MAX8731 Maxim Integrated Products, MAX8731 Datasheet - Page 28

• Manufacturer Part Number: MAX8731
• Description: SMBus Level 2 Battery Charger
• Manufacturer: Maxim Integrated Products

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 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 reverse-recovery charge of the
MOSFET’s body diode and the body diode conduction
losses:
The total power low-side MOSFET dissipation is:
28
GATE
PD
PD
+
PD
QRR
+
t
TRANS
______________________________________________________________________________________
PD
PD
TOTAL
PD
is the peak gate-drive current.
TOTAL
PD
(
TRANS
Low Side
QRR
CONDUCTION
SWITCHING
COSS
=
(
Low Side
(
⎛
⎜
⎝
(
HighSide
HighSide
I
Gsrc
is the driver’s transition time and can be
DS(ON)
1
(
)
HighSide
=
Q
+
(
RR
I
Gsnk
HighSide
)
(
1
)
Low Side
2
≈
)
at low-battery voltages becomes
≈
×
PD
⎞
⎟ ×
⎠
V
) ≈
PD
IN(MAX)
DCIN
CONDUCTION
I
GATE
V
CONDUCTION
2
Q
DCIN
2
)
)
+
×
G
×
=
PD
f
SW
,
⎛
⎜
⎝
, consider choosing
I
1
CHG
×
and f
COSS
−
+
C
2
( .
V
V
RSS
0 05
CSSP
FBS
2
SW
(
×
Low Side
(
(
HighSide
HighSide
R
×
×
_
≈
DS ON
I
f
PEAK
SW
⎞
⎟
⎠
400
(
kHz
RSS
×
)
)
0 4
)
)
.
):
V
)
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 (∆IL):
The ripple current is determined by:
where:
t
or during dropout:
OFF
= 2.5µs (V
V
DCIN
t
OFF
= 0.3µs for V
DCIN
I
∆IL = V
SAT
L
- V
=
= I
V
BATT
BATT
CHG
BATT
0 3 .
BATT
×
) / V
+ (1/2) ∆IL
I
×
× t
CHG
t
OFF
Inductor Selection
OFF
DCIN
> 0.88 V
/ L
for V
DCIN
BATT
< 0.88