Linear Technology DC1587 Demo Manual page 3

QUICK START PROCEDURE
As the output load current increases, the voltage on
V decreases due to the 10Ω input resistor. Figure 2
IN
shows V , V
IN OUT
current is increased over a 2 second time period with
the MPPC programmed for 2V. The output remains at
a regulated 5V until the input voltage drops to the 2V
MPPC threshold. The MPPC prevents the input voltage
from dropping below 2V by reducing the output volt-
age. Without MPPC, the output voltage would drop to
near 0V resulting in zero output power. Connecting the
MPPC pin to GND will disable the MPPC function. Also
shown in Figure 1 is the Power Good (PGOOD) signal
that pulls low when the Boost output voltage drops
10% below the programmed value.
3.5
3.0
2.5
2.0
1.5
Figure 2. Maximum Power Point Control Programmed for 2V,
Operating from a High Impedance Input Power Source (10Ω)
3. To verify very low voltage start-up (250mV to 300mV)
it is necessary to do the following; remove jumper JP2
and add an external 22k resistor between the MPPC ter-
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and PGOOD voltages as the load
V
IN
LOAD CURRENT
(20mA/DIV)
V (2V/DIV)
OUT
PGOOD SIGNAL
2 SECONDS
dc1587 F02
DEMO MANUAL DC1587
minal and ground. Connect a resistor of approximately
0.5Ω between the supply output and V
supply to approximately 250mV to 300mV and verify
that the boost output is in regulation. Since this DC/DC
converter is designed to operate from relatively high
impedance power sources, some source resistance is
neccessary for proper low voltage start-up. At very low
input voltage, excessive input current may result unless
there is some source resistance.
4. Additional demo board features include the following.
Jumper JP4 allows the PGOOD open drain FET pull-up
resistor to be connected to the LDO output voltage or
to an external voltage applied to the VPGOOD termi-
nal. The converter can be shutdown by pulling SHDN
terminal low. MPPC jumper allows the user to select
several preset MPPC voltages or select the DIODE ter-
minal when using diodes located near the solar panel
for MPPC temperature tracking. A small circular pad
on the board allows the LTC3105 AUX pin to be probed
when evaluating the circuit. Do not connect a load to
6
this pad.
4
2
0
Figure 3. Correct Method For Measuring
Input or Output Voltage Ripple
. Adjust the
IN
dc1587f
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