Datasheet

LTC3835-1
14
38351fc
APPLICATIONS INFORMATION
Setting Output Voltage
The LTC3835-1 output voltage is set by an external feed-
back resistor divider carefully placed across the output,
as shown in Figure 1. The regulated output voltage is
determined by:
VV
R
R
OUT
B
A
=+
⎛
⎝
⎜
⎞
⎠
⎟
08 1.•
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 and the SW line.
Tracking and Soft-Start (TRACK/SS Pin)
The start-up of V
OUT
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 LTC3835-1 regu-
lates 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.
LTC3835-1
V
FB
V
OUT
R
B
C
FF
R
A
3835-1 F01
Figure 1. Setting Output Voltage
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 2. 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 1 should be chosen
to be less than 0.8V/I
SENSE
, with I
SENSE
determined from
Figure 2 at the specifi ed output voltage.
V
SENSE
COMMON MODE VOLTAGE (V)
0
–700
INPUT CURRENT (μA)
–600
–400
–300
–200
6789
200
38351 F02
–500
12345 10
–100
0
100
Figure 2. SENSE Pins Input Bias Current
vs Common Mode (Output) Voltage
LTC3835-1
TRACK/SS
C
SS
SGND
3835-1 F03
Figure 3. Using the TRACK/SS Pin to Program Soft-Start
Soft-start is enabled by simply connecting a capacitor
from the TRACK/SS pin to ground, as shown in Figure 3.
An internal 1μA current source charges up the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3835-1 will regulate the V
FB
pin (and hence V
OUT
)
according to the voltage on the TRACK/SS pin, allowing
V
OUT
to rise smoothly from 0V to its fi nal regulated value.
The total soft-start time will be approximately:
tC
V
μA
SS SS
= •
.08
1