Low-Cost Multichemistry Battery Chargers
The 22µF ceramic capacitor has a typical ESR of
CCI Loop Definitions
0.003Ω, which sets the output zero at 2.412MHz.
Compensation of the CCI loop depends on the parame-
ters and components shown in Figure 7. C is the CCI
CI
The output pole is set at:
loop compensation capacitor. A
is the internal gain
CSI
of the current-sense amplifier. RS2 is the charge cur-
1
f
=
= 1.08kHz
P _OUT
rent-sense resistor, RS2 = 15mΩ. R
is the equiva-
OGMI
2πR × C
L
OUT
lent output impedance of the GMI amplifier ≥ 10MΩ.
GMI is the charge-current amplifier transconductance
where:
= 1µA/mV. GM
is the DC-DC converter transcon-
OUT
ductance = 3.3A/V. The CCI loop is a single-pole sys-
∆V
∆I
BATT
CHG
tem with a dominant pole compensation set by f
:
R
=
= Battery ESR
P_CI
L
1
f
=
P _CI
Set the compensation zero (f
) such that it is equiv-
Z_CV
2πR
× C
CI
OGMI
alent to the output pole (f
= 1.08kHz), effectively
P_OUT
producing a pole-zero cancellation and maintaining a
single-pole system response:
The loop transfer function is given by:
R
1
OGMI
LTF = GM
× A ×RS2×GMI
CSI
f
=
OUT
Z _CV
1+sR
×C
2πR
× C
OGMI
CI
CV
CV
Since:
1
C
=
=147nF
1
CV
2πR ×1.08kHz
GM
=
CV
OUT
A
× RS2
CSI
Choose C
= 100nF, which sets the compensation
CV
The loop transfer function simplifies to:
zero (f
) at 1.6kHz. This sets the compensation pole:
Z_CV
1
R
OGMI
f
=
= 0.16Hz
LTF = GMI ×
P _CV
2πR
× C
CV
1+sR
×C
OGMV
OGMI
CI
CCV LOOP PHASE
vs. FREQUENCY
CCV LOOP GAIN
vs. FREQUENCY
80
60
-45
-60
40
-75
20
-90
0
-105
-120
-135
-20
-40
-60
1
10
100
1k
10k
100k
1M
1
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 6. CCV Loop Gain/Phase vs. Frequency
______________________________________________________________________________________ 21
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