Datasheet
LT3598
14
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APPLICATIONS INFORMATION
Table 6. T
SET
Junction Temperature
T
J
(°C) R1 R2
90 100k 68.1k
100 100k 63.4k
110 100k 59k
120 100k 54.9k
Programming Switching Frequency
The switching frequency of the LT3598 should be
programmed between 200kHz and 2.5MHz by an external
resistor connected between the RT pin and ground. Do not
leave this pin open. See Table 7 and Figure 10 for resistor
values and corresponding frequencies.
Selecting the optimum switching frequency depends
on several factors. Inductor size is reduced with higher
frequency, but effi ciency drops slightly due to higher
switching losses. In addition, some applications require
very high duty cycles to drive a large number of LEDs from
a low supply. Low switching frequency allows a greater
operational duty cycle and, hence, a greater number of
LEDs to be driven. In each case, the switching frequency
can be tailored to provide the optimum solution. When
programming the switching frequency, the total power
losses within the IC should be considered.
Table 7. Switching Frequency
SWITCHING FREQUENCY (MHz) R
T
(k)
2.5 14.7
2 20.5
1.5 29.4
1 51.1
0.5 105
0.2 301
Switching Frequency Synchronization
The nominal operating frequency of the LT3598 is
programmed using a resistor from the RT pin to ground
and can be controlled over a 200kHz to 2.5MHz range. In
addition, the internal oscillator can be synchronized to an
external clock applied to the SYNC pin. The synchronizing
clock signal input to the LT3598 must have a frequency
between 250kHz and 3MHz, a duty cycle between 20% and
80%, a low state below 0.4V and a high state above 1.5V.
Synchronization signals outside of these parameters will
cause erratic switching behavior. For proper operation,
an R
T
resistor should be chosen to program a switching
frequency 20% slower than the SYNC pulse frequency.
Synchronization occurs at a fi xed delay after the rising
edge of SYNC.
Figure 8. Programming the T
SET
Pin
Figure 9. T
SET
Pin Threshold
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LT3598
V
REF
T
SET
R2
R1
JUNCTION TEMPERATURE (°C)
0
500
V
TSET
THRESHOLD (mV)
600
700
800
50
100 125
25
75
150
900
550
650
750
850
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Figure 10. Switching Frequency
R
T
(k)
SWITCHING FREQUENCY (MHz)
2.5
2.0
1.5
0
1.0
0.5
1000
10 100
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