LT3688 LINER [Linear Technology], LT3688 Datasheet - Page 14

• Manufacturer Part Number: LT3688
• Description: Dual 800mA Step-Down Switching Regulator with Power-On Reset
• Manufacturer: LINER [Linear Technology]

LT3688
APPLICATIONS INFORMATION
The LT3688 will regulate the output voltage at input volt-
ages greater than V
with an output voltage of 3.3V and switching frequency
of 2.1MHz has a V
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
Typical Performance section.
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:
14
DC =
500mA/DIV
500mA/DIV
50mV/DIV
50mV/DIV
Figure 1. Operation Below Pulse-Skipping
Voltage. V
Figure 2. Operation Above V
V
and Peak Inductor Current Increase
V
OUT
IN
V
V
(AC)
(AC)
OUT
OUT
V
I
I
= 3.3V and f
OUT
– V
L
L
SW
OUT
+ V
IN(PS)
IN(PS)
+ V
IN(PS)
= 3.3V and f
F
F
SW
of 12.7V, as shown in Figure 1.
vs load current is plotted in the
. For example, an application
= 2.1MHz. Output Ripple
2μs/DIV
2μs/DIV
SW
IN(ps)
= 2.1MHz
. V
IN
= 27V,
3688 F01
3688 F02
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:
This leads to a minimum input voltage of:
where V
(~0.4V) and V
(~0.3V at maximum load).
Example: I
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
where V
f
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 0.1Ω. Table 2 lists several vendors and types
that are suitable.
SW
DC
L = V
V
V
is in MHz, and L is in μH. The inductor’s RMS current
IN(MIN)
IN(MIN)
MAX
(
F
F
OUT
is the forward voltage drop of the catch diode
is the voltage drop of the catch diode (~0.4V),
= 90%
SW
=
=
=0.8A and V
SW
+ V
3.3V + 0.4V
V
OUT
DC
is the voltage drop of the internal switch
F
90%
MAX
)
+ V
•
1.8MHz
F
f
SW
– V
OUT
– 0.4 + 0.3V = 4V
F
= 3.3V
+ V
SW
3688f