MAX1870AETJ+ Maxim Integrated Products, MAX1870AETJ+ Datasheet - Page 27

• Manufacturer Part Number: MAX1870AETJ+
• Description: Battery Management Li+ Step Up/Step Down Battery Charger
• Manufacturer: Maxim Integrated Products

Specifications of MAX1870AETJ+

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Table 5 shows the resistive losses and switching losses
in each of the MOSFETs during either step-up or step-
down operation. Table 5 provides a first-order estimate,
but does not consider second-order effects such as
ripple current or nonlinear gate drive.
For typical applications where V
V
since M2 operates at a lower duty cycle. Switching loss-
es are dissipated in M1 when in step-down mode and in
M2 when in step-up mode. Ratio the MOSFETs so that
resistive losses roughly equal switching losses when at
maximum load and typical input/output conditions. The
resistive loss equations are a good approximation in
hybrid mode (V
ing losses apply in hybrid mode.
Switching losses can become a heat problem when the
maximum AC adapter voltage is applied in step-down
operation or minimum AC adapter voltage is applied
with a maximum battery voltage. This behavior occurs
because of the squared term in the CV
equation. Table 5 provides only an estimate and is not
a substitute for breadboard evaluation.
Select the inductor to minimize power dissipation in the
MOSFETs, inductor, and sense resistors. To optimize
resistive losses and RMS inductor current, set the LIR
(inductor current ripple) to 0.3. Because the maximum
resistive power loss occurs at the step-up boundary of
Table 4. Component List
INDUCTORS
P-CHANNEL MOSFETs
N/P-CHANNEL MOSFET PAIRS
N-CHANNEL MOSFETs
BATT
DESIGNATION
, the resistive losses are primarily dissipated in M1
M1/M2
M1
M2
L1
IN
near V
______________________________________________________________________________________
Sumida CDRH104R-100
Sumida CDRH104R-7R0
Sumida CDRH104R-5R2
Sumida CDRH104R-3R8
Siliconix Si4435DY
Fairchild FDC602P
Fairchild FDS4435A
Fairchild FDW256P
Fairchild FDW2520C (8-pin TSSOP)
IRF7811W
MOSFET Power Dissipation
BATT
). Both M1 and M2 switch-
Inductor Selection
BATT
PART
2
/ 2 < V
f switching-loss
IN
< 2 x
hybrid mode, select LIR for operating in this mode. Select
the inductance according to the following equation:
Larger inductance values can be used; however, they
contribute extra resistance that can reduce efficiency.
Smaller inductance values increase RMS currents and
can also reduce efficiency.
The inductor must have a saturation current rating high
enough so it does not saturate at full charge, maximum
output voltage, and minimum input voltage. In step-up
operation, the inductor carries a higher current than in
step-down operation with the same load. Calculate the
inductor saturation current rating by the following
equation:
The input capacitor must meet the ripple current
requirement (I
Nontantalum chemistries (ceramic, aluminum, or OS-
10µH, 4.4A, 35mΩ power inductor
7µH, 4.8A, 27mΩ power inductor
5.2µH, 5.5A, 22mΩ power inductor
3.8µH, 6A, 13mΩ power inductor
P-FET 35mΩ, Q
P-FET 35mΩ, Q
P-FET 25mΩ, Q
P-FET 20mΩ, Q
N-FET 18mΩ, Q
P-FET 35mΩ, Q
N-FET, 9mΩ, Q
Li+ Battery Charger
Step-Up/Step-Down
I
T x V
SAT
G
RMS
G
G
G
G
G
G
≥
= 18nC, V
= 17nC, V
= 14nC, V
= 21nC, V
= 28nC, V
= 14nC, V
= 14nC, V
L
IN MIN
V
) imposed by the switching currents.
_
=
OUT MAX
SPECIFICATIONS
2
Input-Capacitor Selection
_
x V
x
2
LIR I
DSMAX
DSMAX
DSMAX
DSMAX
DSMAX
DSMAX
DSMAX
V
⎛
⎜
⎝
x L
IN MIN
1
IN
Saturation Current Rating
_
−
x I
CHG
x t
= 30V, 8-pin SO
= 30V, 8-pin SO
= 20V, 6-pin SuperSOT
= 30V, 8-pin SO
= 30V, 8-pin TSSOP
= 20V
V
= 20V,
CHG MAX
V
OUT MAX
min
IN MIN
_
_
_
+
⎞
⎟
⎠
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