LTC3709_15 Datasheet, PDF (16/24 Pages) Linear Technology – Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

LTC3709
APPLICATIO S I FOR ATIO
self-consistency between the assumed junction tempera-
ture and the resulting value of I_LIMIT, which heats the
junction.
Caution should be used when setting the current limit based
upon the R_DS(ON) of the MOSFETs. The maximum current
limit is determined by the minimum MOSFET on-resistance.
Data sheets typically specify nominal and maximum values
for R_DS(ON), but not a minimum. A reasonable assumption
is that the minimum R_DS(ON) lies the same amount below
the typical value as the maximum lies above it. Consult the
MOSFET manufacturer for further guidelines.
For a more accurate current limiting, a sense resistor can
be used. Sense resistors in the 1W power range are easily
available with 5%, 2% or 1% tolerance. The temperature
coefficient of these resistors are very low, ranging from
±250ppm/°C to ±75ppm/°C. In this case, the denomina-
tor in the above equation can simply be replaced by the
R_SENSE value.
Minimum Off-Time and Dropout Operation
The minimum off-time t_OFF(MIN) is the smallest amount of
time that the LTC3709 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 250ns. The
minimum off-time limit imposes a maximum duty cycle of
t_ON/(t_ON + t_OFF(MIN)). If the maximum duty cycle is reached,
due to a dropping input voltage for example, then the
output will drop out of regulation in order to maintain the
duty cycle at its limit. The minimum input voltage to avoid
dropout is:
V_IN(MIN)
=
V_OUT
1–
1
t_OFF(MIN)
•
f
A plot of maximum duty cycle vs frequency is shown in
Figure 3.
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.
2.0
1.5
DROPOUT
REGION
1.0
0.5
0
0
0.25
0.50
0.75
1.0
DUTY CYCLE (V_OUT/V_IN)
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Figure 3. Maximum Switching Frequency vs Duty Cycle
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:
( ) ^tDELAY
=
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).
ON THRESHOLD
∆V = 0.6V
RUN/SS
TIME
V_OUT1
TIME
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16
Figure 4. Monotonic Soft-Start Waveforms
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