Datasheet

LT3689/LT3689-5
14
3689fe
For more information www.linear.com/LT3689
Maximum Operating Voltage
The maximum input voltage for LT3689 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 LT3689 can operate from input
voltages up to 36V, and safely withstand input transient
voltages up to 60V. Note that while V
IN
> 38V (typical),
the LT3689 will stop switching, allowing the output to fall
out of regulation.
DC
MIN
= t
ON(MIN)
• f
SW
where t
ON(MIN)
is equal to 130ns (for T
J
> 125°C t
ON(MIN)
is equal to 150ns) 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 voltage and
the minimum duty cycle:
V
IN(PS)
=
V
OUT
+ V
D
DC
MIN
V
D
+ V
SW
Example: f
SW
= 790kHz, V
OUT
= 3.3V,
DC
MIN
= 130ns • 790kHz = 0.103
V
IN(PS)
=
3.3V + 0.4V
0.103
0.4V + 0.4V = 36V
The LT3689 will regulate the output current at input volt-
ages greater than V
IN(PS)
. For example, an application
with an output voltage of 1.8V and switching frequency
of 1.5MHz has a V
IN(PS)
of 11.3V, as shown in Figure 1.
Figure 2 shows operation at 24V. Output ripple and peak
inductor current have significantly increased. A saturating
inductor may further reduce performance. For input volt-
ages over 30V, there are restrictions on the inductor size
and saturation rating. See the Inductor Selection section
for more details. In pulse-skipping mode, the LT3689 skips
switching pulses to maintain output regulation. Above 38V
(typical) switching will stop. Transients of up to 60V are
acceptable, regardless of switching frequency.
Minimum Operating Voltage Range
The minimum input voltage is determined either by the
LT3689’s minimum operating voltage of ~3.4V 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
D
V
IN
V
SW
+ V
D
Unlike many fixed frequency regulators, the LT3689 can
extend its duty cycle by remaining on for multiple cycles.
The LT3689 will not switch off at the end of each clock cycle
if there is sufficient voltage across the boost capacitor (C3
in the Block Diagram). Eventually, the voltage on the boost
capacitor falls and requires refreshing. Circuitry detects
APPLICATIONS INFORMATION
Figure 1. Operation Below V
IN(PS)
. V
IN
= 10V,
V
OUT
= 1.8V and f
SW
= 1.5MHz
Figure 2. Operation Above V
IN(PS)
. V
IN
= 24V,
V
OUT
= 1.8V and f
SW
= 1.5MHz. Output Ripple
and Peak Inductor Current Increase
5µs/DIV
V
OUT
50mV/DIV
(AC)
I
L
0.5A/DIV
3689 F01
5µs/DIV
V
OUT
50mV/DIV
(AC)
I
L
0.5A/DIV
3689 F02