MAX1757EAI+ Maxim Integrated Products, MAX1757EAI+ Datasheet - Page 15

• Manufacturer Part Number: MAX1757EAI+
• Description: IC BATTERY CHRG LI+ 28-SSOP
• Manufacturer: Maxim Integrated Products

Specifications of MAX1757EAI+

• Function: Charge Management
• Battery Type: Lithium-Ion (Li-Ion)
• Voltage - Supply: 6 V ~ 14 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Product: Charge Management
• Operating Supply Voltage: 6 V to 14 V
• Supply Current: 5 mA
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Charge Safety Timers: Yes
• Mounting Style: SMD/SMT
• Temperature Monitoring: Yes
• Uvlo Start Threshold: 2.4 V
• Uvlo Stop Threshold: 2.6 V
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

CELL is the programming input for selecting cell count
N. Table 2 shows how CELL is connected to charge 1,
2, or 3 cells.
A resistor-divider from REF to GND sets the voltage at
ISETOUT (V
rent during the current-regulation (fast-charge) mode.
The full-scale charging current is 1.5A.
The charging current (I
Connect ISETOUT to REF to get the full-scale current
limit.
A resistor-divider from REF to GND sets the voltage at
ISEVTIN (V
rent allowed at any time during charging. The source
current I
R
source current is I
The input current limit (I
Connect ISETIN to REF to get the full-scale input cur-
rent limit. Short CSSP and CSSN if the input source cur-
rent limit is not used.
In choosing the current-sense resistor, it should be noted
that the drop across this resistor adds to the power loss
and thus reduces efficiency. However, too low a resistor
value may degrade input current-limit accuracy.
The inductor value may be changed for more or less
ripple current. The higher the inductance, the lower the
ripple current will be; however, as the physical size is
kept the same, typically, higher inductance will result in
higher series resistance and lower saturation current. A
good tradeoff is to choose the inductor so that the rip-
ple current is approximately 30% to 50% of the DC
average charging current. The ratio of ripple current to
DC charging current (LIR) can be used to calculate the
optimal inductor value:
SOURCE
L
Li+ Battery Charger with Internal 14V Switch
=
FSS
Setting the Charging Current Limit
between CSSP and CSSN. The full-scale
ISETIN
V
ISETOUT
I
DCIN MAX
CHG
is set by the current-sense resistor
V
Setting the Input Current Limit
I
IN
BATT
). This sets the maximum source cur-
(
FSS
______________________________________________________________________________________
=
). This determines the charging cur-
=
1 5
(
= 0.1V / R1 (Figure 1).
)
I
V
.
FSS
CHG
IN
DCIN MAX
A
x f
) is therefore:






) is, therefore:
OSC
V
V
(
ISETOUT
ISETIN
V
V
REF
REF
Inductor Selection
x
)
−
I



CHG



V
BATT
x LIR
)
Stand-Alone, Switch-Mode
where f
The peak inductor current is given by:
The input capacitor shunts the switching current from
the charger input and prevents that current from circu-
lating through the source, typically an AC wall cube.
Thus, the input capacitor must be able to handle the
input RMS current. Typically, at high charging currents,
the converter will operate in continuous conduction (the
inductor current does not go to 0). In this case, the
RMS current of the input capacitor may be approximat-
ed by the equation:
where:
The maximum RMS input current occurs at 50% duty
cycle; thus, the worst-case input ripple current is 0.5
I
PWM controller will never work at 50% duty cycle, then
the worst-case capacitor current will occur where the
duty cycle is nearest 50%.
The input capacitor impedance is critical to preventing
AC currents from flowing back into the wall cube. This
requirement varies depending on the wall cube imped-
ance and the requirements of any conducted or radiat-
ed EMI specifications that must be met. Aluminum
electrolytic capacitors are generally the cheapest, but
usually are a poor choice for portable devices due to
their large size and poor equivalent series resistance
(ESR). Tantalum capacitors are better in most cases, as
are high-value ceramic capacitors. For equivalent size
and voltage rating, tantalum capacitors will have higher
capacitance, but also higher ESR than ceramic capaci-
tors. This makes it more critical to consider RMS cur-
rent and power dissipation ratings when using tantalum
capacitors.
The output filter capacitor is used to absorb the induc-
tor ripple current. The output capacitor impedance
must be significantly less than that of the battery to
ensure that it will absorb the ripple current. Both the
CHG
I
D is the PWM converter duty ratio
(typically V
I
CIN
CHG
. If the input-to-output voltage ratio is such that the
is the input capacitor RMS current.
OSC
is the battery charging current.
is the switching frequency (300kHz).
I
BATT
PEAK
I
CIN
/ V
=
≅
DCIN
I
ISETOUT
I
CHG
).
Capacitor Selection



D
1
−
+
D
LIR
2
2



15
✕