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| LTC3809-1_15 Datasheet, PDF (16/24 Pages) Linear Technology – No RSENSE, Low Input Voltage, Synchronous DC/DC Controller with Output Tracking | |||
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LTC3809-1
APPLICATIONS INFORMATION
For most applications, a 59k resistor is suggested for RA.
In applications where minimizing the quiescent current is
critical, RA should be made bigger to limit the feedback
divider current. If RB then results in very high impedance,
it may be beneï¬cial to bypass RB with a 50pF to 100pF
capacitor CFF.
VOUT
LTC3809-1
VFB
RB
CFF
RA
38091 F03
Figure 3. Setting Output Voltage
Run and Soft-Start/Tracking Functions
The LTC3809-1 has a low power shutdown mode which is
controlled by the RUN pin. Pulling the RUN pin below 1.1V
puts the LTC3809-1 into a low quiescent current shutdown
mode (IQ = 9μA). Releasing the RUN pin, an internal 0.7μA
(at VIN = 4.2V) current source will pull the RUN pin up
to VIN, which enables the controller. The RUN pin can be
driven directly from logic as showed in Figure 4.
3.3V OR 5V
LTC3809-1
RUN
LTC3809-1
RUN
38091 F04
Figure 4. RUN Pin Interfacing
Once the controller is enabled, the start-up of VOUT is con-
trolled by the state of the TRACK/SS pin. If the TRACK/SS
pin is connected to VIN, the start-up of VOUT is controlled
by internal soft-start, which slowly ramps the positive
reference to the error ampliï¬er from 0V to 0.6V, allowing
VOUT to rise smoothly from 0V to its ï¬nal value. The default
internal soft-start time is around 0.74ms. The soft-start
time can be changed by placing a capacitor between the
TRACK/SS pin and GND. In this case, the soft-start time
will be approximately:
tSS
=
CSS
â¢
600mV
1μA
where 1μA is an internal current source which is always on.
When the voltage on the TRACK/SS pin is less than the
internal 0.6V reference, the LTC3809-1 regulates the VFB
voltage to the TRACK/SS pin voltage instead of 0.6V.
Therefore the start-up of VOUT can ratiometrically track
an external voltage VX, according to a ratio set by a resis-
tor divider at TRACK/SS pin (Figure 5a). The ratiometric
relation between VOUT and VX is (Figure 5c):
VOUT = RTA ⢠RA + RB
VX RA RTA + RTB
VX
RTB
RTA
LTC3809-1
VFB
TRACK/SS
VOUT
RB
RA
38091 F5a
Figure 5a. Using the TRACK/SS Pin to Track VX
38091fc
16
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