Datasheet

LT3995
13
3995f
For more information www.linear.com/LT3995
APPLICATIONS INFORMATION
than the absolute maximum ratings of the V
IN
pin. It can
be calculated using the following equation:
V
IN(OP-MAX)
=
V
OUT
+
V
D
f
SW
• t
ON(MIN)
– V
D
+ V
SW
where t
ON(MIN)
is the minimum switch on-time. A lower
switching frequency can be used to extend normal opera-
tion to higher input voltages.
The circuit will tolerate inputs above the maximum op-
erating input voltage and up to the absolute maximum
ratings of the V
IN
and BOOST pins, regardless of chosen
switching frequency. However, during such transients
where V
IN
is higher than V
IN(OP-MAX)
, the LT3995 will enter
pulse-skipping operation where some switching pulses are
skipped to maintain output regulation. The output voltage
ripple and inductor current ripple will be higher than in
typical operation. Do not overload when V
IN
is greater
than V
IN(OP-MAX)
.
Minimum Input Voltage Range
The minimum input voltage is determined by either the
LT3995’s minimum operating voltage of 4.3V, its maximum
duty cycle, or the enforced minimum dropout voltage.
See the Typical Performance Characteristics section for
the minimum input voltage across load for outputs of
3.3V and 5V.
The duty cycle is the fraction of time that the internal
switch is on during a clock cycle. Unlike many fixed fre-
quency regulators, the LT3995 can extend its duty cycle
by remaining on for multiple clock cycles. The LT3995
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. When this occurs,
the switch will turn off, allowing the inductor current to
recharge the boost capacitor. This places a limitation on
the maximum duty cycle as follows:
DC
MAX
=
β
SW
β
SW
+ 1
where β
SW
is equal to the beta of the internal power switch.
The beta of the power switch is typically about 50, which
leads to a DC
MAX
of about 98%. This leads to a minimum
input voltage of approximately:
V
IN(MIN1)
=
V
OUT
+
V
D
DC
MAX
– V
D
+ V
SW
where V
OUT
is the output voltage, V
D
is the catch diode
drop, V
SW
is the internal switch drop and DC
MAX
is the
maximum duty cycle.
The final factor affecting the minimum input voltage is
the minimum dropout voltage. When the OUT pin is tied
to the output, the LT3995 regulates the output such that
it stays 500mV below V
IN
. This enforced minimum drop-
out voltage is due to reasons that are covered in the next
section. This places a limitation on the minimum input
voltage as follows:
V
IN(MIN2)
= V
OUT
+ V
DROPOUT(MIN)
where V
OUT
is the programmed output voltage and
V
DROPOUT(MIN)
is the minimum dropout voltage of 500mV.
Combining these factors leads to the overall minimum
input voltage:
V
IN(MIN)
= Max (V
IN(MIN1)
, V
IN(MIN2)
, 4.3V)
Minimum Dropout Voltage
To achieve a low dropout voltage, the internal power switch
must always be able to fully saturate. This means that the
boost capacitor, which provides a base drive higher than
V
IN
, must always be able to charge up when the part starts
up and then must also stay charged during all operating
conditions.
During start-up if there is insufficient inductor current, such
as during light load situations, the boost capacitor will be
unable to charge. When the LT3995 detects that the boost
capacitor is not charged, it activates a 100mA (typical)
pull-down on the OUT pin. If the OUT pin is connected to
the output, the extra load will increase the inductor current
enough to sufficiently charge the boost capacitor. When
the boost capacitor is charged, the current source turns
off, and the part may re-enter Burst Mode operation.