20
30
40
50
60
70
80
90
100
0.01
0.1
1
10
100
E
ff
ici
e
n
cy
(
%
)
Input Current (mA)
VSTOR = 1.8 V
VSTOR = 3.0 V
VSTOR = 5.5 V
VIN = 0.5 V
Test and Measurment Summary
Figure 4. Charger Efficiency versus Input Current
Because the boost converter regulates input voltage instead of output voltage, uses PFM switching,
operates at very low currents and has MPPT, efficiency cannot be measured using the same test setup as
for an output regulating, higher power, fixed frequency PWM switching boost converter. The VSTOR
output must be held at a fixed voltage (below VBAT_OV threshold) by an external source that is capable
of sinking current, with that sunk current being the measured output current. In addition to filtering bursts
of current due to PFM switching and the ripple voltage voltage on VIN_DC due to input voltage regulation,
the series input current meter and input voltage meter must be set to filtering, or averaging, or both, which
will result in longer than usual measurement times, but not longer than the 16 s MPPT sample time.
Measurements for both VIN and IN will be most accurate when taken at the midpoint of the 16 s MPPT
period. Remote sensing by the source-meters is possible but, on the input side, the source-meter output
regulation loop and the charger MPPT input regulation loop may interfere with each other and cause the
input voltage to oscillate. Adding a large capacitor across VIN_DC and GND will eliminate this oscillation
but the capacitor's leakage current will inflate the input current measurement and result in lower efficiency.
See
for a detailed explanation on how to take these and other measurements with source-
meters.
3.1.2
Buck Converter Efficiency
The test setup is shown in
. The specific equipment used for the test results in
is listed
below:
1. VSTOR was connected to a low impedance power supply with a series current meter to measure
current. The current meter must be able to measure currents in the nA range and may require manual
range adjustments so that the range is always
≤
10 X the expected current for best accuracy. The
voltage meter measures the input voltage as close to the IC VSTOR pin as possible.
2. VOUT was connected to a resistor box with a series current meter to measure the current. (NOTE: The
current meter must be able to measure currents in the nA range and may require manual range
adjustments so that the range is always
≤
10 X the expected current for best accuracy.) The voltage
meter measured the VOUT voltage as close to the IC VOUT pin as possible
10
User's Guide for bq25570 Battery Charger Evaluation Module for Energy
SLUUAA7A – July 2013 – Revised August 2014
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Copyright © 2013–2014, Texas Instruments Incorporated
