MAX8725ETI Maxim Integrated Products, MAX8725ETI Datasheet - Page 28

• 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

Multichemistry Battery Chargers with Automatic
System Power Selector
The ripple current is determined by:
where:
or:
Figure 11 illustrates the variation of the ripple current
vs. battery voltage when the circuit is charging at 3A
with a fixed input voltage of 19V.
Higher inductor values decrease the ripple current.
Smaller inductor values require high-saturation current
capabilities and degrade efficiency. Designs that set
LIR = ΔIL / I
between inductor size and efficiency.
The input capacitor must meet the ripple current
requirement (I
Nontantalum chemistries (ceramic, aluminum, or OS-
CON) are preferred due to their resilience to power-up
surge currents.
The input capacitors should be sized so that the
temperature rise due to ripple current in continuous
conduction does not exceed approximately 10°C. The
maximum ripple current occurs at 50% duty factor or
V
application of interest does not achieve the maximum
value, size the input capacitors according to the
worst-case conditions.
The output capacitor absorbs the inductor ripple cur-
rent and must tolerate the surge current delivered from
the battery when it is initially plugged into the charger.
As such, both capacitance and ESR are important
parameters in specifying the output capacitor as a filter
and to ensure the stability of the DC-DC converter (see
the Compensation section). Beyond the stability
requirements, it is often sufficient to make sure that the
output capacitor’s ESR is much lower than the battery’s
ESR. Either tantalum or ceramic capacitors can be
used on the output. Ceramic devices are preferable
because of their good voltage ratings and resilience to
surge currents.
28
DCIN
______________________________________________________________________________________
= 2
t
OFF
I
RMS
t
OFF
✕
CHG
= 2.5µs (V
V
=
RMS
BATT
= 0.3µs for V
V
I
CHG
BATT
= 0.3 usually result in a good balance
ΔIL = V
) imposed by the switching currents.
, which equates to 0.5
Output-Capacitor Selection
⎛
⎜ ⎜
⎝
< 0.88 V
Input-Capacitor Selection
DCIN
V
BATT DCIN
BATT
BATT
- V
(
DCIN
V
t
V
OFF
BATT
DCIN
> 0.88 V
/ L
) / V
−
V
BATT
DCIN
DCIN
✕
)
I
CHG
for
⎞
⎟ ⎟
⎠
. If the
Bypass DCIN with a 1µF capacitor to ground (Figure 1).
D4 protects the MAX1909/MAX8725 when the DC
power source input is reversed. A signal diode for D4 is
adequate because DCIN only powers the LDO and the
internal reference. Bypass LDO, DHIV, DLOV, and
other pins as shown in Figure 1.
Good PC board layout is required to achieve specified
noise, efficiency, and stable performance. The PC
board layout artist must be given explicit instructions—
preferably, a sketch showing the placement of the
power-switching components and high-current routing.
Refer to the PC board layout in the MAX1909/MAX8725
evaluation kit for examples. A ground plane is essential
for optimum performance. In most applications, the cir-
cuit is located on a multilayer board, and full use of the
four or more copper layers is recommended. Use the
top layer for high-current connections, the bottom layer
for quiet connections, and the inner layers for an unin-
terrupted ground plane.
Use the following step-by-step guide:
1) Place the high-power connections first, with their
It is desirable to charge deeply discharged Li+ batter-
ies at a low rate to improve cycle life. The
MAX1909/MAX8725 automatically reduces the charge
current when the voltage per cell is below 3.1V. The
charge current-sense voltage is set to 4.5mV (I
300mA with RS2 = 15mΩ) until the battery voltage rises
above the threshold. There is approximately 300mV for
3 cell, 400mV for 4 cell of hysteresis to prevent the
charge-current magnitude from chattering between the
two values.
For the MAX8725, control the ICTL voltage to set a con-
ditioning charge rate.
grounds adjacent:
a) Minimize the current-sense resistor trace
b) Minimize ground trace lengths in the high-current
c) Minimize other trace lengths in the high-current
d) Use >5mm wide traces.
lengths, and ensure accurate current sensing
with Kelvin connections.
paths.
paths.
Startup Conditioning Charge for
Applications Information
Layout and Bypassing
Overdischarged Cells
CHG
=