Datasheet
LT3689/LT3689-5
13
3689fe
For more information www.linear.com/LT3689
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose 1% resistors
according to:
R1= R2
V
OUT
0.8V
1
For reference designators, refer to the Block Diagram.
Setting the Switching Frequency
The LT3689 uses a constant-frequency PWM architecture
that can be programmed to switch from 350kHz to 2.2MHz
by using a resistor tied from the RT pin to ground. Table
1 shows the R
T
values for various switching frequencies.
Table 1. Switching Frequency vs R
T
SWITCHING FREQUENCY (MHz) R
T
(kΩ)
0.35 48.7
0.5 31.6
0.6 24.9
0.7 20.5
0.8 16.9
0.9 14.7
1 12.7
1.2 9.53
1.4 7.5
1.6 6.04
1.8 4.87
2 4.02
2.2 3.16
APPLICATIONS INFORMATION
Operating Frequency Trade-Offs
Selection of the operating frequency is a trade-off be-
tween efficiency, component size and maximum input
voltage. The advantage of high frequency operation is
that smaller inductor and capacitor values may be used.
The disadvantages are lower efficiency, and narrower
input voltage range at constant-frequency. The highest
constant-switching frequency (f
SW(MAX)
) for a given ap-
plication can be calculated as follows:
f
SW(MAX)
=
V
OUT
+ V
D
t
ON(MIN)
V
IN
– V
SW
+ V
D
( )
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
D
is the catch diode drop (~0.5V) and V
SW
is
the internal switch drop (~0.5V at maximum load). If the
LT3689 is programmed to operate at a frequency higher
than f
SW(MAX)
for a given V
IN
input voltage, the LT3689
enters pulse-skipping mode, where it skips switching
cycles to maintain regulation. At frequencies higher than
f
SW(MAX)
, the LT3689 no longer operates with constant-
frequency. The LT3689 enters pulse-skipping mode at
frequencies higher than f
SW(MAX)
because of the limita-
tion on the LT3689’s minimum on-time of 130ns. As the
switching frequency is increased above f
SW(MAX)
, the part
is required to switch for shorter periods of time to maintain
the same duty cycle. Delays associated with turning off
the power switch dictate the minimum on-time of the part.
When the required on-time decreases below the minimum
on-time of 130ns, the switch pulse width remains fixed at
130ns (instead of becoming narrower to accommodate
the duty cycle requirement). The inductor current ramps
up to a value exceeding the load current and the output
ripple increases. The part then remains off until the output
voltage dips below the programmed value before it begins
switching again.