MAX8731A Maxim Integrated Products, MAX8731A Datasheet - Page 28

• Manufacturer Part Number: MAX8731A
• Description: SMBus Level 2 Battery Charger
• Manufacturer: Maxim Integrated Products

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SMBus Level 2 Battery Charger
with Remote Sense
where t
calculated as follows:
I
The following is the power dissipated due to the high-
side n-channel MOSFET’s output capacitance (C
The following high-side MOSFET’s loss is due to the
reverse-recovery charge of the low-side MOSFET’s
body diode:
Ignore PD
parallel to the low-side MOSFET.
The total high-side MOSFET power dissipation is:
Switching losses in the high-side MOSFET can become
an insidious heat problem when maximum AC adapter
voltages are applied. If the high-side MOSFET chosen
for adequate R
hot when biased from V
another MOSFET with lower parasitic capacitance. For
the low-side MOSFET (N2), the worst-case power dissi-
pation always occurs at maximum input voltage:
The following additional loss occurs in the low-side
MOSFET due to the body diode conduction losses:
28
GATE
+PD
PD
+
t
TRANS
______________________________________________________________________________________
PD
PD
PD
is the peak gate-drive current.
TOTAL
PD
TRANS
QRR
QRR
CONDUCTION
SWITCHING
COSS
QRR
PD
=
(HighSide) = Q
(HighSide)
⎛
⎜
⎝
(
BDY
HighSide
I
Gsrc
is the driver’s transition time and can be
DS(ON)
1
(
(HighSide) if a Schottky diode is used
HighSide
(
Low Side
+
I
(
Gsnk
HighSide
(
1
Low Side
)
at low-battery voltages becomes
≈
⎞
⎟ ×
⎠
) ≈
PD
)
IN(MAX)
=
RR2
I
0 05
GATE
V
CONDUCTION
2
.
DCIN
Q
)
2
)
+
G
x V
×
=
×
PD
,
I
⎛
⎜
⎝
PEAK
, consider choosing
I
DCIN
1
CHG
and f
×
COSS
−
C
2
V
V
RSS
FBS
CSSP
2
×
SW
x f
0 4
×
(
HighSide
(
.
R
×
HighSide
SW
_
≈
V
DS ON
f
SW
⎞
⎟
⎠
400
x 0.5
(
kHz
RSS
)
)
)
):
The total power low-side MOSFET dissipation is:
These calculations provide an estimate and are not a sub-
stitute for breadboard evaluation, preferably including a
verification using a thermocouple mounted on the MOSFET.
The charge current, ripple, and operating frequency
(off-time) determine the inductor characteristics. For
optimum efficiency, choose the inductance according
to the following equation:
This sets the ripple current to 1/3 the charge current
and results in a good balance between inductor size
and efficiency. Higher inductor values decrease the rip-
ple current. Smaller inductor values save cost but
require higher saturation current capabilities and
degrade efficiency.
Inductor L1 must have a saturation current rating of at
least the maximum charge current plus 1/2 the ripple
current (ΔIL):
The ripple current is determined by:
where:
t
or during dropout:
OFF
PD
+
PD
= 2.5µs (V
TOTAL
V
BDY
DCIN
t
OFF
(
(
Low Side
Low Side
= 0.3µs for V
DCIN
I
SAT
ΔIL = V
L
- V
=
)
)
= I
≈
V
BATT
PD
BATT
CHG
0 3 .
BATT
CONDUCTION
BATT
×
) / V
+ (1/2) ΔIL
I
×
CHG
× t
t
OFF
Inductor Selection
OFF
DCIN
> 0.88 V
/ L
for V
(
Low Side
DCIN
BATT
< 0.88
)