Datasheet

LT3746
20
3746fa
For more information www.linear.com/3746
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
f
SW
(kHz) R
T
* (kΩ)
200 280
300 182
400 133
500 105
600 84.5
700 71.5
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
wa
ve 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
m
o
de input voltage of the current sense amplifier ranges
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)
–
Δ I
L
2
where the maximum inductor current I
L(MAX)
is set by:
I
L(MAX)
=
46.5mV
R
S
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
t
ol
erances, R
S
can be estimated as:
R
S
=
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.