (V)
3827 F04
Figure 4. SENSE Pins Input Bias Current
vs Common Mode (Output) Voltage
1/2 LTC3827
TRACK/SS
C_SS
SGND
3827 F05
Figure 5. Using the TRACK/SS Pin to Program Soft-Start
resistor divider to compensate for the current comparator’s
negative input bias current. Since V_FB is servoed to the
0.8V reference voltage, R_A in Figure 3 should be chosen
to be less than 0.8V/I_SENSE, with I_SENSE determined from
Figure 4 at the specified output voltage.
Tracking and Soft-Start (TRACK/SS Pins)
The start-up of each V_OUT is controlled by the voltage on
the respective TRACK/SS pin. When the voltage on the
TRACK/SS pin is less than the internal 0.8V reference, the
LTC3827 regulates the V_FB 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 V_OUT 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 1μA current source charges up the capacitor,
3827ff
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