
Principles of Analog Electronics
526
3.6 V. So we have to reduce the voltage but not the
current, typically 100 mA for a USB socket, otherwise
the battery will not be charged. The point is that an
ad-hoc circuit is necessary to reduce the voltage, and
to maintain it at a constant value, without reducing
the current. It cannot be made by a simple resistor
voltage divider, otherwise the current will heat up the
resistors rather than charge the battery.
The circuit shown in Figure 15.7 can solve this.
The diodes have a negative temperature coefcient,
that is, their voltage drop decreases increasing the
temperature, typically
mV C
for silicon ...