LTC3835-1 Linear Technology, LTC3835-1 Datasheet - Page 14

• Manufacturer Part Number: LTC3835-1
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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LTC3835-1
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:
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
fl ow out of both SENSE pins to the main output. In these
cases, the output can be easily pre-loaded by the V
resistor divider to compensate for the current comparator’s
negative input bias current. Since V
0.8V reference voltage, R
to be less than 0.8V/I
Figure 2 at the specifi ed output voltage.
APPLICATIONS INFORMATION
14
V
OUT
+
and SENSE
=
FF
0 8
, may be used. Great care should be taken to
FB
.
line away from noise sources, such as the
Figure 1. Setting Output Voltage
V
LTC3835-1
•
⎛
⎝ ⎜
1
–
+
SENSE
Pins
V
R
R
FB
B
A
A
⎞
⎠ ⎟
, with I
in Figure 1 should be chosen
V
OUT
R
R
3835-1 F01
B
A
SENSE
FB
C
is servoed to the
FF
determined from
OUT
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 LTC3835-1 regu-
lates the V
pin instead of 0.8V. The TRACK/SS pin can be used to
program an external soft-start function or to allow V
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 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
according to the voltage on the TRACK/SS pin, allowing
V
The total soft-start time will be approximately:
OUT
t
SS
Figure 3. Using the TRACK/SS Pin to Program Soft-Start
to rise smoothly from 0V to its fi nal regulated value.
=
C
–100
–200
–300
–400
–500
–600
–700
200
100
FB
SS
Figure 2. SENSE Pins Input Bias Current
vs Common Mode (Output) Voltage
0
0
pin voltage to the voltage on the TRACK/SS
•
. 0 8
1
1
V
SENSE
μA
OUT
C
SS
V
2
COMMON MODE VOLTAGE (V)
is controlled by the voltage on the
3
4
5
TRACK/SS
SGND
LTC3835-1
6
FB
7
3835-1 F03
pin (and hence V
8
38351 F02
9
10
38351fc
OUT
OUT
)