MAX8725ETI Maxim Integrated Products, MAX8725ETI Datasheet - Page 27

• Manufacturer Part Number: MAX8725ETI
• Description: Battery Management Multichemistry Battery Charger
• Manufacturer: Maxim Integrated Products

Specifications of MAX8725ETI

• Product: Charge Management
• Battery Type: Li-Ion, Li-Polymer, NiCd, NiMH, Lead Acid, Universal
• Operating Supply Voltage: 8 V to 28 V
• Supply Current: 2.7 mA
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Package / Case: TQFN-28
• Charge Safety Timers: No
• Mounting Style: SMD/SMT
• Temperature Monitoring: No
• Uvlo Start Threshold: 9.18 V
• Uvlo Stop Threshold: 9.42 V
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Choose a low-side MOSFET that has the lowest possi-
ble on-resistance (R
sized package, and is reasonably priced. Make sure
that the DLO gate driver can supply sufficient current to
support the gate charge and the current injected into
the parasitic gate-to-drain capacitor caused by the
high-side MOSFET turning on; otherwise, cross-con-
duction problems can occur.
The MAX1909/MAX8725 have an adaptive dead-time cir-
cuit that prevents the high-side and low-side MOSFETs
from conducting at the same time (see the MOSFET
Drivers section). Even with this protection, it is still possi-
ble for delays internal to the MOSFET to prevent one
MOSFET from turning off when the other is turned on.
Select devices that have low turn-off times. To be
conservative, make sure that P1(t
N1(t
efficiency-killing shoot-through currents. If delay mis-
match causes shoot-through currents, consider adding
extra capacitance from gate to source on N1 to slow
down its turn-on time.
Worst-case conduction losses occur at the duty factor
extremes. For the high-side MOSFET, the worst-case
power dissipation (PD) due to resistance occurs at the
minimum supply voltage:
Generally, a small high-side MOSFET is desired to
reduce switching losses at high input voltages.
However, the R
power-dissipation limits often limits how small the
MOSFET can be. The optimum occurs when the switch-
ing (AC) losses equal the conduction (I
losses. High-side switching losses do not usually
become an issue until the input is greater than approxi-
mately 15V. Switching losses in the high-side MOSFET
can become an insidious heat problem when maximum
AC adapter voltages are applied, due to the squared
term in the CV
side MOSFET that was chosen for adequate R
low supply voltages becomes extraordinarily hot when
subjected to V
lower losses. Calculating the power dissipation in P1
due to switching losses is difficult since it must allow for
difficult quantifying factors that influence the turn-on
and turn-off times. These factors include the internal
gate resistance, gate charge, threshold voltage, source
inductance, and PC board layout characteristics. The
Multichemistry Battery Chargers with Automatic
DON(MIN)
PD P
( )
) < 40ns. Failure to do so may result in
1
DCIN(MAX),
2
DS(ON)
=
f switching-loss equation. If the high-
⎛
⎜
⎝
______________________________________________________________________________________
V
V
MOSFET Power Dissipation
BATT
DCIN
DS(ON)
required to stay within package
⎞
⎟
⎠
then choose a MOSFET with
⎛
⎜
⎝
I
LOAD
), comes in a moderate-
2
⎞
⎟
⎠
2
×
R
DOFF(MAX)
DS ON
(
2
R
DS(ON)
)
DS(ON)
) -
at
)
System Power Selector
following switching-loss calculation provides only a very
rough estimate and is no substitute for breadboard
evaluation, preferably including a verification using a
thermocouple mounted on P1:
where C
and I
For the low-side MOSFET (N1), the worst-case power
dissipation always occurs at maximum input voltage:
Choose a Schottky diode (D1, Figure 2) with a forward
voltage low enough to prevent the N1 MOSFET body
diode from turning on during the dead time. As a gen-
eral rule, a diode with a DC current rating equal to 1/3rd
the load current is sufficient. This diode is optional and
can be removed if efficiency is not critical.
The charge current, ripple, and operating frequency
(off-time) determine the inductor characteristics.
Inductor L1 must have a saturation current rating of at
least the maximum charge current plus 1/2 of the ripple
current (ΔIL):
Figure 11. Ripple Current vs. Battery Voltage (MAX1909)
PD P
( _
GATE
1
PD N
Switching
RSS
( )
is the peak gate-drive source/sink current.
1
1.5
1.0
0.5
0
=
is the reverse transfer capacitance of P1,
8
⎡
⎢
⎢
⎣
V
VCTL = ICTL = LDO
1
DCIN
I
−
9
SAT
)
⎛
⎜
⎝
= 19V
=
V
V
10 11 12 13
BATT
DCIN
V
= I
DCIN MAX
3 CELLS
CHG
⎞
⎟
⎠
V
(
⎤
⎥
⎥
⎦
BATT
⎛
⎜
⎝
+ (1/2) ΔIL
I
LOAD
(V)
)
14 15 16 17 18
Inductor Selection
2
2
4 CELLS
2
×
C
I
⎞
⎟ ×
⎠
GATE
RSS
2
R
×
DS ON
f
SW
(
×
)
I
LOAD
27