MAX1535 Maxim Integrated Products, MAX1535 Datasheet - Page 36

• Manufacturer Part Number: MAX1535
• Description: Highly Integrated Level 2 Smbus Battery Charger
• Manufacturer: Maxim Integrated Products

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Highly Integrated Level 2 SMBus
Battery Charger
The input current limit should be set based on the cur-
rent capability of the AC adapter and the tolerance of
the input current limit. The upper limit of the input cur-
rent threshold should never exceed the adapter’s mini-
mum available output current. For example, if the
adapter’s output current rating is 5A ±10%, the input
current limit should be selected so that its upper limit is
less than 5A × 0.9 = 4.5A. Since the input current-limit
accuracy of the MAX1535 is ±5%, the typical value of
the input current limit should be set at 4.5A / 1.05 ≈
4.28A. The lower limit for input current must also be
considered. For chargers at the low end of the specifi-
cation, the input current limit for this example could be
4.28A × 0.95, or approximately 4.07A.
The VMAX and IMAX limits should be determined
based on the design values of the maximum current
and voltage in the battery and the accuracy of VMAX
and IMAX limits. The VMAX function is intended to be a
secondary protection mechanism. So it is not relevant
whether SMBus or VMAX controls the charger, so long
as the charge voltage is below the battery manufactur-
er’s maximum ratings. The SMBus and VMAX thresh-
olds can therefore overlap slightly because charge-
voltage accuracy is critical for safely and efficiently
charging the battery. The lower limit of the VMAX
threshold should be equal to the normal charge voltage
of 4.20V per cell. Assuming the accuracy of the VMAX
threshold is ±5%, the typical value of the VMAX limit
should be set at 4.20 / 0.95 = 4.42V per cell. For a 4-
cell battery pack, the nominal VMAX limit is 4 × 4.42V =
17.68V.
The accuracy of the charge current is not as critical as
the charge voltage. Small variations (such as ±5%) in
the absolute value of the charge current do not have a
significant effect on the total charge time. It is accept-
able to have the maximum charge current set by
SMBus and the IMAX limit overlap slightly. Assuming
the maximum charge current is 5A ±5% and the accu-
racy of the IMAX threshold is ±5%, the lower limit of the
IMAX threshold can be set equal to the upper limit of
the maximum charge current (5.25A), or slightly lower.
Therefore, the typical value of the IMAX limit should be
5.25A / 0.95 = 5.53A, or slightly lower.
36
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Setting VMAX and IMAX Limits
Setting Input Current Limit
Figure 17. Typical Smart-Battery System
Bypass DCIN with a 1µF ceramic capacitor to ground
(Figure 1). D4 protects the MAX1535 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 V
DLOV, SRC, DAC, and REF as shown in Figure 1.
Good PC board layout is required to achieve specified
noise immunity, efficiency, and stable performance.
The PC board layout artist must be given explicit
instructions—preferably a sketch showing the place-
ment of the power-switching components and high-cur-
rent routing. Refer to the PC board layout in the
MAX1535 evaluation kit for examples. A ground plane
is essential for optimum performance. In most applica-
tions, the circuit is located on a multilayer board, and
full use of the four or more copper layers is recom-
mended. Use the top layer for high-current connec-
tions, the bottom layer for quiet connections, and the
inner layers for uninterrupted ground planes.
AC LINE
SMART-BATTERY
POWER-SOURCE
CONVERTER
(ADAPTER)
CHARGER/
SELECTOR
AC-TO-DC
MAX1535
CONTROL
SIGNALS
BATTERY
SMBus
FOR
(KEYBOARD CONTROLLER)
SYSTEM HOST
BATT+
BATT-
Layout and Bypassing
DD
SUPPLIES
BATTERY
CONTROL
, DCIN, LDO, DHIV,
SYSTEM
SIGNALS
BATTERY
POWER
SMART
SMBus
FOR