MAX8731 Maxim Integrated Products, MAX8731 Datasheet - Page 26

• Manufacturer Part Number: MAX8731
• Description: SMBus Level 2 Battery Charger
• Manufacturer: Maxim Integrated Products

SMBus Level 2 Battery Charger with
Remote Sense
The loop-transfer function is given by:
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
1/10 the switching frequency:
C
ing frequency.
Values for C
can slow down the current-loop response. Choosing C
= 10nF yields a crossover frequency of 15.9kHz. Figure
10 shows the Bode plot of the current-loop frequency
response using the values calculated above.
Figure 10. CCI Loop Response
26
CI
LTF GM
> 10 × GMI / (2π f
______________________________________________________________________________________
=
100
-20
-40
80
60
40
20
0
OUT
CI
0.1
LTF GMI
greater than 10 times the minimum value
×
=
GM
A
CSI
f
OUT
CO CI
10
OSC
1
FREQUENCY (Hz)
×
_
+
RS
=
sR
) = 4nF, for a 400kHz switch-
2
A
R
=
OGMI
×
CSI
OGMI
2π
1k
GMI
GMI
1
C
×
CI
×
RS
1
C
+
2
CI
MAG
PHASE
sR
100k
R
OGMI
OGMI
0
-45
-90
×
C
CI
CI
The simplified schematic in Figure 11 is sufficient to
describe the operation of the MAX8731 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
gain of the current-sense resistor; RS1 = 10mΩ in the
typical application circuits. R
output impedance of the GMS amplifier, which is
greater than 10MΩ. GMS is the charge-current amplifier
transconductance = 1µA/mV. GM
verter’s input-referred transconductance = (1/D) x
GM
The loop-transfer function is given by:
Since:
the loop-transfer function simplifies to:
Figure 11. CCS Loop Diagram
LTF GM
OUT
=
= (1 / D) x 5A/V.
InputCurrent( )
IN
CCS
LTF GMS
×
A
=
GMS
CSS
GM
C
CS
IN
×
RSI GMS
1
R
=
OGMS
+
SR
×
A
CCS Loop Compensation
CSS
R
OGMS
OGMS
GM
OGMS
CSS
1
IN
×
IN
1
RS
+
×
is the DC-DC con-
SR
CSS
C
2
is the equivalent
CS
ADAPTER
R
SYSTEM
OGMS
INPUT
LOAD
OGMS
is the internal
CSSP
CSSI
RS1
×
C
CS