MAX8731 Maxim Integrated Products, MAX8731 Datasheet - Page 24

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

SMBus Level 2 Battery Charger with
Remote Sense
The charge-voltage and charge-current regulation
loops are independent and compensated separately at
the CCV, CCI, and CCS.
The simplified schematic in Figure 7 is sufficient to
describe the operation of the MAX8731 when the volt-
age loop (CCV) is in control. The required compensa-
tion network is a pole-zero pair formed with C
R
formed by the output capacitor and the load. R
the equivalent series resistance (ESR) of the charger
output capacitor (C
output load, where R
lent output impedance of the GMV amplifier, R
greater than 10MΩ. The voltage amplifier transconduc-
Figure 7. CCV Loop Diagram
Table 5. CCV Loop Poles and Zeros
24
CV
CCV Pole
CCV Zero
Output
Output
NAME
Zero
Pole
. The zero is necessary to compensate the pole
______________________________________________________________________________________
R
CCV
C
CV
CV
f
P CV
R
f
f
f
OGMV
P OUT
P OUT
_
Z CV
OUT
_
_
GM
_
L
OUT
=
= ∆V
GMV
). R
EQUATION
2π
=
=
=
2π
R
2π
2π
ChargeVoltage( )
L
CCV Loop Compensation
BATT
OGMV
R
is the equivalent charger
R
R
CV
L
L
1
1
×
×
1
1
×
C
C
/ ∆I
FBS_
×
C
OUT
OUT
C
CV
Compensation
CHG
CV
R
C
ESR
. The equiva-
OUT
OGMV
CV
Lowest frequency pole created by C
Voltage-loop compensation zero. If this zero is at the same frequency or
lower than the output pole f
approximates a single-pole response near the crossover frequency. Choose
C
adequate phase margin.
Output pole formed with the effective load resistance R
capacitance C
stability of the system or the crossover frequency.
Output ESR Zero. This zero can keep the loop from crossing unity gain if
f
capacitor with an ESR zero greater than the crossover frequency.
R
ESR
Z_OUT
L
CV
and
, is
to place this zero at least 1 decade below crossover to ensure
is
is less than the desired crossover frequency; therefore, choose a
tance, GMV = 0.125µA/mV. The DC-DC converter
transconductance is dependent upon the charge-cur-
rent sense resistor RS2:
where A
application circuits, so GM
fer function is given by:
The poles and zeros of the voltage loop-transfer func-
tion are listed from lowest frequency to highest frequen-
cy in Table 5.
Near crossover C
R
inates the parallel impedance near crossover.
Additionally, R
and dominates the series combination of R
so near crossover:
OUT
OGMV
. R
LTF GM
L
. Since C
influences the DC gain but does not affect the
CSI
×
=
P_OUT
R
= 20V/V, and RS2 = 10mΩ in the typical
GM
(
(
OGMV
1
1
CV
+
(
DESCRIPTION
+
1
OUT
CV
OUT
sC
sC
+
, then the loop-transfer function
is much higher impedance than C
sC
CV
CV
OUT
is in parallel with R
CV
×
×
=
CV
(
R
1
×
is much lower impedance than
and GMV’s finite output resistance.
L
+
A
R
×
×
sC
×
CSI
OGMV
R
R
GMV R
OUT
ESR
OGMV
CV
1
×
RS
×
)(
)(
×
= 5A/V. The loop-trans-
1
R
1
2
+
)
CV
+
OGMV
sC
sC
L
)
and the output
CV
≅
OUT
OGMV
R
CV
×
R
CV
×
CV
, C
R
L
and C
)
CV
)
dom-
CV
CV
,