MAX1909ETIT Maxim Integrated Products, Inc., MAX1909ETIT Datasheet - Page 23
MAX1909ETIT
Manufacturer Part Number
MAX1909ETIT
Description
Multichemistry Battery Chargers With Automatic System Power Selector
Manufacturer
Maxim Integrated Products, Inc.
Datasheet
1.MAX1909ETIT.pdf
(30 pages)
The poles and zeros of the voltage-loop transfer function
are listed from lowest frequency to highest frequency in
Table 1.
Near crossover, C
than R
dominates the parallel impedance near crossover.
Additionally, R
C
C
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:
Multichemistry Battery Chargers with Automatic
Figure 6. CCV Loop Response
CV
CV
OUT
ESR
, so:
and dominates the series combination of R
also has a much lower impedance than R
OGMV
is small enough, its associated output zero has
-20
-40
80
60
40
20
0
0.1
. Since C
R
LTF GM
OGMV
CV
(
1
(
1
+
1
=
+
has a much higher impedance than
sC
______________________________________________________________________________________
MAG
PHASE
sC
CV
× +
10
OUT
R
CV
CV
OUT
(
L
FREQUENCY (Hz)
1
has a much lower impedance
100
is in parallel with R
×
×
sC
R
×
R
OGMV
L
CV
sC
1k
)
R
≅
CV
OUT
×
sC
R
10k
)
CV
OUT
1
GMV
100k
)
≅
R
CV
1M
0
-45
-90
-135
OGMV,
CV
L
near
C
and
CV
System Power Selector
Setting the LTF = 1 to solve for the unity-gain frequency
yields:
For stability, choose a crossover frequency lower than
1/10th of the switching frequency. Choosing a
crossover frequency of 30kHz and solving for R
using the component values listed in Figure 1 yields:
MODE = V
GMV = 0.125µA/mV
C
V
R
GM
f
f
To ensure that the compensation zero adequately can-
cels the output pole, select f
where C
charge current).
Figure 6 shows the Bode plot of the voltage-loop fre-
quency response using the values calculated above.
The simplified schematic in Figure 7 is sufficient to
describe the operation of the MAX1909/MAX8725 when
the battery current loop (CCI) is in control. Since the
output capacitor’s impedance has little effect on the
response of the current loop, only a single pole is
required to compensate this loop. A
gain of the current-sense amplifier. RS2 is the charge
current-sense resistor, RS2 = 15mΩ. R
equivalent output impedance of the GMI amplifier,
which is greater than 10MΩ. GMI is the charge-current
amplifier transconductance = 1µA/mV. GM
DC-DC converter transconductance = 3.3A/V.
The loop transfer function is given by:
CO_CV
OSC
BATT
L
OUT
LTF GM
= 0.2Ω
OUT
= 400kHz
= 22µF
= 16.8V
=
= 30kHz
= 3.33A/V
CV
f
R
CC
CO CV
CV
≥ 4nF (assuming 4 cells and 4A maximum
OUT
_
(4 cells)
=
C
×
2
=
π
CV
A
CSI
GM
GMV GM
×
≥ (R
C
OUT
×
OUT
RS
L
×
/R
×
Z_CV
2
CV
GMV
CCI Loop Compensation
×
×
OUT
GMI
f
) C
CO CV
≤ f
OUT
_
2π
1
P_OUT
+
CSI
sR
R
×
CV
=
R
C
OGMI
is the internal
OUT
OGMI
:
10
OGMI
k
OUT
Ω
×
C
is the
is the
CI
CV
23