isl88731c Intersil Corporation, isl88731c Datasheet - Page 21

• Manufacturer Part Number: isl88731c
• Description: Smbus Level 2 Battery Charger
• Manufacturer: Intersil Corporation

The loop response equations, bode plots and the
selection of C
control loop with loop gain reduced by the duty cycle and
the ratio of R
the duty cycle D = 50%, the loop gain will be 6dB lower
than the loop gain in Figure 25. This gives lower
crossover frequency and higher phase margin in this
mode. If R
adapter current loop gain will be identical to the gain in
Figure 25.
A filter should be added between R
to 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)
takes control of the output (assuming that the adapter
DCIN
FIGURE 24. CHARGE CURRENT LOOP BODE PLOTS
R
FIGURE 25. ADAPTER CURRENT LIMIT LOOP
C
-20
-40
-60
C
60
40
20
S1
F1
0
0.01k
ICOMP
ICOMP
CSSN
CSSP
S1
R
F1
/R
S1
ICOMP
COMPENSATOR
MODULATOR
LOOP
S2
Σ
S
/R
0.1k
S2
= 2 and the duty cycle is 50% then the
are the same as the charge current
11
+
-
. In other words, if R
20
0.25
GMS
CA1
F
-
+
PHASE
+
-
POLE1
FREQUENCY (Hz)
21
1k
R
F
CA2
FET_RDSON
ZERO
DACS
20X
+
-
F
FILTER
10k
S1
F
CSOP
CSON
POLE2
and CSIP and CSIN
L
S1
C
100k
R
C
R
= R
F2
ESR
O
L_DCR
R
F2
S2
ISL88731C
and
1M
R
R
S2
BAT
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 26.
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 27. 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
low frequency pole and a zero at F
current information is added to the feedback to create a
second zero F
between R
ω
LC
VCOMP
ESR
=
VCOMP
=
---------------------------------------------------------- -
⎛
⎜
⎝
C
R
----------- -
ω
VCOMP
VCOMP
s
-------------------------------- -
(
DP
which give the loop very high DC gain, a very
R
FIGURE 26. VOLTAGE CONTROL LOOP
2
LC
ESR
Σ Σ
S
S2
⎛
⎝
+
1
(s) is shown in Equation 22:
1
------------------------ -
(
–
and ISL88731C add a pole at F
ω
ZERO2
⋅
---------------
ω
LC
C
ESR
SENSE
11
s
s
O
o
0.25
GMV
)
⋅
is a combination of MOSFET r
Q
⎞
⎠
-
+
+
-
. The low pass filter R
)
ω
DACV
PHASE
+
LC
1
and the internal resistance of the
⎞
⎟
⎠
=
R
----------------------- -
(
CA2
FET_RDSON
R4
20x
L C
R3
1
⋅
+
-
o
)
ZERO1
CSON
CSOP
Q
L
=
R
. Inductor
F2
o
R
September 9, 2010
⋅
L_DCR
, C
C
C
R
FILTER
F2
O
ESR
------ -
C
L
F2
R
o
F2
DS(ON)
VCOMP
(EQ. 22)
FN6978.1
. R
R
R
S2
BAT
3
,
,