LTC3709 Linear Technology, LTC3709 Datasheet - Page 17

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LTC3709

Manufacturer Part Number
LTC3709
Description
No RSENSE Synchronous DC/DC Controller
Manufacturer
Linear Technology
Datasheet

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APPLICATIO S I FOR ATIO
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
the pin allows an internal 1.2µA internal current source to
charge the external capacitor C
pulled all the way to ground, there is a delay before starting
of about:
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
circuit timer. After the RUN/SS pin charges above 3V, and
t
DELAY
Figure 3. Maximum Switching Frequency vs Duty Cycle
=
2.0
1.5
1.0
0.5
1 2
0
1 4
.
0
.
µ
V
A
U
0.25
C
DUTY CYCLE (V
SS
U
=
(
0.50
1 2
.
OUT
SS
s F C
DROPOUT
/
REGION
/V
. If RUN/SS has been
µ
W
IN
0.75
Q
SS
)
< 30µA). Releasing
)
is used as a short-
SS
3709 F03
1.0
U
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
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
antee that the output is in regulation by the time C
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:
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
the soft-start period. Using a resistor to V
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.
ON THRESHOLD
C
SS
> C
Figure 4. Monotonic Soft-Start Waveforms
OUT
V
OUT
∆V = 0.6V
R
SS
SENSE
. If the fault condition persists until
SS
SS
be made large enough to guar-
during a fault and also shortens
(10
–4
[F/V
RUN/SS
V
TIME
TIME
S
OUT1
3709 F04
])
LTC3709
IN
as shown in
17
SS
has
3709f

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