isl88731c Intersil Corporation, isl88731c Datasheet - Page 22

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

and R
output voltage. For a 3-cell battery, R
R
compensation network’s poles, zeros and gain to the
components in Figure 26. Figure 28 shows an asymptotic
Bode plot of the DC/DC converter’s gain vs. frequency. It
is strongly recommended that F
30% of F
FIGURE 28. ASYMPTOTIC BODE PLOT OF THE VOLTAGE
FIGURE 27. FREQUENCY RESPONSE OF THE LC OUTPUT
4
= 100kΩ. The following equations relate the
-20
-40
-60
60
40
20
4
0
0.1k
are internal divider resistors that set the DC
LC
and F
CONTROL LOOP GAIN
FILTER
COMPENSATOR
MODULATOR
LOOP
R
= 200m Ω
F
ZERO1
BATTERY
ZERO2
R
= 50m Ω
1k
BATTERY
FREQUENCY (Hz)
is approximately 70% of F
FREQUENCY (Hz)
22
F
10k
ZERO2
ZERO1
F
LC
is approximately
3
F
= 500kΩ and
ESR
100k
NO BATTERY
F
FILTER
F
POLE1
LC
1M
ISL88731C
.
Compensation Break Frequency Equations
F
F
F
F
F
Choose R
from Equation 29.
R
Next, choose C
calculated from Equation 30.
C
PCB Layout Considerations
Power and Signal Layers Placement on the
PCB
As a general rule, power layers should be close together,
either on the top or bottom of the board, with signal
layers on the opposite side of the board. As an example,
layer arrangement on a 4-layer board is shown in the
following:
Separate the power voltage and current flowing path
from the control and logic level signal path. The
controller IC will stay on the signal layer, which is isolated
by the signal ground to the power signal traces.
Component Placement
The power MOSFET should be close to the IC so that the
gate drive signal, the LGATE, UGATE, PHASE, and BOOT,
traces can be short.
Place the components in such a way that the area under
the IC has less noise traces with high dv/dt and di/dt,
such as gate signals and phase node signals.
F
1. Top Layer: signal lines, or half board for signal lines
2. Signal Ground
3. Power Layers: Power Ground
4. Bottom Layer: Power MOSFET, Inductors and other
ZERO1
ZERO2
LC
POLE1
ESR
FILTER
VCOMP
VCOMP
and the other half board for power lines
Power traces
=
=
-------------------------------
(
2π L C
------------------------------------------- -
(
=
=
=
=
2π C
=
=
VCOMP
---------------------------------------- -
(
-----------------------------------------------------------------------
(
⎛
⎜
⎝
------------------------------------------ -
(
2π RS2 C
1
(
-------------------------------------------------------------------------- -
(
2π C
----------------------------------- -
2π RS2 C
⋅
2π R
0.7 F
0.3 F
⋅
R
⋅
⋅
⋅
o
VCOMP
⋅
1
o
⋅
⋅
⋅
VCOMP
)
1
VCOMP
R
F2
equal or lower than the value calculated
LC
1
LC
ESR
⋅
⋅
⋅
)
)
C
⋅
⋅
1
o
)
(
(
o
F2
2π C
)
1
2π R
⎞
⎟
⎠
equal or higher than the value
⋅
)
⋅
R
⎛
⎜
⎝
⋅
⋅
VCOMP
-------------------- -
R
4
o
VCOMP
R
+
⋅
4
RS2
R
3
)
⎞
⎟
⎠
⋅
)
)
⎛
⎝
⋅
gm1
----------- -
⎛
⎝
----------- -
gm1
5
5
⎞
⎠
⎞
⎠
⋅
⎛
⎜
⎝
R
-------------------- -
September 9, 2010
3
R
+
4
R
4
(EQ. 23)
(EQ. 24)
(EQ. 25)
(EQ. 27)
(EQ. 28)
(EQ. 29)
(EQ. 30)
(EQ. 26)
⎞
⎟
⎠
FN6978.1