isl6252a Intersil Corporation, isl6252a Datasheet - Page 19

• Manufacturer Part Number: isl6252a
• Description: Highly Integrated Battery Charger Controller For Notebook Computers
• Manufacturer: Intersil Corporation

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 30:
CHARGE CURRENT CONTROL LOOP
When the battery voltage is less than the fully charged
voltage, the voltage error amplifier goes to it’s maximum
output (limited to 1.2V above ICOMP) and the ICOMP
voltage controls the loop through the minimum voltage
buffer. Figure 19 shows the charge current control loop.
ICOMP
The compensation capacitor (C
amplifier (GMI) a pole at a very low frequency (<<1Hz) and a
a zero at f
ICOMP. The frequency of can be calculated from
Equation 31:
Placing this zero at a frequency equal to the pole calculated
in Equation 29 will result in maximum gain at low frequencies
and phase margin near 90°. If the zero is at a higher
frequency (smaller C
the phase margin will be lower. Use a capacitor on ICOMP
that is equal to or greater than the value calculated in
Equation 32:
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 33:
The cross over frequency is determined by the DC gain of
the modulator and output filter and the pole in Equation 23.
f
f
f
C
POLE2
ZERO
FILTER
ICOMP
C
S
=
Σ
ICOMP
=
FIGURE 19. CHARGE CURRENT LIMIT LOOP
=
=
---------------------------------------
(
Z1
2π C
-------------------------------------- -
2π C
------------------------------------------ -
(
----------------------------------------------------------------------------------------- -
(
2π C
. f
R
4 gm2
⋅
⋅
11
Z1
S2
⋅
0.25
⋅
gm2
o
ICOMP
is created by the 0.25*CA2 output added to
1
+
F2
⋅
PHASE
1
+
-
-
+
R
r
DS ON
BAT
⋅
4
R
ICOMP
(
)
⋅
F2
CHLIM
(
50μA V
F2
)
R FET_r
)
should be small (<10Ω) to
+
), the DC gain will be higher but
R
CA2
gm2
19
DCR
⁄
20
DS(ON)
ICOMP
)
+
-
=
+
S2
+
-
R
50μA
-------------- -
CSON
CSOP
BAT
and CSOP and CSON
V
) gives the error
L
)
R L_DCR
C F2
R
C O
ESR
R F2
ISL6252, ISL6252A
(EQ. 33)
(EQ. 30)
(EQ. 31)
(EQ. 32)
R S2
R BAT
The DC gain is calculated in Equation 34 and the cross over
frequency is calculated with Equation 35.
The bode plot of the loop gain, the compensator gain and
the power stage gain is shown in Figure 20:
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 ACLIM pin, ISL6252 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 gm3 and
ICOMP controls the charger output. Figure 21 shows the
adapter current limit control loop.
A
f
CO
DC
DCIN
=
FIGURE 20. CHARGE CURRENT LOOP BODE PLOTS
=
-20
-40
-60
60
40
20
R S1
A
C F1
0.01k
0
--------------------------------------------------------------------------------------------------------- -
(
C ICOMP
DC
FIGURE 21. ADAPTER CURRENT LIMIT LOOP
R
S2
ICOMP
⋅
CSIN
CSIP
f
+
R F1
POLE
r
DS ON
0.1k
Σ
(
=
f
POLE1
)
11 R
--------------------- -
11 R
11
+
-
+
2π L
20
gm3
⋅
0.25
R
⋅
FREQUENCY (Hz)
CA1
⋅
DCR
-
+
PHASE
S2
S2
+
-
1k
f
ZERO
+
ACLIM
R FET_r
f
R
FILTER
CA2
BATTERY
20
f
+
-
POLE2
10k
DS(ON)
+
-
CSON
CSOP
COMPENSATOR
MODULATOR
LOOP
)
L
100k
R L_DCR
C F2
R ESR
C O
July 19, 2007
R F2
(EQ. 35)
(EQ. 34)
FN6498.1
1M
R