isl88731c Intersil Corporation, isl88731c Datasheet - Page 20

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

In most cases the Battery resistance is very small
(<200mΩ) resulting in a very low Q in the output filter.
This results in a frequency response from the input of the
PWM to the inductor current with a single pole at the
frequency calculated in Equation 15:
F
The output capacitor creates a pole at a very high
frequency due to the small resistance in parallel with it.
The frequency of this pole is calculated in Equation 16:
F
Charge Current Control Loop
When the battery is less than the fully charged, the
voltage error amplifier goes to it’s maximum output
(limited to 0.3V above ICOMP) and the ICOMP voltage
controls the loop through the minimum voltage buffer.
Figure 24 shows the charge current control loop.
The compensation capacitor (C
amplifier (GMI) a pole at a very low frequency (<<1Hz)
and a a zero at F
output added to ICOMP. The frequency can be calculated
from Equation 17:
Placing this zero at a frequency equal to the pole
calculated in Equation 16 will result in maximum gain at
low frequencies and phase margin near 90°. If the zero is
at a higher frequency (smaller C
be higher but the phase margin will be lower. Use a
capacitor on ICOMP that is equal to or greater than the
value calculated in Equation 18. The factor of 1.5 is to
F
POLE1
POLE2
ZERO
V
GAIN = 11
INPUT
PWM
RAMP
PWM
RAMP GEN
INPUT
=
PWM
=
=
= V
FIGURE 22. SMALL SIGNAL AC MODEL
---------------------------------------
(
(
------------------------------------------------------------------------------------------ -
-------------------------------------- -
2π C
11
2π C
RS2
ADAPTER
4 gm2
⋅
⋅
+
-
⋅
R
+
o
ICOMP
1
FET_RDSON
/11
⋅
r
DS ON
R
Z1
BAT
. F
(
)
Z1
2π L
)
is created by the 0.25*CA2
+
⋅
R
L
20
DCR
V
gm2
ADAPTER
R
+
ICOMP
L_DCR
R
ICOMP
=
BAT
50μA V
) gives the error
)
L
R
), the DC gain will
RS2
C
⁄
C
ESR
O
O
(EQ. 15)
(EQ. 16)
(EQ. 17)
R
ISL88731C
BAT
ensure the zero is at a frequency lower than the pole
including tolerance variations.
C
A filter should be added between R
CSON to reduce switching noise. The filter roll-off
frequency should be between the crossover frequency
and the switching frequency (~100kHz). R
small (<10Ω) to minimize offsets due to leakage current
into CSOP. The filter cutoff frequency is calculated using
Equation 19:
F
The crossover frequency is determined by the DC gain of
the modulator and output filter and the pole in
Equation 16. The DC gain is calculated in Equation 20
and the cross over frequency is calculated with
Equation 21:
A
F
The Bode plot of the loop gain, the compensator gain and
the power stage gain is shown in Figure 24.
Adapter Current Limit Control Loop
If the combined battery charge current and system load
current draws current that equals the adapter current
limit set by the InputCurrent register, ISL88731C will
reduce the current to the battery and/or reduce the
output voltage to hold the adapter current at the limit.
Above the adapter current limit the minimum current
buffer equals the output of GMS and ICOMP controls the
charger output. Figure 25 shows the adapter current limit
control loop.
FILTER
CO
ICOMP
DC
ICOMP
=
=
FIGURE 23. CHARGE CURRENT LIMIT LOOP
A
-------------------------------------------------------------------------------------------
(
RS2
Σ
DC
=
=
C
Σ
Σ
S
ICOMP
------------------------------------------------------------------------------------------ -
(
------------------------------------------ -
(
RS2
⋅
2π C
+
F
POLE
r
⋅
DS ON
11
+
0.25
1.5 4
F2
GMI
1
(
r
11 RS2
DS ON
+
-
-
+
⋅
PHASE
⋅
=
R
⋅
)
(
11 RS2
---------------------- -
F2
+
⋅
(
2π L
R
50μA V
)
)
⋅
DCR
+
⋅
R
R
FET_RDSON
DACI
DCR
CA2
⁄
+
20X
R
) L
BAT
⋅
+
+
-
R
BAT
)
S2
CSOP
CSON
L
and CSOP and
)
R
F2
L_DCR
September 9, 2010
C
C
R
F2
O
ESR
should be
R
F2
(EQ. 18)
(EQ. 19)
(EQ. 20)
(EQ. 21)
FN6978.1
R
R
S2
BAT