isl88731 Intersil Corporation, isl88731 Datasheet - Page 19

• Manufacturer Part Number: isl88731
• Description: Smbus Level 2 Battery Charger
• Manufacturer: Intersil Corporation

The loop response equations, bode plots and the selection
of C
with loop gain reduced by the duty cycle and the ratio of
R
D = 50%, the loop gain will be 6dB lower than the loop gain
in Figure 22. This gives lower crossover frequency and
higher phase margin in this mode. If R
duty cycle is 50% then the adapter current loop gain will be
identical to the gain in Figure 22.
A filter should be added between R
reduce switching noise. The filter roll off frequency should be
between the cross over frequency and the switching
frequency (~100kHz).
Voltage Control Loop
When the battery is charged to the voltage set by
ChargeVoltage register the voltage error amplifier (GMV)
DCIN
S1
ICOMP
/R
FIGURE 21. CHARGE CURRENT LOOP BODE PLOTS
R
-20
-40
-60
C
60
40
20
C
S2
S1
F1
0
ICOMP
0.01
FIGURE 22. ADAPTER CURRENT LIMIT LOOP
. In other words, if R
ICOMP
are the same as the charge current control loop
CSSN
CSSP
R
F1
S
Σ
0.1
11
+
-
F
20
POLE1
GMS
0.25
CA1
-
+
PHASE
+
-
FREQUENCY (kHz)
1
19
S1
R
FET_RDSON
CA2
DACS
= R
20X
+
-
S1
F
S2
ZERO
10
and CSIP and CSIN to
CSOP
CSON
S1
and the duty cycle
F
F
/R
FILTER
POLE2
L
S2
C
R
Compensator
Modulator
Loop
C
= 2 and the
100
R
F2
ESR
O
L_DCR
R
F2
R
1000
R
S2
BAT
ISL88731
takes control of the output (assuming that the adapter
current is below the limit set by ACLIM). The voltage error
amplifier (GMV) discharges the cap on VCOMP to limit the
output voltage. The current to the battery decreases as the
cells charge to the fixed voltage and the voltage across the
internal battery resistance decreases. As battery current
decreases the 2 current error amplifiers (GMI and GMS)
output their maximum current and charge the capacitor on
ICOMP to its maximum voltage (limited to 0.3V above
VCOMP). With high voltage on ICOMP, the minimum voltage
buffer output equals the voltage on VCOMP.
The voltage control loop is shown in Figure 23.
Output LC Filter Transfer Functions
The gain from the phase node to the system output and
battery depend entirely on external components. Typical
output LC filter response is shown in Figure 24. Transfer
function A
A
The resistance R
inductor DCR, R
battery (normally between 50mΩ and 200mΩ) The worst
case for voltage mode control is when the battery is absent.
This results in the highest Q of the LC filter and the lowest
phase margin.
The compensation network consists of the voltage error
amplifier GMV and the compensation network R
C
frequency pole and a zero at F
ω
LC
VCOMP
ESR
VCOMP
=
C
R
=
---------------------------------------------------------- -
⎛
⎜
⎝
VCOMP
VCOMP
----------- -
ω
s
-------------------------------- -
(
DP
R
which give the loop very high DC gain, a very low
2
Σ
S
LC
FIGURE 23. VOLTAGE CONTROL LOOP
ESR
⎛
⎝
(s) is shown in Equation 22:
+
1
1
–
------------------------ -
(
ω
⋅
---------------
ω
11
LC
SENSE
C
O
0.25
ESR
GMV
s
s
o
is a combination of MOSFET r
⋅
)
+
-
-
+
Q
PHASE
⎞
⎠
DACV
)
+
ω
and the internal resistance of the
1
LC
⎞
⎟
⎠
R
FET_RDSON
CA2
=
R4
20X
R3
ZERO1
----------------------- -
(
+
-
L C
1
⋅
o
. Inductor current
CSOP
CSON
)
L
Q
R
=
L_DCR
November 20, 2006
C
R
C
R
VCOMP
F2
ESR
O
o
DS(ON)
R
⋅
F2
(EQ. 22)
------ -
C
FN9258.0
L
o
,
R
R
,
S2
BAT