Datasheet

LT3688
13
3688f
APPLICATIONS INFORMATION
Setting the Output Voltage
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resistors
according to:
R1= R2
V
OUT
0.8V
–1
⎛
⎝
⎜
⎞
⎠
⎟
For reference designators, refer to the Block Diagram.
Setting the Switching Frequency
The LT3688 uses a constant-frequency PWM architecture
that can be programmed to switch from 350 kHz to 2.2 MHz
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)
0.35 165
0.5 110
0.6 88.7
0.7 75
0.8 64.9
0.9 56.2
1 49.9
1.2 40.2
1.4 33.2
1.6 27.4
1.8 23.2
2.1 20
2.3 17.4
Operating Frequency Tradeoffs
Selection of the operating frequency is a tradeoff between
effi ciency, 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 effi ciency, and narrower input
voltage range at constant-frequency. The highest constant-
switching frequency (f
SW(MAX)
) for a given application can
be calculated as follows:
f
SW(MAX)
=
V
OUT
+ V
F
t
ON(MIN)
V
IN
+ V
F
–V
SW
()
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
F
is the catch diode drop (~0.5V) and V
SW
is
the internal switch drop (~0.3V at maximum load). If the
LT3688 is programmed to operate at a frequency higher
than f
SW(MAX)
for a given input voltage, the LT3688 enters
pulse skip mode, where it skips switching cycles to maintain
regulation. At frequencies higher than f
SW(MAX)
, the LT3688
no longer operates with constant frequency. The LT3688
enters pulse skip mode at frequencies higher than f
SW(MAX)
because of the limitation on the LT3688’s minimum on time
of 140ns (180ns for T
J
> 125°C). As the switching frequency
is increased above f
SW(MAX)
, the part is required to switch
for shorter periods 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 140ns, the switch
pulse width remains fi xed at 140ns (instead of becoming
narrower) to accommodate the same duty cycle require-
ment. 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.
Maximum Operating Voltage Range
The maximum input voltage for LT3688 applications
depends on switching frequency, the absolute maximum
ratings of the V
IN
and BST pins, and by the minimum
duty cycle (DC
MIN
). The LT3688 can operate from input
voltages up to 36V.
DC
MIN
= t
ON(MIN)
• f
SW
where t
ON(MIN)
is equal to 140ns and f
SW
is the switching
frequency. Running at a lower switching frequency allows
a lower minimum duty cycle. The maximum input voltage
before pulse-skipping occurs depends on the output volt-
age and the minimum duty cycle:
V
IN(PS)
=
V
OUT
+ V
F
DC
MIN
–V
F
+ V
SW
Example: f = 2.1MHz, V
OUT
= 3.3V
DC
MIN
= 140ns • 2.1MHz = 0.294
V
IN(PS)
=
3.3V + 0.5V
0.294
– 0.5V + 0.3V = 12.7V