Datasheet

LT3975
14
3975f
APPLICATIONS INFORMATION
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 LT3975 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.
To keep the boost capacitor charged regardless of load
during dropout conditions, a minimum dropout voltage
is enforced. When the OUT pin is tied to the output, the
LT3975 regulates the output such that:
V
IN
– V
OUT
> V
DROPOUT(MIN)
where V
DROPOUT(MIN)
is 500mV. The 500mV 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 3 shows
the overall V
IN
to V
OUT
performances during start-up and
dropout conditions.
measured dropout voltage, can be significantly reduced.
Additionally, when operating in dropout at high currents,
high ripple voltage on the input and output can generate
audible noise. This noise can also be significantly reduced
by adding bulk capacitance to the input and output to
reduce the voltage ripple.
Inductor Selection and Maximum Output Current
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current increases with higher V
IN
or V
OUT
and
decreases with higher inductance and faster switching
frequency. A good first choice for the inductor value is:
L =
V
OUT
+ V
D
1.5• f
SW
where f
SW
is the switching frequency in MHz, V
OUT
is the
output voltage, V
D
is the catch diode drop (~0.5V) and L
is the inductor value is μH.
The inductor’s RMS current rating must be greater than
the maximum load current and its saturation current
should be about 30% higher. For robust operation in fault
conditions (start-up or overload) and high input voltage
(>30V), the saturation current should be above 8.5A.
To keep the efficiency high, the series resistance (DCR)
should be less than 0.1Ω, and the core material should
be intended for high frequency applications. Table 2 lists
several inductor vendors.
Table 2. Inductor Vendors
VENDOR URL
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Toko www.tokoam.com
Würth Elektronik www.we-online.com
Coiltronics www.cooperet.com
Murata www.murata.com
The inductor value must be sufficient to supply the desired
maximum output current (I
OUT(MAX)
), which is a function
of the switch current limit (I
LIM
) and the ripple current.
I
OUT(MAX)
= I
LIM
–
∆I
L
2
Figure 3. V
IN
to V
OUT
Performance
V
IN
1V/DIV
V
OUT
1V/DIV
V
OUT
V
IN
100ms/DIV1kΩ LOAD
(5mA IN REGULATION)
3975 F03
It is important to note that the 500mV dropout voltage
specified is the minimum difference between V
IN
and
V
OUT
. When measuring V
IN
to V
OUT
with a multimeter,
the measured value will be higher than 500mV because
you have to add half the ripple voltage on the input and
half the ripple voltage on the output. With the normal
ceramic capacitors specified in the data sheet, this mea-
sured dropout voltage can be as high as 650mV at high
load. If some bulk electrolytic capacitance is added to the
input and output the voltage ripple, and subsequently the