ltc3827ig-1 Linear Technology Corporation, ltc3827ig-1 Datasheet - Page 16

• Manufacturer Part Number: ltc3827ig-1
• Description: Low Iq, Dual, 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3827-1
Setting Output Voltage
The LTC3827-1 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:
To improve the frequency response, a feed-forward ca-
pacitor, C
route the V
inductor or 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 4. 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 4 at the specified output voltage.
16
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 3. Setting Output Voltage
•
1/2 LTC3827-1
⎛
⎜
⎝
1
U
+
–
Pins
R
R
V
B
A
FB
⎞
⎟
⎠
U
V
OUT
R
R
3827-1 F03
B
A
W
SENSE
C
FF
A
, with I
in Figure 3
U
SENSE
FB
is
Tracking and Soft-Start (TRACK/SS Pins)
The start-up of each V
the respective TRACK/SS pin. When the voltage on the
TRACK/SS pin is less than the internal 0.8V reference, the
LTC3827-1 regulates the V
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
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,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3827-1 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 5. Using the TRACK/SS Pin to Program Soft-Start
to rise smoothly from 0V to its final regulated value.
OUT
=
C
SS
to “track” another supply during start-up.
–100
–200
–300
–400
–500
–600
–700
200
100
Figure 4. SENSE Pins Input Bias Current
vs Common Mode Voltage
0
•
C
0
SS
. 0 8
1μ
1
V
A
SENSE
V
2
OUT
COMMON MODE VOLTAGE (V)
3
1/2 LTC3827-1
TRACK/SS
SGND
is controlled by the voltage on
FB
4
pin voltage to the voltage on
5
38271 F05
FB
6
pin (and hence V
7
8
9
38271 F04
10
38271fd
OUT
)