LTC3105
10
3105fb
For more information www.linear.com/LTC3105
When the converter is placed in shutdown mode, the LDO
is forced into reverse-blocking mode with reverse current
limited to under 1µA. After the shutdown event has ended,
the LDO remains in reverse-blocking mode until V
AUX
has
risen above the LDO voltage.
MPPC Operation
The maximum power point control circuit allows the user
to set the optimal input voltage operating point for a given
power source. The MPPC circuit dynamically regulates
the average inductor current to prevent the input voltage
from dropping below the MPPC threshold. When V
IN
is
greater than the MPPC voltage, the inductor current is
increased until V
IN
is pulled down to the MPPC set point.
If V
IN
is less than the MPPC voltage, the inductor current
is reduced until V
IN
rises to the MPPC set point.
Automatic Power Adjust
The LTC3105 incorporates a feature that maximizes ef-
ficiency at light load while providing increased power
capability at heavy load by adjusting the peak and valley
of the inductor current as a function of load. Lowering the
peak inductor current to 100mA at light load optimizes
efficiency by reducing conduction losses. As the load
increases, the peak inductor current is automatically in
-
creased to a maximum of 500mA. At intermediate loads,
the peak inductor current can vary between 100mA to
500mA. This function is overridden by the MPPC function
and will only be obser
ved when the power source can
deliver more power than the load requires.
PGOOD Operation
The power good output is used to indicate that V
OUT
is
in regulation. PGOOD is an open-drain output, and is
disabled in shutdown. PGOOD will indicate that power
is good at the beginning of the first sleep event after
the output voltage has risen above 90% of its regulation
value. PGOOD remains asserted until V
OUT
drops below
90% of its regulation value at which point PGOOD will
pull low.
OPERATION
Component Selection
Low DCR power inductors with values between 4.7µH
and 30µH are suitable for use with the LTC3105. For
most applications, a 10µH inductor is recommended. In
applications where the input voltage is very low, a larger
value inductor can provide higher efficiency and a lower
start-up voltage. In applications where the input voltage
is relatively high (V
IN
> 0.8V), smaller inductors may be
used to provide a smaller overall footprint. In all cases,
the inductor must have low DCR and sufficient saturation
current rating. If the DC resistance of the inductor is too
high, efficiency will be reduced and the minimum operating
voltage will increase.
Input capacitor selection is highly important in low voltage,
high source resistance systems. For general applications,
a 10µF ceramic capacitor is recommended between V
IN
and GND. For high impedance sources, the input capacitor
APPLICATIONS INFORMATION
should be large enough to allow the converter to complete
start-up mode using the energy stored in the input ca-
pacitor. When using bulk input capacitors that have high
ESR, a small valued parallel ceramic capacitor should be
placed between V
IN
and GND as close to the converter
pins as possible.
A 1µF ceramic capacitor should be connected between
AUX and GND. Larger capacitors should be avoided to
minimize start-up time. A low ESR output capacitor should
be connected between V
OUT
and GND. The main output
capacitor should be 10µF or larger. The main output can
also be used to charge energy storage devices including
tantalum capacitors, supercapacitors and batteries. When
using output bulk storage devices with high ESR, a small
valued ceramic capacitor should be placed in parallel and
located as close to the converter pins as possible.
