Datasheet

LTC3708
18
3708fb
APPLICATIONS INFORMATION
The current limit value should be checked to ensure that
I
LIMIT(MIN)
> I
OUT(MAX)
. The minimum value of current
limit generally occurs with the largest V
IN
at the highest
ambient temperature, conditions which cause the largest
power loss in the converter. Note that it is important to
check for self-consistency between the assumed junction
temperature 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 can
be easily available in the 5%, 2% or 1% tolerance. The
temperature coeffi cient of these resistors is very low,
ranging from ±250ppm/°C to ±75ppm/°C. In this case, the
(R
DS(ON)
• ρ
T
) product 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 LTC3708 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 270ns.
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. The minimum
input voltage to avoid dropout is:
VV
tt
t
IN MIN OUT
ON OFF MIN
ON
()
()
=
+
A plot of maximum frequency vs duty cycle is shown in
Figure 5.
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC3708 as well as a timer for soft-start and overcurrent
latchoff.
Pulling the RUN/SS pin below 0.8V shuts down the
LTC3708. Releasing the pin allows an internal 1.2μA in-
ternal 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
V
μA
CsμFC
DELAY SS SS
==
()
13
12
11
.
.
•./
When the RUN/SS voltage reaches the ON threshold
(typically 1.3V), the LTC3708 begins operating with a
clamp on channel 1’s reference voltage. The clamp level
is one threshold voltage below RUN/SS. As the voltage on
RUN/SS continues to rise, channel 1’s reference is raised
at the same rate, achieving monotonic output voltage
soft-start (Figure 6). When RUN/SS rises 0.6V above the
ON threshold, the reference clamp is invalidated and the
internal precision reference takes over. When channel 2 is
tracked to channel 1, soft-start on channel 2 is automati-
cally achieved (see Output Voltage Tracking).
2.0
1.5
1.0
0.5
0
0 0.25 0.50 0.75
3708 F05
1.0
DROPOUT
REGION
DUTY CYCLE (V
OUT
/V
IN
)
SWITCHING FREQUENCY (MHz)
Figure 5. Maximum Switching Frequency vs Duty Cycle