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LTC3351

3351f

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APPLICATIONS INFORMATION

instantaneous drop in voltage due to the high frequency

ESR and a slower settling back to the original voltage

due to the “DC” ESR. The following figure illustrates this.

Figure 15.

VCAP

ICAP

REAL

ESR ONLY

MODEL

3351 F15

This results in an ESR measurement dependent on the

frequencyatwhichitismeasured.Thereisahighfrequency

ESR and a larger “DC” ESR. In energy storage applica-

tions where the capacitors will supply a sustained load,

the “DC” ESR limits the deliverable power. Supercapacitor

manufacturers have various methods for measuring ESR.

Even when specifying a “DC” ESR, the measurement is

often done at a frequency significantly higher than “DC”,

often near 100Hz, thereby reporting an intermediate

frequency ESR.

ESR Measurement Timing Adjustments

The LTC3351 default ESR measurement timing is set to

measure between the DC and high frequency ESR, closer

to the high frequency ESR for a wide variety of superca-

pacitors. The timing can be adjusted to measure closer to

the DC ESR, however this must done empirically based on

the exact switching controller configuration and capaci-

tors used. Values appropriate for some capacitors would

result in failed measurements for others and there are

no meaningful defaults preprogrammed in the LTC3351.

Using an DC coupled oscilloscope with a precision offset,

observe the supercapacitor stack voltage (VCAP) during

the ESR test. AC coupling will not work as the signal

time scales are too long to cross the oscilloscope’s AC

coupling high pass filter without distortion. Set the oscil-

loscope to trigger on SMBALERT falling and enabling the

mon_meas_activealertusingthemask_mon_meas_active

bit. Then set the ctl_start_cap_esr_meas to start the ESR

measurement. The SMBAlert will need to be cleared each

time to re-trigger the oscilloscope. Alternatively, a current

probe can be used to trigger on input current, since the

input current will be high during the ESR test.

The esr_i_override setting is available to prevent the

automatic adjustment of the ESR test current. The au-

tomatically selected ESR test current is available in the

next_esr_iregister.Afterseveralmeasurementshavebeen

completed, next_esr_i may be used as a starting point to

manually set esr_i_override. While adjusting the timing

of the ESR measurement, either set esr_i_override so the

test current won’t change between successive measure-

ments or do each measurement twice for each setting and

ignore the first result.

Referring to Figure 4, using the oscilloscope you can now

see the effects of esr_i_on_settling and esr_i_off_settling

on the captured waveform. Begin by adjusting esr_i_off_

settling so that VCAP is no longer decreasing significantly

after esr_i_off_settling time. This time will likely be sig-

nificantly longer than the default time.

The esr_i_on_settling time is a little more complicated

to adjust as the capacitors are being charged during this

time. If the esr_i_on_settling time is too short, the internal

chemistry of the supercapacitor will not have settled and

the ESR measured will be closer to the high frequency

ESR than the DC ESR. If esr_i_on_settling is too long

and esr_i_override is not set, then charge current will be

automatically adjusted downward, resulting in reduced

signal for the ESR measurement. If esr_i_on_settling is

too long while esr_i_override is set, the capacitors will

charge to constant voltage and charge current will fall,

corrupting the measurement. This can be observed in the

input current waveform. During an ESR test, it should not

decrease, it should turn off cleanly. The charge current can

also be observed by comparing esr_m1_i and esr_m2_i.

They should be about the same, esr_m2_i should not be

significantly less than esr_m1_i. If it is, esr_i_on_settling

is too long.

Ifesr_i_overrideisset,monitortheincreaseinstackvoltage

during the ESR test. Ideally it should be about the same

as the Δv in the capacitance test (as set by cap_delta_v_

setting). If the change in voltage during the ESR test is

less than in the capacitance test, esr_i_override can be

increased to maximize the signal available for ESR test. If

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