LTC3834-1 Datasheet, PDF (14/28 Pages) Linear Technology – 30μA IQ Synchronous Step-Down Controller

LTC3834-1
APPLICATIO S I FOR ATIO
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:
V_OUT
=
0.8V •^{⎛⎝⎜}1+
R_B
R_A
⎞
⎠⎟
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.
LTC3834-1
V_FB
V_OUT
R_B
C_FF
R_A
3834-1 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 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.
60
30
0
–30
–60
–90
–120
–150
–180
–210
–240
–270
–300
0
1 2 3 4 5 6 7 8 9 10
V_SENSE COMMON MODE VOLTAGE (V)
38341 G14
Figure 2. SENSE Pins Input Bias Current
vs Common Mode (Output) Voltage
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.
LTC3834-1
TRACK/SS
C_SS
SGND
3834-1 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 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:
t_SS
=
C_SS
•
0.8V
1μA
38341f
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