lt3695 Linear Technology Corporation, lt3695 Datasheet - Page 13

• Manufacturer Part Number: lt3695
• Description: 1a Fault Tolerant Micropower Step-down Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
A good choice of switching frequency should allow an
adequate input voltage range (see Input Voltage Range sec-
tion) and keep the inductor and capacitor values small.
Input Voltage Range
The minimum input voltage is determined by either the
LT3695’s minimum operating voltage of ~3.6V (V
or by its maximum duty cycle (see equation in Operating
Frequency Trade-Offs section). The minimum input voltage
due to duty cycle is:
where V
is the minimum switch off time. Note that a higher switch-
ing frequency will increase the minimum input voltage.
If a lower dropout voltage is desired, a lower switching
frequency should be used.
The maximum input voltage for LT3695 applications
depends on switching frequency, the Absolute Maximum
Ratings of the V
The LT3695 can operate from continuous input voltages
up to 36V. Input voltage transients of up to 60V are also
safely withstood. However, note that while V
(overvoltage lockout, 38V typical), the LT3695 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 fi xed, the maximum
input voltage that guarantees optimum output voltage
ripple for that application can be found by applying the
following expression:
where V
V
(~0.5V), V
f
the minimum switch on time (~150ns). Note that a higher
SW
OUT
V
V
is the switching frequency (set by R
IN MIN
IN MAX
is the output voltage, V
(
(
IN(MIN)
IN(MAX)
SW
)
)
=
=
is the internal switch drop (~0.5V at max load),
1
f
−
is the minimum input voltage, and t
IN
SW ON MIN
V
is the maximum operating input voltage,
V
OUT
f
and BOOST pins and the operating mode.
SW OFF MIN
OUT
t
t
+
(
+
V
V
D
(
D
)
−
)
V
−
D
D
V
+
is the catch diode drop
D
V
+
SW
V
SW
T
) and t
IN
ON(MIN)
> V
BD
OFF(MIN)
> 3V)
OVLO
is
switching frequency will reduce the maximum operating
input voltage. Conversely, a lower switching frequency
will be necessary to achieve optimum operation at high
input voltages.
Special attention must be paid when the output is in start-
up, short-circuit or other overload conditions. During these
events, the inductor peak current might easily reach and
even exceed the maximum current limit of the LT3695,
especially in those cases where the switch already operates
at minimum on-time. The circuitry monitoring the current
through the catch diode via the DA pin prevents the switch
from turning on again if the inductor valley current is above
1.6A nominal. In these cases, the inductor peak current is
therefore the maximum current limit of the LT3695 plus
the additional current overshoot during the turn off delay
due to minimum on time:
where I
the maximum expected input voltage, L is the inductor
value, t
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 3.5A. Inductor current saturation and excessive
junction temperature may further limit performance.
Input voltage transients of up to V
regardless of the switching frequency. In this case, the
LT3695 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.
Input voltage transients above V
be tolerated. However, since the part will stop switching
during these transients, the output will fall out of regulation
and the output capacitor may eventually be completely
discharged. This case must be treated then as a start-up
condition as soon as V
and the part starts switching again.
I
L PEAK
(
ON(MIN)
L(PEAK)
)
=
2
A
is the minimum on time and V
is the peak inductor current, V
+
V
IN MAX
(
IN
returns to values below V
)
−
L
V
OUT OL
OVLO
(
OVLO
)
and up to 60V can
•
t
ON MIN
are acceptable
LT3695
(
OUT(OL)
IN(MAX)
)
13
is the
OVLO
3695f
is