MAX8725ETI Maxim Integrated Products, MAX8725ETI Datasheet - Page 23

• Manufacturer Part Number: MAX8725ETI
• Description: Battery Management Multichemistry Battery Charger
• Manufacturer: Maxim Integrated Products

Specifications of MAX8725ETI

• Product: Charge Management
• Battery Type: Li-Ion, Li-Polymer, NiCd, NiMH, Lead Acid, Universal
• Operating Supply Voltage: 8 V to 28 V
• Supply Current: 2.7 mA
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Package / Case: TQFN-28
• Charge Safety Timers: No
• Mounting Style: SMD/SMT
• Temperature Monitoring: No
• Uvlo Start Threshold: 9.18 V
• Uvlo Stop Threshold: 9.42 V
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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
CV
sC
10
× +
OUT
R
CV
CV
OUT
(
L
FREQUENCY (Hz)
1
has a much lower impedance
100
is in parallel with R
×
×
sC
R
R
×
L
OGMV
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 = LDO (4 cells)
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
= 0.2Ω
LTF GM
OUT
= 400kHz
= 22µF
= 16.8V
=
= 30kHz
= 3.33A/V
CV
f
R
CO CV
CV
≥ 4nF (assuming 4 cells and 4A maximum
OUT
_
=
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
CI
is the
CV
23