MAX8731 Maxim Integrated Products, MAX8731 Datasheet - Page 25

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

C
crossover so the parallel impedance is mostly capaci-
tive and:
If R
a negligible effect near crossover and the loop-transfer
function can be simplified as follows:
Setting LTF = 1 to solve for the unity-gain frequency
yields:
For stability, choose a crossover frequency lower than
1/10 the switching frequency. For example, choose a
crossover frequency of 50kHz and solve for R
the component values listed in Figure 1 to yield R
10kΩ:
GMV = 0.125µA/mV
Figure 8. CCV Loop Response
OUT
ESR
is also much lower impedance than R
is small enough, its associated output zero has
-20
-40
f
80
60
40
20
CO CV
0
R
0.1
CV
_
LTF GM
(
1
=
+
1
SMBus Level 2 Battery Charger with
2
=
=
sC
______________________________________________________________________________________
π
MAG
PHASE
GM
GMV GM
×
10
R
OUT
C
L
FREQUENCY (Hz)
OUT
OUT
OUT
⋅
100
×
×
R
×
×
×
L
G
sC
1k
f
)
CO CV
OUT
R
MV
≅
CV
OUT
sC
_
10k
×
2π
OUT
1
G
100k
≅
MV
×
R
10
CV
C
k
OUT
1M
Ω
0
-45
-90
-135
VC
L
using
CV
near
=
GM
C
F
R
F
To ensure that the compensation zero adequately can-
cels the output pole, select f
C
charge current.)
Figure 8 shows the Bode plot of the voltage-loop fre-
quency response using the values calculated above.
The simplified schematic in Figure 9 is sufficient to
describe the operation of the MAX8731 when the bat-
tery current loop (CCI) is in control. Since the output
capacitor’s impedance has little effect on the response
of the current loop, only a simple single pole is required
to compensate this loop. A
current-sense amplifier. RS2 is the charge current-
sense resistor (10mΩ). R
impedance of the GMI amplifier, which is greater than
10MΩ. GMI is the charge-current amplifier transcon-
ductance = 1µA/mV. GM
transconductance = 5A/V.
Figure 9. CCI Loop Diagram
OSC
CO_CV
L
OUT
CV
= 0.2Ω
OUT
≥ 400pF (assuming 2 cells and 2A maximum
= 400kHz
= 2 x 10µF
= 5A/V
= 50kHz
CCI
C
CI
Remote Sense
C
CV
R
OGMI
≥ (R
GM
OUT
L
GMI
OGMI
OUT
/ R
CSI
Z_CV
CCI Loop Compensation
CV
ChargeCurrent( )
is the internal gain of the
is the DC-DC converter
is the equivalent output
) C
≤ f
CSIP
OUT
P_OUT
CSI
RS2
:
CSIN
25