isl88731 Intersil Corporation, isl88731 Datasheet - Page 18

• Manufacturer Part Number: isl88731
• 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 21 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
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
F
POLE1
POLE2
V
ZERO
INPUT
GAIN=11
RAMP
PWM
RAMP GEN
PWM
INPUT
PWM
=
= VDD/11
=
=
11
---------------------------------------
(
Z1
(
------------------------------------------------------------------------------------------------------ -
-------------------------------------- -
2π C
2π C
FIGURE 19. SMALL SIGNAL AC MODEL
R
. F
SENSE
4 gm2
R
⋅
⋅
-
+
⋅
Z1
FET_RDSON
o
ICOMP
1
is created by the 0.25*CA2 output added to
⋅
R
+
BAT
r
DS ON
)
(
2π L
L
⋅
)
18
+
gm2
ICOMP
VDD
R
R
DCR
L_DCR
=
50μA V
+
) gives the error
R
R
BAT
L
R
SENSE
C
C
ESR
⁄
O
O
)
(EQ. 17)
(EQ. 15)
(EQ. 16)
R
BAT
ISL88731
frequency (smaller C
the phase margin will be lower. Us a capacitor on ICOMP
that is equal to or greater than the value calculated in
Equation 18. The factor of 1.5 is to ensure the zero is at a
frequency lower than the pole including tolerance variations.
C
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). R
minimize offsets due to leakage current into CSOP. The filter
cut off frequency is calculated using Equation 19:
F
The cross over 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 21.
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, ISL88731 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 22
shows the adapter current limit control loop.
FILTER
CO
ICOMP
DC
ICOMP
ICOMP
=
=
Σ
------------------------------------------------------------------------------------------------------ -
(
A
C
C
R
S
S
Σ
FIGURE 20. CHARGE CURRENT LIMIT LOOP
DC
=
=
ICOMP
ICOMP
SENSE
------------------------------------------------------------------------------------------------------ -
(
------------------------------------------ -
(
⋅
R
2π C
F
SENSE
POLE
11
11
⋅
0.25
0.25
GMI
GMI
+
F2
1
r
PHASE
PHASE
11 R
+
+
-
-
-
-
+
+
1.5 4
DS ON
⋅
=
+
ICOMP
R
⋅
(
r
⋅
11 R
---------------------------------- -
F2
DS ON
F2
SENSE
⋅
)
⋅
)
(
R
R
(
2π L
should be small (<10Ω) to
+
50μA V
FET_RDSON
FET_RDSON
), the DC gain will be higher but
DACI
DACI
SENSE
R
⋅
CA2
CA2
)
DCR
+
20X
20X
R
⁄
+
+
-
-
DCR
+
) L
S2
⋅
R
CSOP
CSOP
CSON
CSON
and CSOP and CSON
BAT
+
L
L
R
BAT
)
R
R
)
L_DCR
L_DCR
C
C
R
R
C
C
November 20, 2006
F2
F2
ESR
ESR
O
O
R
R
F2
F2
(EQ. 21)
(EQ. 19)
(EQ. 20)
(EQ. 18)
FN9258.0
R
R
R
R
S2
S2
BAT
BAT