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LT3697
21
3697f
applicaTions inForMaTion
minimum duty cycle rather than the absolute maximum
ratings of the V
IN
pin. It can be calculated using the fol-
lowing equation:
V
IN(OP−MAX)
=
SYS
D
f
SW
• t
ON(MIN)
– V
D
+ V
SW
where V
D
is the catch diode drop and V
SW
is the internal
switch drop. V
SYS
can vary between 5V and 6.1V depending
on if cable drop compensation is used and how USB5V is
tied to SYS. A lower switching frequency can be used to
extend normal operation 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 LT3697 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 the output when V
IN
is
greater than V
IN(OP-MAX)
, unless the ISP and ISN pins are
connected such as to limit the output current.
Minimum Input Voltage Range
The minimum input voltage for full frequency operation is
determined by either the LT3697’s maximum duty cycle
or the enforced minimum dropout voltage. See the Typi
-
cal Performance Characteristics section for the minimum
input voltage across load.
The
LT3697 will continue to switch and pull the output as
high as possible down to its minimum operating voltage
of 4.5V. The duty cycle is the fraction of time that the
internal switch is on during a clock cycle. Unlike many
fixed frequency regulators, the LT3697 can extend its
duty cycle by remaining on for multiple clock cycles. The
LT3697 will not switch off at the end of each clock cycle
if there is sufficient voltage across the boost capacitor
(C
BST
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.
At low V
IN
, the LT3697 regulates the SYS voltage such
that it stays 600mV below V
IN
. This enforced minimum
dropout voltage is due to reasons that are covered in the
next section. This places a limitation on the minimum
input voltage as follows:
IN(MIN)
SYS
DROPOUT(MIN)
where V
DROPOUT(MIN)
is the minimum dropout voltage of
600mV. V
SYS
can vary between 5V and 6.1V depending
on if cable drop compensation is used and how USB5V
is tied to SYS.
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 LT3697 detects that
the boost capacitor is not charged, it activates a 200mA
(typical) load on the SYS pin. If the SYS 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.
To keep the boost capacitor charged regardless of load
during dropout conditions, a minimum dropout voltage
is enforced. When the
SYS pin is tied to the output, the
LT3697 regulates the output such that:
IN
SYS
DROPOUT(MIN)
where V
DROPOUT(MIN)
is 600mV. The 600mV dropout volt-
age limits the duty cycle and forces the switch to turn off
regularly
to charge the boost capacitor. Since sufficient
voltage across the boost capacitor is maintained, the switch
is allowed to fully saturate and the internal switch drop
stays low for good dropout performance. Figure 8 shows
the overall V
IN
to V
OUT
performances during start-up and
dropout conditions.
