LTC3709 Datasheet, PDF (17/24 Pages) Linear Technology – Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

APPLICATIO S I FOR ATIO
2.0
1.5
DROPOUT
REGION
1.0
0.5
LTC3709
ON THRESHOLD
∆V = 0.6V
RUN/SS
TIME
V_OUT1
0
0
0.25
0.50
0.75
1.0
DUTY CYCLE (V_OUT/V_IN)
3709 F03
Figure 3. Maximum Switching Frequency vs Duty Cycle
TIME
3709 F04
Figure 4. Monotonic Soft-Start Waveforms
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC3709 as well as a timer for soft-start and overcurrent
latchoff.
Pulling the RUN/SS pin below 1.4V puts the LTC3709 into
a low quiescent current shutdown (I_Q < 30µA). Releasing
the pin allows an internal 1.2µA internal current source to
charge the external capacitor C_SS. If RUN/SS has been
pulled all the way to ground, there is a delay before starting
of about:
( ) ^t_DELAY
=
1.4V
1.2µA
•
C_SS
=
1.2s /µF C_SS
When the RUN/SS voltage reaches the ON threshold
(typically 1.4V), the LTC3709 begins operating with a
clamp on EA’s reference voltage. The clamp level is one ON
threshold voltage below RUN/SS. As the voltage on RUN/SS
continues to rise, EA’s reference is raised at the same rate,
achieving monotonic output voltage soft-start (Figure 4).
When RUN/SS rises 0.6V above the ON threshold, the
reference clamp is invalidated and the internal precision
reference takes over.
After the controller has been started and given adequate
time to charge the output capacitor, C_SS is used as a short-
circuit timer. After the RUN/SS pin charges above 3V, and
if the output voltage falls below 67% of its regulated value,
then a short-circuit fault is assumed. A 2µA current then
begins discharging C_SS. If the fault condition persists until
the RUN/SS pin drops to 2.5V, then the controller turns off
both power MOSFETs, shutting down the converter per-
manently. The RUN/SS pin must be actively pulled down
to ground in order to restart operation.
The overcurrent protection timer requires that the soft-
start timing capacitor C_SS be made large enough to guar-
antee that the output is in regulation by the time C_SS has
reached the 3V threshold. In general, this will depend upon
the size of the output capacitance, output voltage and load
current characteristic. A minimum soft-start capacitor can
be estimated from:
C_SS > C_OUT V_OUT R_SENSE (10^–4 [F/V_S])
Overcurrent latchoff operation is not always needed or
desired and can prove annoying during troubleshooting.
The feature can be overridden by adding a pull-up current
of >5µA to the RUN/SS pin. The additional current pre-
vents the discharge of C_SS during a fault and also shortens
the soft-start period. Using a resistor to V_IN as shown in
Figure 5 is simple, but slightly increases shutdown cur-
rent. Any pull-up network must be able to pull RUN/SS
above the 3V threshold that arms the latchoff circuit and
overcome the 2µA maximum discharge current.
3709f
17