Datasheet

14
LTC3834-1
38341f
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 LTC3834-1 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 final regulated value.
The total soft-start time will be approximately:
tC
V
A
SS SS
= •
.08
1μ
Figure 3. Using the TRACK/SS Pin to Program Soft-Start
LTC3834-1
TRACK/SS
C
SS
SGND
3834-1 F03
APPLICATIO S I FOR ATIO
WUUU
Setting Output Voltage
The LTC3834-1 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 deter-
mined 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 LTC3834-1
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.
Figure 1. Setting Output Voltage
Figure 2. SENSE Pins Input Bias Current
vs Common Mode (Output) Voltage
LTC3834-1
V
FB
V
OUT
R
B
C
FF
R
A
3834-1 F01
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 flow 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 specified output voltage.
V
SENSE
COMMON MODE VOLTAGE (V)
–120
–150
–180
–210
–240
30
0
–30
–60
–90
60
6789
38341 G14
–270
32104
510
–300
INPUT BIAS CURRENT (μA)