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LT3746
APPLICATIONS INFORMATION
frequency is usually chosen to keep the switching noise
out of a sensitive frequency band.
Switching Frequency Setting and Synchronization
The LT3746 uses a constant switching frequency that can
be programmed from 200kHz to 1MHz with a resistor
from the RT pin to ground. Table 2 shows R T values for
common switching frequencies.
Table 2. Switching Frequency f SW vs R T Value
from 0V to (V IN – 2V) or 13V absolute maximum value,
whichever is lower. The current sense amplifier not only
provides current information to form the current-mode
control, but also a 46.5mV threshold. The 46.5mV threshold
across the R S resistor imposes an accurate current limit
to protect both P-channel MOSFET M1 and catch diode
D1, and also to prevent inductor current saturation. Good
Kelvin sensing is required for accurate current limit. The
R S resistor value can be determined by:
I OUT(MAX) = I L(MAX) –
f SW (kHz)
200
300
400
R T * (k Ω)
280
182
133
Δ I L
2
where the maximum inductor current I L(MAX) is set by:
500
600
700
105
84.5
71.5
I L(MAX) =
46.5mV
R S
R S =
800 60.4
900 53.6
1000 46.4
* Recommend 1% Standard Values
Synchronizing the LT3746 oscillator to an external fre-
quency can be achieved using the SYNC pin. The square
wave amplitude, compatible to TTL/CMOS logic, should
have valleys that are below 0.6V and peaks that are above
2.4V. The synchronization frequency also ranges from
200kHz to 1MHz, in which the R T resistor should be chosen
to set the internal switching frequency around 20% below
the synchronization frequency. In the case of 200kHz
synchronization frequency, R T = 348k is recommended.
It is also important to note that when the synchroniza-
tion frequency is much higher than the R T programmed
internal frequency, the internal slope compensation will
be significantly reduced, which may trigger sub-harmonic
oscillation at duty cycles greater than 50%.
Inductor Current Sense Resistor R S and Current Limit
The current sense resistor, R S , monitors the inductor
current between the ISP and ISN pins, which are the in-
puts to the internal current sense amplifier. The common
mode input voltage of the current sense amplifier ranges
I OUT(MAX) is the maximum output load current, and ?I L
is the inductor peak-to-peak ripple current. Allowing ad-
equate margin for ripple current and external component
tolerances, R S can be estimated as:
35mV
I OUT(MAX)
Inductor Selection
The critical parameters for selection of an inductor are
inductance value, DC or RMS current, saturation current,
and DCR resistance. For a given input and output voltage,
the inductor value and switching frequency will determine
the peak-to-peak ripple current, ?I L . The ?I L value usually
ranges from 20% to 50% of the maximum output load
current, I OUT(MAX) . Lower values of ?I L require larger and
more costly inductors; higher values of ?I L increase the
peak currents and the inductor core loss. An inductor
current ripple of 30% to 40% offers a good compromise
between inductor performance and inductor size and cost.
However, for high duty cycle applications, a ?I L value of
~20% should be used to prevent sub-harmonic oscillation
due to insufficient slope compensation.
20
For more information www.linear.com/3746
3746fa
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