Datasheet

LTC3809-1
16
38091fc
For most applications, a 59k resistor is suggested for R
A
.
In applications where minimizing the quiescent current is
critical, R
A
should be made bigger to limit the feedback
divider current. If R
B
then results in very high impedance,
it may be benefi cial to bypass R
B
with a 50pF to 100pF
capacitor C
FF
.
APPLICATIONS INFORMATION
Figure 3. Setting Output Voltage
LTC3809-1
V
FB
V
OUT
R
B
C
FF
R
A
38091 F03
Once the controller is enabled, the start-up of V
OUT
is con-
trolled by the state of the TRACK/SS pin. If the TRACK/SS
pin is connected to V
IN
, the start-up of V
OUT
is controlled
by internal soft-start, which slowly ramps the positive
reference to the error amplifi er from 0V to 0.6V, allowing
V
OUT
to rise smoothly from 0V to its fi 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:
tC
mV
A
SS SS
=
μ
•
600
1
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 V
FB
voltage to the TRACK/SS pin voltage instead of 0.6V.
Therefore the start-up of V
OUT
can ratiometrically track
an external voltage V
X
, according to a ratio set by a resis-
tor divider at TRACK/SS pin (Figure 5a). The ratiometric
relation between V
OUT
and V
X
is (Figure 5c):
V
V
R
R
RR
RR
OUT
X
TA
A
AB
TA TB
=
+
+
•
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 (I
Q
= 9μA). Releasing the RUN pin, an internal 0.7μA
(at V
IN
= 4.2V) current source will pull the RUN pin up
to V
IN
, which enables the controller. The RUN pin can be
driven directly from logic as showed in Figure 4.
Figure 4. RUN Pin Interfacing
3.3V OR 5V
38091 F04
LTC3809-1
RUN
LTC3809-1
RUN
Figure 5a. Using the TRACK/SS Pin to Track V
X
LTC3809-1
V
FB
V
OUT
V
X
TRACK/SS
R
B
R
A
38091 F5a
R
TB
R
TA