Datasheet

MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
28 ______________________________________________________________________________________
where t
TRANS
is the driver’s transition time and can be
calculated as follows:
I
GATE
is the peak gate-drive current.
The following is the power dissipated due to the high-
side n-channel MOSFET’s output capacitance (C
RSS
):
The following high-side MOSFET’s loss is due to the
reverse-recovery charge of the low-side MOSFET’s
body diode:
PD
QRR
(HighSide) = Q
RR2
x V
DCIN
x f
SW
x 0.5
Ignore PD
QRR
(HighSide) if a Schottky diode is used
parallel to the low-side MOSFET.
The total high-side MOSFET power dissipation is:
+PD
QRR
(HighSide)
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
DS(ON)
at low-battery voltages becomes
hot when biased from V
IN(MAX)
, consider choosing
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:
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.
Inductor Selection
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):
I
SAT
= I
CHG
+ (1/2) ΔIL
The ripple current is determined by:
ΔIL = V
BATT
× t
OFF
/ L
where:
t
OFF
= 2.5µs (V
DCIN
- V
BATT
) / V
DCIN
for V
BATT
< 0.88
V
DCIN
or during dropout:
t
OFF
= 0.3µs for V
BATT
> 0.88 V
DCIN
L
Vt
I
BATT OFF
CHG
=
×
×03.
PD Low Side PD LowSide
PD Low Side
TOTAL CONDUCTION
BDY
() ()
()
≈
+
PD Low Side I V
BDY PEAK
(). .=× ×005 04
PD Low Side
V
V
IR
CONDUCTION
FBS
CSSP
CHG
DS ON
()
_
()
=−
⎛
⎝
⎜
⎞
⎠
⎟
××
1
2
PD HighSide PD HighSide
PD HighSide PD HighSide
TOTAL CONDUCTION
SWITCHING COSS
() ()
() ()
≈
++
PD HighSide
VCf
COSS
DCIN
RSS SW
()≈
××
2
2
t
II
Q
I
and f kHz
TRANS
Gsrc Gsnk
G
GATE
SW
=+
⎛
⎝
⎜
⎞
⎠
⎟
×≈
112
400,