Datasheet

LT3975
13
3975f
APPLICATIONS INFORMATION
A good choice of switching frequency should allow ad-
equate input voltage range (see next two sections) and
keep the inductor and capacitor values small.
Maximum Input Voltage Range
The LT3975 can operate from input voltages of up to 42V.
Often the highest allowed V
IN
during normal operation
(V
IN(OP-MAX)
) is limited by the minimum duty cycle rather
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 LT3975 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
LT3975’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 LT3975 can extend its duty cycle
by remaining on for multiple clock cycles. The LT3975
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 LT3975 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.