lt8410-1 Linear Technology Corporation, lt8410-1 Datasheet - Page 10

• Manufacturer Part Number: lt8410-1
• Description: Ultralow Power Boost Converter With Output Disconnect
• Manufacturer: Linear Technology Corporation

LT8410/LT8410-1
APPLICATIONS INFORMATION
Step 4. Calculate the nominal output current:
Step 5. Derate output current:
For low output voltages the output current capability will
be increased. When using output disconnect (load current
taken from V
drop in the PMOS switch to be higher resulting in lower
output current capability than predicted by the preceding
equations.
Inrush Current
When V
while the output capacitor is discharged, a high level of
inrush current may fl ow through the inductor and Schottky
diode into the output capacitor. Conditions that increase
inrush current include a larger more abrupt voltage step
at V
an inductor with a low saturation current. While the chip is
designed to handle such events, the inrush current should
not be allowed to exceed 0.3A. For circuits that use output
capacitor values within the recommended range and have
input voltages of less than 6V, inrush current remains low,
posing no hazard to the device. In cases where there are
large steps at V
is used at the CAP pin, inrush current should be measured
to ensure safe operation.
Soft-Start
The LT8410 series contains a soft-start circuit to limit peak
switch currents during start-up. High start-up current
is inherent in switching regulators in general since the
feedback loop is saturated due to V
its fi nal value. The regulator tries to charge the output
capacitor as quickly as possible, which results in large
peak current.
When the FBP pin voltage is generated by a resistor divider
from the V
connecting an external capacitor (typically 47nF to 220nF)
to the V
10
I
I
OUT(NOM)
OUT
CC
, a larger output capacitor tied to the CAP pin and
REF
= I
CC
OUT(NOM)
is stepped from ground to the operating voltage
REF
pin. When the part is brought out of shutdown,
=
OUT
pin, the start-up current can be limited by
CC
I
IN(AVG)
), these higher currents will cause the
(more than 6V) and/or a large capacitor
• 0.8
V
OUT
• V
IN
• 0.7
mA
OUT
being far from
this capacitor is fi rst discharged for about 70μs (providing
protection against pin glitches and slow ramping), then
an internal 10μA current source pulls the V
to 1.235V. Since the V
voltage, the V
regulated voltage, which results in lower peak inductor
current. The voltage ramp rate on the pin can be set by
the value of the V
Output Disconnect
The LT8410 series has an output disconnect PMOS that
blocks the load from the input during shutdown. The
maximum current through the PMOS is limited by circuitry
inside the chip, helping the chip survive output shorts.
SHDN Pin Comparator and Hysteresis Current
An internal comparator compares the SHDN pin voltage
with an internal voltage reference (1.3V) which gives a
precise turn-on voltage level. The internal hysteresis of this
turn-on voltage is about 60mV. When the chip is turned on,
and the SHDN pin voltage is close to this turn-on voltage,
0.1μA current fl ows out of the SHDN pin. This current is
called SHDN pin hysteresis current, and will go away when
the chip is off. By connecting the external resistors as in
Figure 2, a user-programmable enable voltage function
can be realized.
The turn-on voltage for the confi guration is:
1.30 • (1 + R1/R2)
and the turn-off voltage is:
(1.24 – R3 • 10
where R1, R2 and R3 are resistance value in Ω.
Figure 2. Programming Enable Voltage by Using External Resistors
OUT
ENABLE VOLTAGE
– 7
REF
voltage will also slowly increase to the
) • (1 + R1/R2) – (R1 • 10
R1
R2
pin capacitor.
OUT
R3
voltage is set by the FBP pin
CONNECT TO
SHDN PIN
REF
– 7
pin slowly
)
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