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isl88731a
isl88731a  

Manufacturer Part Number  isl88731a 
Description  Smbus Level 2 Battery Charger 
Manufacturer  Intersil Corporation 
isl88731a datasheet 

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Low switching loss requires low draintogate charge Q
Generally, the lower the draintogate charge, the higher the
ONresistance. Therefore, there is a tradeoff between the
ONresistance and draintogate charge. Good MOSFET
selection is based on the Figure of Merit (FOM), which is a
product of the total gate charge and ONresistance. Usually,
the smaller the value of FOM, the higher the efficiency for
the same application.
For the lowside MOSFET, the worstcase power dissipation
occurs at minimum battery voltage and maximum input
voltage (Equation 10):
⎛
⎞
V
2
OUT
⋅
⋅
⎜
⎟
P
1
 
I
r
=
–
(
)
Q2
BAT
DS ON
⎝
V
⎠
IN
Choose a lowside MOSFET that has the lowest possible
ONresistance with a moderatesized package like the SO8
and is reasonably priced. The switching losses are not an
issue for the lowside MOSFET because it operates at
zerovoltageswitching.
Ensure that the required total gate drive current for the
selected MOSFETs should be less than 24mA. So, the total
gate charge for the highside and lowside MOSFETs is
limited by Equation 11:
I
GATE
≤
Q
 
GATE
f
sw
Where I
is the total gate drive current and should be
GATE
less than 24mA. Substituting I
= 24mA and f
GATE
into Equation 11 yields that the total gate charge should be
less than 80nC. Therefore, the ISL88731A easily drives the
battery charge current up to 8A.
Snubber Design
ISL88731A's buck regulator operates in discontinuous
current mode (DCM) when the load current is less than half
the peaktopeak current in the inductor. After the lowside
FET turns off, the phase voltage rings due to the high
impedance with both FETs off. This can be seen in Figure 9.
Adding a snubber (resistor in series with a capacitor) from
the phase node to ground can greatly reduce the ringing. In
some situations a snubber can improve output ripple and
regulation.
The snubber capacitor should be approximately twice the
parasitic capacitance on the phase node. This can be
estimated by operating at very low load current (100mA) and
measuring the ringing frequency.
C
and R
can be calculated from Equations 12
SNUB
SNUB
and 13:
2
 
C
=
R
SNUB
2
SNUB
(
)
⋅
2πF
L
ring
(EQ. 12)
18
ISL88731A
.
Input Capacitor Selection
gd
The input capacitor absorbs the ripple current from the
synchronous buck converter, which is given by Equation 14:
=
I
I
rms
This RMS ripple current must be smaller than the rated RMS
current in the capacitor datasheet. Nontantalum chemistries
(ceramic, aluminum, or OSCON) are preferred due to their
resistance to powerup surge currents when the ACadapter
is plugged into the battery charger. For Notebook battery
charger applications, it is recommended that ceramic
(EQ. 10)
capacitors or polymer capacitors from Sanyo be used due to
their small size and reasonable cost.
Loop Compensation Design
ISL88731A has three closed loop control modes. One
controls the output voltage when the battery is fully charged
or absent. A second controls the current into the battery
when charging and the third limits current drawn from the
adapter. The charge current and input current control loops
are compensated by a single capacitor on the ICOMP pin.
The voltage control loop is compensated by a network on the
VCOMP pin. Descriptions of these control loops and
guidelines for selecting compensation components will be
(EQ. 11)
given in the following sections. Which loop controls the
output is determined by the minimum current buffer and the
= 400kHz
minimum voltage buffer shown in the “Functional Block
s
Diagram” (see Figure 1) on page 2. These three loops will be
described separately.
Transconductance Amplifiers GMV, GMI and GMS
ISL88731A uses several transconductance amplifiers (also
known as gm amps). Most commercially available op amps
are voltage controlled voltage sources with gain expressed
as A = V
sources with gain expressed as gm = I
appear in some of the equations for poles and zeros in the
compensation.
PWM Gain F
The Pulse Width Modulator in the ISL88731A converts
voltage at VCOMP to a duty cycle by comparing VCOMP to
a triangle wave (duty = VCOMP/V
filter formed by L and C
output voltage (Vo = V
wave amplitude is proportional to V
amplitude proportional to DCIN makes the gain from
VCOMP to the PHASE output a constant 11 and is
independent of DCIN. For small signal AC analysis, the
⋅
2 L
battery is modeled by its internal resistance. The total output
=
 
C
SNUB
resistance is the sum of the sense resistor and the internal
(EQ. 13)
resistance of the MOSFETs, inductor and capacitor.
Figure20 shows the small signal model of the pulse width
modulator (PWM), power stage, output filter and battery.
(
)
−
V
V
V
OUT
IN
OUT
BAT
V
IN
/V
. gm amps are voltage controlled current
OUT
IN
/V
OUT
m
). The lowpass
PP RAMP
convert the duty cycle to a DC
O
*duty). In ISL88731A, the triangle
DCIN
. Making the ramp
DCIN
(EQ. 14)
. gm will
IN
FN6738.1
January 7, 2009
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