MCP73831/2
6.1.1.2
Thermal Considerations
6.1.1.5
Charge Inhibit
The worst-case power dissipation in the battery
charger occurs when the input voltage is at the
maximum and the device has transitioned from the
Preconditioning mode to the Constant-Current mode.
In this case, the power dissipation is:
The current regulation set input pin (PROG) can be
used to terminate a charge at any time during the
charge cycle, as well as to initiate a charge cycle or
initiate a recharge cycle.
Placing a programming resistor from the PROG input
to VSS enables the device. Allowing the PROG input to
float or by applying a logic-high input signal, disables
the device and terminates a charge cycle. When
disabled, the device’s supply current is reduced to
25 μA, typically.
PowerDissipation = (V
– V
) × I
PTHMIN REGMAX
DDMAX
Where:
VDDMAX
IREGMAX
VPTHMIN
=
=
=
the maximum input voltage
the maximum fast charge current
6.1.1.6
Charge Status Interface
the minimum transition threshold
voltage
A status output provides information on the state of
charge. The output can be used to illuminate external
LEDs or interface to a host microcontroller. Refer to
Table 5-1 for a summary of the state of the status
output during a charge cycle.
Power dissipation with a 5V, 10% input voltage
source is:
PowerDissipation = (5.5V – 2.7V) × 550mA = 1.54W
6.2
PCB Layout Issues
This power dissipation with the battery charger in the
SOT23-5 package will cause thermal regulation to be
entered as depicted in Figure 6-3. Alternatively, the
2mm x 3mm DFN package could be utilized to reduce
charge cycle times.
For optimum voltage regulation, place the battery pack
as close as possible to the device’s VBAT and VSS
pins. This is recommended to minimize voltage drops
along the high current-carrying PCB traces.
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. Figures 6-4 and 6-5 depict a
typical layout with PCB heatsinking.
6.1.1.3
External Capacitors
The MCP73831/2 are stable with or without a battery
load. In order to maintain good AC stability in the Con-
stant-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 capac-
itance may be necessary to compensate for the
inductive nature of the battery pack.
R
LED
LED
R
PROG
V
V
SS
C
MCP73831 C
V
DD
OUT
BAT
IN
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 output
currents up to a 500 mA.
FIGURE 6-4:
Typical Layout (Top).
V
SS
V
V
6.1.1.4
Reverse-Blocking Protection
BAT
DD
The MCP73831/2 provide protection from a faulted or
shorted input. Without the protection, a faulted or
shorted input would discharge the battery pack
through the body diode of the internal pass transistor.
FIGURE 6-5:
Typical Layout (Bottom).
DS21984B-page 14
© 2006 Microchip Technology Inc.
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