Installation and Operation
© 2009 Microchip Technology Inc. DS51812B-page 15
2.3.3 Voltage Proportional Charge Control (VPCC) Function
If the input voltage drops to a preset value, determined by the threshold established at
the VPCC input (1.23V at pin #2), due to a limited amount of input current or input
source impedance, the battery charging current is reduced. The VPCC control tries to
reach a steady state condition where the system load has priority and the battery is
charged with the remaining current. Therefore, if the system demands more current
that the input can provide, the MCP73871 changes its status and the battery is able to
supplement the input current to the system load.
The VPCC function sustains the system load as its highest priority. It does this by
reducing the noncritical charge current while maintaining the maximum power output
of the adapter. Further demand from the system is supported by the battery, if possible.
The VPCC feature functions are identical for USB port or AC-DC adapter inputs. This
feature can be disabled by removing resistor R
7
and replacing R
8
with a zero ohms
0603 resistor.
The demo board is designed to operate with 5.5V with +/-0.5V tolerance input voltage,
by setting accordingly the voltage on VPCC pin by R7 and R8 resistors (see
A.2 “Board – Schematic”). Additional information on the configuration of the VPCC
function is available in the MCP73871 Data Sheet.
2.3.3.1 EXAMPLE
A system is designed with a 5.0V rated DC power supply with ±0.5V tolerance. The
worst condition of 4.5V is selected, which is used to calculate the VPCC supply voltage
with divider. The voltage divider equation is shown below:
The calculated R
8
equals to 292.43 k when 110 k is selected for R
7
. The 294 k
resistor is selected for R
8
to build the voltage divider for VPCC pin.
Figure 2-4 illustrates the influence of various input voltages.
FIGURE 2-4: Input and Output Parameters when Input Voltage is Variable,
from 2.5V to 5.5V. See the Influence of VPCC Pin.
VPCC
R7
R7 R8+
-------------------- -
V
DDmin
×
=
1.23V
110k
110k R8+
------------------------- -
4.5V
×
=
R8 292.43k
Ω
=
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
3.5 4 4.5 5 5.5
Input Voltage (V)
Battery Volatge (V)
VPCC pin Voltage (V)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Input Current (A)
V
BAT
I
IN
V
VPCC
