Datasheet

LT3690
13
3690fa
applicaTions inForMaTion
Input Voltage Range
The minimum input voltage is determined by either the
LT3690’s minimum operating voltage of 3.9V (V
BIAS
>
3V) or by its maximum duty cycle (see equation in the
Operating Frequency Trade-offs section). The minimum
input voltage due to duty cycle limitation is:
V
IN(MIN)
=
V
OUT
+ V
LS
1− ƒ
SW
• t
OFF(MIN)
− V
LS
+ V
SW
where V
IN(MIN)
is the minimum input voltage, and t
OFF(MIN)
is the minimum switch off-time (210ns). Note that higher
switching frequency will increase the minimum input volt-
age. If a lower dropout voltage is desired, a lower switching
frequency should be used.
The maximum input voltage for LT3690 applications
depends on switching frequency, the absolute maximum
ratings of the V
IN
and BST pins, and the operating mode.
The LT3690 can operate from continuous input voltages
up to 36V. If the operating junction temperature is below
125°C, the LT3690 will tolerate input voltage transients of
up to 60V. However, note that while V
IN
> V
OVLO
(typically
38V), the LT3690 will stop switching, allowing the output
to fall out of regulation.
For a given application where the switching frequency
and the output voltage are already fixed, the maximum
input voltage that guarantees optimum output voltage
ripple for that application can be found by applying the
following expression:
V
IN(MAX)
=
V
OUT
+
V
LS
ƒ
SW
• t
ON(MIN)
− V
LS
+ V
SW
where V
IN(MAX)
is the maximum operating input voltage,
V
OUT
is the output voltage, V
LS
is the LS switch drop (0.12V
at maximum load), V
SW
is the internal switch drop (0.37V
at maximum load), f
SW
is the switching frequency (set by
RT), and t
ON(MIN)
is the minimum switch on-time (210ns;
250ns for T
J
> 125°C). Note that a higher switching fre-
quency will reduce the maximum operating input voltage.
Conversely, a lower switching frequency will be necessary
to achieve optimum operation at high input voltages. The
maximum operating voltage is 36V (minimum overvoltage
lockout threshold).
Special attention must be paid when the output is in start-
up, short-circuit, or other overload conditions. In these
cases, the LT3690 tries to bring the output in regulation
by driving current into the output load. During these
events, the inductor peak current might easily reach and
even exceed the maximum current limit of the LT3690,
especially in those cases where the switch already operates
at minimum on-time. The circuitry monitoring the current
through the LS switch prevents the HS switch from turn-
ing on again if the inductor valley current is
above I
PSDLIM
(5A nominal). In these cases, the inductor peak current is
therefore the maximum current limit of the LT3690 plus
the additional current overshoot during the turn-off delay
due to minimum on-time:
I
L(PEAK)
= 8A +
V
IN(MAX)
− V
OUTOL
L
• t
ON(MIN)
where I
L(PEAK)
is the peak inductor current, V
IN(MAX)
is
the maximum expected input voltage, L is the inductor
value, t
ON(MIN)
is the minimum on-time and V
OUTOL
is
the output voltage under the overload condition. The part
is robust enough to survive prolonged operation under
these conditions as long as the peak inductor current does
not exceed 9A. Inductor current saturation and excessive
junction temperature may further limit performance.
If the output is in regulation and no short-circuit, start-
up, or overload events are expected, then input voltage
transients of up to V
OVLO
are acceptable regardless of the
switching frequency. In this case, the LT3690 may enter
pulse-skipping operation where some switching pulses
are skipped to maintain output regulation. In this mode,
the output voltage ripple and inductor current ripple will
be higher than in normal operation.