Datasheet

LT3688
14
3688f
APPLICATIONS INFORMATION
The LT3688 will regulate the output voltage at input volt-
ages greater than V
IN(PS)
. For example, an application
with an output voltage of 3.3V and switching frequency
of 2.1MHz has a V
IN(PS)
of 12.7V, as shown in Figure 1.
Figure 2 shows operation at 27V. Output ripple and peak
inductor current have signifi cantly increased. A saturating
inductor may further reduce performance. In pulse skip
mode, the LT3688 skips switching pulses to maintain
output regulation. The LT3688 will also skip pulses at very
low load currents. V
IN(PS)
vs load current is plotted in the
Typical Performance section.
V
OUT
50mV/DIV
(AC)
I
L
500mA/DIV
2µs/DIV
3688 F01
Figure 1. Operation Below Pulse-Skipping
Voltage. V
OUT
= 3.3V and f
SW
= 2.1MHz
V
OUT
50mV/DIV
(AC)
I
L
500mA/DIV
2µs/DIV
3688 F02
Figure 2. Operation Above V
IN(ps)
. V
IN
= 27V,
V
OUT
= 3.3V and f
SW
= 2.1MHz. Output Ripple
and Peak Inductor Current Increase
Minimum Operating Voltage Range
The minimum input voltage is determined either by the
LT3688’s minimum operating voltage of ~3.6V or by its
maximum duty cycle. The duty cycle is the fraction of
time that the internal switch is on and is determined by
the input and output voltages:
DC =
V
OUT
+ V
F
V
IN
–V
SW
+ V
F
Unlike many fi xed frequency regulators, the LT3688 can
extend its duty cycle by remaining on for multiple cycles.
The LT3688 will not switch off at the end of each clock
cycle if there is suffi cient voltage across the boost capacitor
(C3 in the Block Diagram). Eventually, the voltage on the
boost capacitor falls and requires refreshing. Circuitry
detects this condition and forces the switch to turn off,
allowing the inductor current to charge up the boost
capacitor. This places a limitation on the maximum duty
cycle as follows:
DC
MAX
= 90%
This leads to a minimum input voltage of:
V
IN(MIN)
=
V
OUT
+ V
F
DC
MAX
–V
F
+ V
SW
where V
F
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
(~0.3V at maximum load).
Example: I
SW
=0.8A and V
OUT
= 3.3V
V
IN(MIN)
=
3.3V + 0.4V
90%
– 0.4 + 0.3V = 4V
For best performance in dropout, use a 1µF or larger
boost capacitor.
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is
L = V
OUT
+ V
F
()
•
1.8MHz
f
SW
where V
F
is the voltage drop of the catch diode (~0.4V),
f
SW
is in MHz, and L is in H. The inductor’s RMS current
rating must be greater than the maximum load current
and its saturation current should be at least 30% higher.
For robust operation in fault conditions (start-up or short-
circuit) and high input voltage (>30V), use an 8.2µH or
greater inductor with a saturation rating of 2.2A, or higher.
For highest effi ciency, the series resistance (DCR) should
be less than 0.1. Table 2 lists several vendors and types
that are suitable.