LTC4041 - Unexpected low charge current

Question

Hello, 
 I&rsquo;m experiencing an issue with the LTC4041. 
 I want to charge two supercapacitors (2 &times; 50 F / 2.7 V in series) at 2 A. I&rsquo;ve attached the circuit diagram. 
 
 Currently, there is no load connected to Vsys. However, the charging process takes a very long time, and the charge current only increases very slowly. The desired 2 A is reached only shortly before the final charge voltage. This can be clearly seen in the attached oscilloscope screenshot. 
 
 The first trace (blue) shows Vsys. The circuit is powered by a lab power supply set to 5.1 V, with a current limit of 5 A, so that should not be an issue. 
 The second trace (red) shows the current; 1 V corresponds to approximately 1 A. You can see that the current rises gradually throughout the entire charging period. I expected it to be limited initially and then switch to the full 2 A. 
 The green trace shows the voltage at Scap. 
 The third trace (yellow) shows the voltage at the PROG pin. 
 
 The entire charging process takes more than one minute, which is too long for my application&mdash;this is exactly why I chose the LTC4041, to charge as quickly as possible with a constant 2 A. 
 Any help would be greatly appreciated!

BlaiseParker · Accepted Answer

Hi,

Just looking at the capacitor voltage, the part seems to be working as expected. If you have a constant 2A charging current, two 50F caps, and assuming ideal conditions, I expect the caps to fully charge in around 56s. The math is included below:

For a capacitor, I = C * dV/dt

dV/dt = I / C

dV/dt = 2A / 25F (two 50F caps in series equivalent to one 25F cap)

dV/dt = 1/12.5 V/s

Time to charge assuming constant current:

(Vfull - Vempty) / (dV/dt) ~= tCharge

(4.5 - 0) / (1/12.5 V/s) = 4.5 * 12.5 = 56.25s = tCharge

The slope of the cap voltage waveform you sent appears to match up exactly with our expectation assuming a constant current (2V / 25s, or 1/12.5 V/s).

Your caps also seem to charge in roughly 60s, which again aligns with the math above. In practice, the charging time takes a little longer due to parasitics, cap balancing, and other non-idealities.

If you need faster charging time, you may want to explore a part like the LTC3350, which allows for higher charging currents due to the external power FETs

LTC3350 Datasheet and Product Info | Analog Devices

Best,

Blaise

BlaiseParker · Answer

Hi,

Just looking at the capacitor voltage, the part seems to be working as expected. If you have a constant 2A charging current, two 50F caps, and assuming ideal conditions, I expect the caps to fully charge in around 56s. The math is included below:

For a capacitor, I = C * dV/dt

dV/dt = I / C

dV/dt = 2A / 25F (two 50F caps in series equivalent to one 25F cap)

dV/dt = 1/12.5 V/s

Time to charge assuming constant current:

(Vfull - Vempty) / (dV/dt) ~= tCharge

(4.5 - 0) / (1/12.5 V/s) = 4.5 * 12.5 = 56.25s = tCharge

The slope of the cap voltage waveform you sent appears to match up exactly with our expectation assuming a constant current (2V / 25s, or 1/12.5 V/s).

Your caps also seem to charge in roughly 60s, which again aligns with the math above. In practice, the charging time takes a little longer due to parasitics, cap balancing, and other non-idealities.

If you need faster charging time, you may want to explore a part like the LTC3350, which allows for higher charging currents due to the external power FETs

LTC3350 Datasheet and Product Info | Analog Devices

Best,

Blaise

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LTC4041 - Unexpected low charge current

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