Datasheet

LT3514
13
3514fa
For more information www.linear.com/LT3514
APPLICATIONS INFORMATION
peak inductor current. Provided that the inductor does not
saturate and that the switch current remains below 2A
(SW1, SW4) or below 4A (SW3), operation above V
IN(PS)
is safe and will not damage the part. For a more detailed
discussion on minimum on-time and pulse-skipping, re
-
fer to the Applications Information section of the LT3505
data sheet.
Avoid starting up the LT3514 at input voltages greater
than 36V, as the LT3514 must simultaneously conduct
maximum currents at high V
IN
. The maximum operating
junction temperature of 125°C may be exceeded due to
the high instantaneous power dissipation.
Frequency Selection
The maximum frequency that the LT3514 can be pro
-
grammed to is 2.5MHz. The minimum frequency is 250kHz.
The
switching
frequency can be programmed in two ways.
The first method is by tying a 1% resistor (R
T
) from the
RT/SYNC pin to ground. Table 1 can be used to select the
value of R
T
. The second method is to synchronize (sync)
the internal oscillator to an external clock. The external
clock must have a minimum amplitude from 0V to 1.5V
and a minimum pulse-width of 50ns.
Table 1. RT/SYNC Pin Resistance to Program Oscillator
Frequency
FREQUENCY (MHz) RT/SYNC PIN RESISTANCE (kΩ)
0.20 140
0.3 82.5
0.4 56.2
0.5 43.2
0.6 34.8
0.7 28.0
0.8 23.7
0.9 20.5
1.0 18.2
1.1 16.9
1.2 14.7
1.3 13.0
1.4 11.5
Figure 2. Driving the RT/SYNC Pin From a Port That
Is in a High Impedance State When it Is Inactive
Figure 3. Driving the RT/SYNC Pin from a Port That Is
in a Low Impedance State When it Is Inactive
FREQUENCY (MHz) RT/SYNC PIN RESISTANCE (kΩ)
1.5 10.7
1.6 9.76
1.7 8.66
1.8 8.06
1.9 7.32
2.0 6.81
2.1 6.34
2.2 6.04
2.3 5.62
2.4 5.36
2.5 4.99
In certain applications, the LT3514 may be required to be
alive and switching for a period of time before it begins
to receive a sync signal. If the sync signal is in a high
impedance state when it is inactive then the solution is to
simply tie an R
T
resistor from the RT/SYNC pin to ground
(Figure 2). The sync signal should be capable of driving the
R
T
resistor. If the sync signal is in a low impedance state
or an unknown state when it is inactive, then the solution
is to tie the R
T
resistor from the RT/SYNC pin to ground
and then to drive the RT/SYNC pin with the sync signal
through a 1nF capacitor as shown in Figure 3.
3514 F02
PORT
GND
LT3514
RT/SYNC
R
T
3514 F03
PORT
1nF
GND
LT3514
RT/SYNC
R
T