Datasheet

LTC3827
19
3827ff
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
fl ow out of both SENSE pins to the main output. In these
cases, the output can be easily pre-loaded by the V
OUT
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 specifi ed 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,
Figure 3. Setting Output Voltage
Figure 5. Using the TRACK/SS Pin to Program Soft-Start
Figure 4. SENSE Pins Input Bias Current
vs Common Mode (Output) Voltage
1/2 LTC3827
V
FB
V
OUT
R
B
C
FF
R
A
3827 F03
V
SENSE
COMMON MODE VOLTAGE (V)
0
–700
INPUT CURRENT (µA)
–600
–400
–300
–200
6789
200
3827 F04
–500
12345 10
–100
0
100
1/2 LTC3827
TRACK/SS
C
SS
SGND
3827 F05