ISL88731CHRTZ-T Intersil, ISL88731CHRTZ-T Datasheet - Page 20

• Manufacturer Part Number: ISL88731CHRTZ-T
• Description: IC BATT CHRGR SMBUS LVL2 28TQFN
• Manufacturer: Intersil

Specifications of ISL88731CHRTZ-T

• Function: Charge Management
• Battery Type: Lithium-Ion (Li-Ion)
• Voltage - Supply: 8 V ~ 26 V
• Operating Temperature: -10°C ~ 100°C
• Mounting Type: Surface Mount
• Package / Case: 28-WFQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
(GMI) a pole at a very low frequency (<<1Hz) and a a zero at F
F
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
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 ensure the zero is at a frequency lower than the pole
including tolerance variations.
C
A filter should be added between R
reduce switching noise. The filter roll-off frequency should be
between the crossover frequency and the switching frequency
(~100kHz). R
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
POLE2
Z1
F
FILTER
ICOMP
ZERO
is created by the 0.25*CA2 output added to ICOMP. The
ICOMP
=
=
=
=
Σ
C
Σ
S
ICOMP
Σ
----------------------------------- -
2π C
------------------------------------ -
(
------------------------------------------------------------------------------------ -
(
---------------------------------------- -
(
ICOMP
2π C
RS2
2π C
FIGURE 23. CHARGE CURRENT LIMIT LOOP
⋅
4 gm2
F2
⋅
⋅
⋅
should be small (<10Ω) to minimize offsets due
o
1.5 4
+
1
), the DC gain will be higher but the phase
11
ICOMP
F2
⋅
1
0.25
r
R
DS ON
GMI
⋅
⋅
BAT
R
+
-
-
+
(
PHASE
⋅
F2
)
(
50μA V ⁄
)
)
+
R
DCR
R
20
FET_RDSON
DACI
ICOMP
CA2
) L
gm2
+
⋅
20X
R
S2
+
-
BAT
) gives the error amplifier
=
and CSOP and CSON to
50μA V ⁄
)
CSON
CSOP
L
R
L_DCR
C
C
R
F2
O
ESR
R
F2
ISL88731C
(EQ. 16)
(EQ. 18)
(EQ. 19)
(EQ. 17)
R
R
S2
BAT
Z1
.
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.
CO
DC
DCIN
=
=
R
A
-20
-40
-60
C
------------------------------------------------------------------------------------ -
(
60
40
20
C
RS2
S1
DC
F1
FIGURE 24. CHARGE CURRENT LOOP BODE PLOTS
0
0.01k
ICOMP
⋅
ICOMP
FIGURE 25. ADAPTER CURRENT LIMIT LOOP
CSSN
CSSP
F
+
R
POLE
F1
r
DS ON
COMPENSATOR
MODULATOR
LOOP
S
Σ
(
11 RS2
=
0.1k
⋅
)
11 RS2
---------------------- -
11
+
+
-
2π L ⋅
20
R
0.25
⋅
GMS
DCR
CA1
-
+
F
PHASE
+
-
POLE1
FREQUENCY (Hz)
+
1k
R
R
F
BAT
CA2
FET_RDSON
ZERO
DACS
20X
)
+
-
F
FILTER
10k
F
CSOP
CSON
POLE2
L
100k
C
R
C
R
F2
ESR
O
L_DCR
February 8, 2011
R
F2
FN6978.2
(EQ. 20)
(EQ. 21)
R
1M
R
S2
BAT