isl88731a Intersil Corporation, isl88731a Datasheet - Page 18

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isl88731a

Manufacturer Part Number
isl88731a
Description
Smbus Level 2 Battery Charger
Manufacturer
Intersil Corporation
Datasheet
Low switching loss requires low drain-to-gate charge Q
Generally, the lower the drain-to-gate charge, the higher the
ON-resistance. Therefore, there is a trade-off between the
ON-resistance and drain-to-gate charge. Good MOSFET
selection is based on the Figure of Merit (FOM), which is a
product of the total gate charge and ON-resistance. Usually,
the smaller the value of FOM, the higher the efficiency for
the same application.
For the low-side MOSFET, the worst-case power dissipation
occurs at minimum battery voltage and maximum input
voltage (Equation 10):
Choose a low-side MOSFET that has the lowest possible
ON-resistance with a moderate-sized package like the SO-8
and is reasonably priced. The switching losses are not an
issue for the low-side MOSFET because it operates at
zero-voltage-switching.
Ensure that the required total gate drive current for the
selected MOSFETs should be less than 24mA. So, the total
gate charge for the high-side and low-side MOSFETs is
limited by Equation 11:
Where I
less than 24mA. Substituting I
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 peak-to-peak current in the inductor. After the low-side
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 13:
C
P
Q
SNUB
SNUB
Q2
GATE
=
=
GATE
and R
1
I
---------------- -
GATE
------------------------------------ -
(
f
2πF
sw
V
--------------- -
V
OUT
IN
is the total gate drive current and should be
SNUB
ring
2
)
2
(EQ. 12)
I
BAT
can be calculated from Equations 12
L
2
r
DS ON
18
GATE
(
R
SNUB
)
= 24mA and f
=
------------------- -
C
2 L
SNUB
s
= 400kHz
(EQ. 10)
(EQ. 13)
(EQ. 11)
gd
.
ISL88731A
I
Input Capacitor Selection
The input capacitor absorbs the ripple current from the
synchronous buck converter, which is given by Equation 14:
This RMS ripple current must be smaller than the rated RMS
current in the capacitor datasheet. Non-tantalum chemistries
(ceramic, aluminum, or OSCON) are preferred due to their
resistance to power-up surge currents when the AC-adapter
is plugged into the battery charger. For Notebook battery
charger applications, it is recommended that ceramic
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
given in the following sections. Which loop controls the
output is determined by the minimum current buffer and the
minimum voltage buffer shown in the “Functional Block
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
battery is modeled by its internal resistance. The total output
resistance is the sum of the sense resistor and the internal
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.
rms
=
I
BAT
OUT
/V
V
OUT
m
IN
. gm amps are voltage controlled current
(
V
V
IN
IN
DCIN
O
V
convert the duty cycle to a DC
OUT
*duty). In ISL88731A, the triangle
)
P-P RAMP
DCIN
OUT
. Making the ramp
/V
). The low-pass
IN
. gm will
January 7, 2009
(EQ. 14)
FN6738.1

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