Rainbow Electronics MAX8724 - Manual (Page 21)

The 22µF ceramic capacitor has a typical ESR of
0.003Ω, which sets the output zero at 2.412MHz.
The output pole is set at:
where:
Set the compensation zero (f Z_CV ) such that it is equiv-
alent to the output pole (f P_OUT = 1.08kHz), effectively
producing a pole-zero cancellation and maintaining a
single-pole system response:
Choose C CV = 100nF, which sets the compensation
zero (f Z_CV ) at 1.6kHz. This sets the compensation pole:
CCI Loop Definitions
Compensation of the CCI loop depends on the parame-
ters and components shown in Figure 7. C
CI
is the CCI
loop compensation capacitor. A CSI is the internal gain
of the current-sense amplifier. RS2 is the charge cur-
rent-sense resistor, RS2 = 15mΩ. R
OGMI
is the equiva-
lent output impedance of the GMI amplifier ≥ 10MΩ.
GMI is the charge-current amplifier transconductance
= 1µA/mV. GM
OUT is the DC-DC converter transcon-
ductance = 3.3A/V. The CCI loop is a single-pole sys-
tem with a dominant pole compensation set by f
P_CI :
The loop transfer function is given by:
Since:
The loop transfer function simplifies to:
LTF GMI
R
sR C
OGMI
OGMI CI
= Ч
+ Ч 1
GM
A RS
OUT
CSI
=
×
1
2
LTF GM A RS GMI
R
sR C
OUT CSI
OGMI
OGMI CI
= Ч Ч Ч
+ Ч
2
1
f
R C
P CI
OGMI CI
_
=
×
1
2π
f
R C
Hz
P CV
OGMV CV
_
. =
×
=
1
2
0 16
π
C
R kHz
nF
CV
CV
=
×
=
1
2 1 08
147
π .
f
R C
Z CV
CV CV
_
=
×
1
2π
R
V
I
Battery ESR
L
BATT
CHG
= =
∆
∆
f
R C
kHz
P OUT
L OUT
_
. =
×
=
1
2
1 08
π
MAX1908/MAX8724
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 21
CCV LOOP GAIN
vs. FREQUENCY
FREQUENCY (Hz)
GAIN (dB)
100k 10k 1k 100 10
-40
-20
0
20
40
60
80
-60
1 1M
CCV LOOP PHASE
vs. FREQUENCY
FREQUENCY (Hz)
PHASE (DEGREES)
100k 10k 1k 100 10
-120
-105
-90
-75
-60
-45
-135
1 1M
Figure 6. CCV Loop Gain/Phase vs. Frequency