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LT3745
APPLICATIONS INFORMATION
I OUT(MAX) = I L(MAX) –
Besidesthesecommonconsiderations,thespecificappli-
cation also plays an important role in switching frequency
choice. In a noise-sensitive system, the switching frequency
is usually chosen to keep the switching noise out of a
sensitive frequency band.
Switching Frequency Setting and Synchronization
The LT3745 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 S W vs R T Value
f SW (kHz) R T * (kΩ)
200 280
300 182
400 133
500 105
600 84.5
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
from 0V to (V IN – 2.1V) or 36V 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 L
2
where the maximum inductor current I L(MAX) is set by:
700
800
900
71.5
60.4
53.6
I L(MAX) =
46.5mV
R S
R S =
1000                                            46.4
* Recommend 1% Standard Values
Synchronizing the LT3745 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 cho-
sen 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%.
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)
3745f
20
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