ltc3834-1 Linear Technology Corporation, ltc3834-1 Datasheet - Page 14

• Manufacturer Part Number: ltc3834-1
• Description: 30?a Iq Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3834-1
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:
To improve the frequency response, a feed-forward ca-
pacitor, C
route the V
inductor and the SW line.
SENSE
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
comparator’s negative input bias current. Since V
servoed to the 0.8V reference voltage, R
should be chosen to be less than 0.8V/I
determined from Figure 2 at the specified output voltage.
14
V
OUT
+
OUT
and SENSE
=
FF
0 8
FB
, may be used. Great care should be taken to
resistor divider to compensate for the current
.
line away from noise sources, such as the
V
Figure 1. Setting Output Voltage
•
⎛
⎜
⎝
LTC3834-1
1
U
+
–
Pins
R
R
B
A
V
FB
⎞
⎟
⎠
U
V
OUT
R
R
3834-1 F01
W
B
A
SENSE
C
FF
A
, with I
in Figure 1
U
SENSE
FB
is
Tracking and Soft-Start (TRACK/SS Pin)
The start-up of V
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
SS pin instead of 0.8V. The TRACK/SS pin can be used to
program an external soft-start function or to allow V
“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 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
according to the voltage on the TRACK/SS pin, allowing
V
The total soft-start time will be approximately:
OUT
t
Figure 3. Using the TRACK/SS Pin to Program Soft-Start
SS
to rise smoothly from 0V to its final regulated value.
=
–120
–150
–180
–210
–240
–270
–300
C
–30
–60
–90
60
30
Figure 2. SENSE Pins Input Bias Current
vs Common Mode (Output) Voltage
SS
0
0
•
FB
1
C
V
. 0 8
1μ
SS
SENSE
pin voltage to the voltage on the TRACK/
OUT
2
A
V
COMMON MODE VOLTAGE (V)
3
is controlled by the voltage on the
4
TRACK/SS
SGND
5
LTC3834-1
6
3834-1 F03
7
FB
pin (and hence V
8
38341 G14
9
10
OUT
OUT
38341f
to
)