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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

TheLTC3128canchargetwooutputcapacitorsinseriesthat

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

(smallercap)toacapacitorwithalowervoltage(biggercap).

scope as shown in Figure 5. The trace 1 in Figure 5 is

1

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