MCP73861/2/3/4
Power dissipation with a 5V, ±10% input voltage source
is:
6.2
PCB Layout Issues
For optimum voltage regulation, place the battery pack
as close as possible to the device’s VBAT and VSS pins,
recommended to minimize voltage drops along the
high current-carrying PCB traces.
PowerDissipation = (5.5V – 2.7V) × 575mA = 1.61W
With the battery charger mounted on a 1 in2 pad of
1 oz. copper, the junction temperature rise is 60°C,
approximately. This would allow for a maximum operat-
ing ambient temperature of 50°C before thermal
regulation is entered.
If the PCB layout is used as a heatsink, adding many
vias in the heatsink pad can help conduct more heat to
the backplane of the PCB, thus reducing the maximum
junction temperature.
6.1.1.3
External Capacitors
The MCP7386X is stable with or without a battery load.
In order to maintain good AC stability in the Constant-
voltage mode, a minimum capacitance of 4.7 µF is
recommended to bypass the VBAT pin to VSS. This
capacitance provides compensation when there is no
battery load. In addition, the battery and interconnec-
tions appear inductive at high frequencies. These
elements are in the control feedback loop during
Constant-voltage mode. Therefore, the bypass capaci-
tance may be necessary to compensate for the
inductive nature of the battery pack.
Virtually any good quality output filter capacitor can be
used, independent of the capacitor’s minimum
Effective Series Resistance (ESR) value. The actual
value of the capacitor (and its associated ESR)
depends on the output load current. A 4.7 µF ceramic,
tantalum or aluminum electrolytic capacitor at the
output is usually sufficient to ensure stability for up to a
1A output current.
6.1.1.4
Reverse-Blocking Protection
The MCP7386X provides protection from a faulted or
shorted input, or from a reversed-polarity input source.
Without the protection, a faulted or shorted input would
discharge the battery pack through the body diode of
the internal pass transistor.
6.1.1.5
Enable Interface
In the stand-alone configuration, the enable pin is
generally tied to the input voltage. The MCP7386X
automatically enters a Low-power mode when voltage
on the VDD input falls below the UVLO voltage (VSTOP),
reducing the battery drain current to 0.23 µA, typically.
6.1.1.6
Charge Status Interface
Two status outputs provide information on the state of
charge. The current-limited, open-drain outputs can be
used to illuminate external LEDs. Refer to Table 5-1 for
a summary of the state of the status outputs during a
charge cycle.
© 2005 Microchip Technology Inc.
DS21893C-page 19
MICROCHIP [ MICROCHIP ]
