3
dc1887af
DEMO MANUAL DC1887A
Quick start proceDure
Figure 3. The Output Current Decreases as the Supercap Voltage
Increases Due to Precise Input Current Limit.
Figure 4. The Supercap Supplies Most of the Pulsed Load Current.
The LTC3128 Makes a Small Contribution to Load Current as
Shown in Figure 3.
Figure 5. The DC1887 Ramps the Output Slowly at Start-Up
without Generating an Input Current Surge.
CONVERTER OUTPUT CURRENT
The DC1887 output current capability depends on the
input voltage and programmed input current limit. Typical
performance of the DC1887A is shown in Figure 3. As can
be seen from Figure 3, the output current decreases as
the output voltage is approaching the programmed output
voltage of 4.2V.
OUTPUT LOAD STEP RESPONSE
The load step response of DC1887A is dependent on the
supercap used. If higher load steps need to be handled
more output capacitance can be added in order to keep
the voltage transients at the desired level. The load step
transients are shown in Figure 4.
START-UP AND CURRENT LIMIT FUNCTION
The DC1887 features an input current limit circuit that
controls the inrush current and output voltage ramp at
start-up and during recharge cycles. The input current
limit of DC1887 is set to 900mA and it should be suitable
for operation from a USB port. The input current limit can
be seen in Figure 5. While charging the supercap, the cur-
rent is constant. Once the output voltage is close to the
set point the current limit is reduced in order to prevent
output voltage overshoot.
The input current limit can be programmed up to 3A by
changing the resistor R8 in the Schematic Diagram.
OUTPUT VOLTAGE BALANCING
The LTC3128 can charge two output capacitors in series that
are not identical. The internal capacitor voltage balancing
circuit can compensate for the difference in capacitance by
moving the charge from the capacitor at a higher voltage
(smaller cap) to a capacitor with a lower voltage (bigger cap).
To see this effect, monitor V
MID
(VC3) and V
OUT
with a
scope as shown in Figure 5. The trace 1 in Figure 5 is
showing the top cap voltage (VC3 = V
OUT
– V
MID
).
V
OUT
(V)
1
I
OUT
(A)
1.2
0.8
0.6
1.0
0.4
0.2
0
4
2
3
dc1887 F03
4.5
3.5
1.5
2.5
2V INPUT
3.3V INPUT
5V INPUT
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