Datasheet
MCP73833/4
DS22005B-page 20 © 2009 Microchip Technology Inc.
6.1.1.2 Thermal Considerations
The worst-case power dissipation in the battery char-
ger 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:
Power dissipation with a 5V, ±10% input voltage source
is:
This power dissipation with the battery charger in the
MSOP-10 package will cause thermal regulation to be
entered as depicted in Figure 6-3. Alternatively, the
DFN-10 (3 mm x 3 mm) package could be utilized to
reduce charge cycle times.
6.1.1.3 External Capacitors
The MCP73833/4 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 V
BAT
pin to V
SS
.
This capacitance provides compensation when there is
no battery load. In addition, the battery and
interconnections appear inductive at high frequencies.
These elements are in the control feedback loop during
Constant-voltage mode. Therefore, the bypass
capacitance 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 output
currents up to a 500 mA.
6.1.1.4 Reverse-Blocking Protection
The MCP73833/4 provides 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.
6.1.1.5 Charge Inhibit
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
V
SS
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
100 µA, typically.
6.1.1.6 Temperature Monitoring
The charge temperature window can be set by placing
fixed value resistors in series-parallel with a thermistor.
The resistance values of R
T1
and R
T2
can be
calculated with the following equations in order to set
the temperature window of interest.
For NTC thermistors:
For example, by utilizing a 10 kΩ at 25C NTC
thermistor with a sensitivity index, β, of 3892, the
charge temperature range can be set to 0C - 50C by
placing a 1.54 kΩ resistor in series (R
T1
), and a
69.8 kΩ resistor in parallel (R
T2
) with the thermistor as
depicted in Figure 6-1.
6.1.1.7 Charge Status Interface
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.
PowerDissipation V
DDMAX
V
PTHMIN
–()I
REGMAX
×=
Where:
V
DDMAX
= the maximum input voltage
I
REGMAX
= the maximum fast charge current
V
PTHMIN
= the minimum transition threshold
voltage
PowerDissipation 5.5V 2.7V–()550mA× 1.54W==
24k
Ω
R
T1
R
T2
R
COLD
×
R
T2
R
COLD
+
---------------------------------+=
5k
Ω
R
T1
R
T2
R
HOT
×
R
T2
R
HOT
+
-----------------------------+=
Where:
R
T1
= the fixed series resistance
R
T2
= the fixed parallel resistance
R
COLD
= the thermistor resistance at the
lower temperature of interest
R
HOT
= the thermistor resistance at the
upper temperature of interest