MAX1909 Maxim Integrated Products, MAX1909 Datasheet - Page 24

• Manufacturer Part Number: MAX1909
• Description: (MAX8725 / MAX1909) Multichemistry Battery Charger
• Manufacturer: Maxim Integrated Products

Multichemistry Battery Chargers with Automatic
System Power Selector
Figure 7. CCI Loop Diagram
This describes a single-pole system. Since:
the loop transfer function simplifies to:
The crossover frequency is given by:
For stability, choose a crossover frequency lower than
1/10th of the switching frequency:
Choosing a crossover frequency of 30kHz and using the
component values listed in Figure 1 yields C
Values for C
may slow down the current-loop response excessively.
Figure 8 shows the Bode plot of the current-loop fre-
quency response using the values calculated above.
24
______________________________________________________________________________________
CCI
C
CI
CI
LTF GMI
greater than 10 times the minimum value
C
GM
=
CI
R
OGMI
OUT
f
= GMI / (2π f
CO CI
1
_
GM
GMI
+
=
sR
OUT
A
=
R
CSI
OGMI
2π
OGMI
GMI
C
1
×
ICTL
O_CI
CI
RS
CSIP
×
C
2
)
CI
CSI
RS2
CSIN
CI
> 5.4nF.
The simplified schematic in Figure 9 is sufficient to
describe the operation of the MAX1909/MAX8725 when
the input current-limit loop (CCS) is in control. Since the
output capacitor’s impedance has little effect on the
response of the input current-limit loop, only a single
pole is required to compensate this loop. A
internal gain of the current-sense amplifier. RS1 is the
input current-sense resistor; RS1 = 10mΩ in the typical
operating circuits. R
impedance of the GMS amplifier, which is greater than
10MΩ. GMS is the charge-current amplifier transcon-
ductance = 1µA/mV. GM
input-referred transconductance = (1/D) GM
(1/D) 3.3A/V.
Figure 8. CCI Loop Response
Figure 9. CCS Loop Diagram
CCS
100
-20
-40
CLS
80
60
40
20
0
0.1
GMS
C
CS
R
OGMS
10
OGMS
FREQUENCY (Hz)
IN
CCS Loop Compensation
is the DC-DC converter’s
CSS
GM
is the equivalent output
1k
IN
ADAPTER
SYSTEM
INPUT
LOAD
MAG
PHASE
100k
CSSP
CSSN
RS1
CSS
0
-45
-90
OUT
is the
=