MAX8724 Maxim Integrated Products, MAX8724 Datasheet - Page 22

• Manufacturer Part Number: MAX8724
• Description: (MAX1908 - MAX87xx) Low-Cost Multichemistry Battery Chargers
• Manufacturer: Maxim Integrated Products

Compensation of the CCV loop depends on the para-
meters and components shown in Figure 5. C
R
series resistor. R
(ESR) of the charger output capacitor (C
equivalent charger output load, where R
I
amplifier, R
transconductance, GMV = 0.125µA/mV. The DC-DC
converter transconductance, GM
where A
sense resistor in the Typical Application Circuits.
The compensation pole is given by:
The compensation zero is given by:
The output pole is given by:
Low-Cost Multichemistry Battery Chargers
Figure 5. CCV Loop Diagram
22
CHG
CV
______________________________________________________________________________________
are the CCV loop compensation capacitor and
. The equivalent output impedance of the GMV
CCV
CSI
R
C
CV
CV
= 20, and RS2 is the charging current-
OGMV
f
P CV
f
f
P OUT
GM
_
Z CV
ESR
R
_
_
OGMV
OUT
≥ 10MΩ. The voltage amplifier
GM
=
is the equivalent series resistance
=
GMV
OUT
2π
=
=
2π
R
2π
A
OGMV
R
CCV Loop Definitions
R
CSI
CV
L
1
1
×
REF
1
1
×
×
OUT
C
×
C
RS
BATT
OUT
C
CV
2
CV
= 3.33A/V:
OUT
R
C
ESR
OUT
L
). R
= V
CV
L
R
BATT
L
is the
and
/
where R
Output zero due to output capacitor ESR:
The loop transfer function is given by:
Assuming the compensation pole is a very low
frequency, and the output zero is a much higher fre-
quency, the crossover frequency is given by:
To calculate R
Cells = 4
C
V
I
GMV = 0.125µA/mV
GM
R
f = 400kHz
Choose crossover frequency to be 1/5th the
MAX1908’s 400kHz switching frequency:
Solving yields R
Conservatively set R
frequency at:
Choose the output-capacitor ESR so the output-capacitor
zero is 10 times the crossover frequency:
CHG
BATT
OGMV
OUT
OUT
= 2.5A
= 22µF
= 16.8V
f
LTF GM
CO CV
L
= 10MΩ
= 3.33A/V
R
f
varies with load according to R
ESR
Z ESR
_
_
f
=
CO CV
CV
(
=
(
f
1
1
Z ESR
_
+
=
CV
2
+
_
and C
=
OUT
π
sC
sC
GMV R
2
×
= 26kΩ.
π
=
CV
10
OUT
CV
f
CO_CV
R
×
GMV R
=
ESR
CV
R
×
×
×
= 1kΩ, which sets the crossover
2
2π
L
f
R
CO CV
×
π
values of the circuit of Figure 2:
1
×
CV
OGMV
C
×
R
R
×
1
GMV R
_
ESR
= 3kHz
C
ESR
2π
OUT
×
OUT
CV
C
GM
1
)
)
OUT
×
×
(
×
1
(
1
×
C
C
+
=
OUT
+
GM
OUT
OGMV
OUT
sC
2 412
sC
.
L
CV
OUT
OUT
=
= V
=
80
MHz
×
×
0 24
.
BATT
R
kHz
×
CV
R
Ω
L
)
)
/ I
CHG.