MAX1909ETI Maxim Integrated Products, MAX1909ETI Datasheet - Page 17

• Manufacturer Part Number: MAX1909ETI
• Description: Battery Management
• Manufacturer: Maxim Integrated Products

Specifications of MAX1909ETI

• 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

The MAX1909/MAX8725 include all of the functions
necessary to charge Li+, NiMH, and NiCd batteries. A
high-efficiency, synchronous-rectified step-down DC-
DC converter is used to implement a precision con-
stant-current, constant-voltage charger with input
current limiting. The DC-DC converter uses external
p-channel/n-channel MOSFETs as the buck switch and
synchronous rectifier to convert the input voltage to the
required charge current and voltage. The charge cur-
rent and input current-limit sense amplifiers have low-
input-referred offset errors and can use small-value
sense resistors. The MAX1909/MAX8725 feature a volt-
age-regulation loop (CCV) and two current-regulation
loops (CCI and CCS). The CCV voltage-regulation loop
monitors BATT to ensure that its voltage never exceeds
the voltage set by VCTL. The CCI battery current-regu-
lation loop monitors current delivered to BATT to ensure
that it never exceeds the current limit set by ICTL. A
third loop (CCS) takes control and reduces the charge
current when the sum of the system load and the input-
referred charge current exceeds the power source cur-
rent limit set by CLS. Tying CLS to the reference
voltage provides a 7.5A input current limit with a 10mΩ
sense resistor.
The ICTL, VCTL, and CLS analog inputs set the charge
current, charge voltage, and input current limit, respec-
tively. For standard applications, internal set points for
ICTL and VCTL provide a 3A charge current using a
15mΩ sense resistor and a 4.2V per-cell charge volt-
age. The variable for controlling the number of cells is
set with the MODE input. The MAX8725 includes a
PKPRES input used for battery-pack detection.
Based on the presence or absence of the AC adapter,
the MAX1909/MAX8725 automatically provide an open-
drain logic output signal ACOK and select the appropri-
ate source for supplying power to the system. A
p-channel load switch controlled from the PDL output and
a similar p-channel source switch controlled from the PDS
output are used to implement this function. Using the
MODE control input, the MAX1909/MAX8725 can be pro-
grammed to perform a relearning, or conditioning, cycle
in which the battery is isolated from the charger and com-
pletely discharged through the system load. When the
battery reaches 100% depth of discharge, it is recharged
to full capacity.
The circuit shown in Figure 1 demonstrates a simple
hardwired application, while Figure 2 shows a typical
application for smart-battery systems with variable
charge current and source switch configuration that sup-
ports battery conditioning. Smart-battery systems typical-
ly use a host µC to achieve this added functionality.
Multichemistry Battery Chargers with Automatic
______________________________________________________________________________________
Detailed Description
System Power Selector
The MAX1909/MAX8725 use a high-accuracy voltage
regulator for charge voltage. The VCTL input adjusts
the battery output voltage. In default mode (VCTL =
LDO), the overall accuracy of the charge voltage is
±0.5%. VCTL is allowed to vary from 0 to 3.6V, which
provides a 10% adjustment range of the battery volt-
age. Limiting the adjustment range reduces the sensi-
tivity of the charge voltage to external resistor
tolerances from ±1% to ±0.05%. The overall accuracy
of the charge voltage is better than ±1% when using
±1% resistors to divide down the reference to establish
VCTL. The per-cell battery termination voltage is a func-
tion of the battery chemistry and construction. Consult
the battery manufacturer to determine this voltage. The
battery voltage is calculated by the equation:
where V
selected with the MAX1909/MAX8725s’ trilevel MODE
control input. When MODE is tied to the LDO output,
CELL = 4. When MODE is left floating, CELL = 3. When
MODE is tied to ground, the charger enters condition-
ing mode, which is used to isolate the battery from the
charger and discharge it through the system load. See
the Conditioning Mode section. The internal error ampli-
fier (GMV) maintains voltage regulation (see Figure 3
for the Functional Diagram ). The voltage-error amplifier
is compensated at CCV. The component values shown
in Figures 1 and 2 provide suitable performance for
most applications. Individual compensation of the volt-
age regulation and current-regulation loops allow for
optimal compensation. See the Compensation section.
The voltage on the ICTL input sets the maximum
voltage across current-sense resistor RS2, which in turn
determines the charge current. The full-scale differen-
tial voltage between CSIP and CSIN is 75mV; thus, for a
0.015Ω sense resistor, the maximum charge current is
5A. In default mode (ICTL = LDO), the sense voltage is
45mV with an overall accuracy of ±5%. The charge cur-
rent is programmed with ICTL using the equation:
REF
V
BATT
= 4.2235V, and CELL is the number of cells
=
I
CELL V
CHG
Setting the Charge Voltage
Setting the Charge Current
=
⎛
⎜
⎝
0 075
REF
RS
.
2
+
×
⎛
⎜
⎝
V
V
3 6
VCTL
ICTL
.
V
9 52
.
−
1 8
.
V
⎞
⎟
⎠
⎞
⎟
⎠
17