AN817 Datasheet, PDF (1/5 Pages) Vishay Siliconix – A Discrete Approach to Battery Charging for Cellular Phones

AN817
Vishay Siliconix
A Discrete Approach to Battery Charging for Cellular Phones
Guy Moxey and Michael Speed
INTRODUCTION
All portable cordless appliances must receive power from an
external source, whether it’s a wall cube adapter, car charger,
or docking station. This external source will then charge, in a
predetermined fashion, the equipment’s internal battery.
In the case of a portable phone, the power management
system will incorporate charging control circuitry to regulate
the voltage supplied to the battery from the external charger.
External charging equipment—whether wall cubes or
chargers that utilize car cigarette lighters—will supply a
continuous but unregulated voltage to the phone, typically
4.2 V for a single Lithium-ion (Li+) cell. A typical charging
design is explored in Appendix A.
Charge control for a Li+ cell is most commonly implemented
by a discrete MOSFET in series with a Schottky diode,
controlled via the onboard power management ASIC or
system microprocessor. Integration of these two discrete
power components into a single power package, such as the
ChipFETt, reduces size and simplifies the assembly.
Charger
+
–
LITTLE FOOT Plust
PA
Battery
+
–
FIGURE 1.
LITTLE FOOT Plust Schottky—The Integrated
Solution of MOSFET and Schottky in One Power
Package
Regardless of the charging device selected, the designer is
still bound by space, cost, and efficiency considerations.
There is therefore an obvious desire to increase levels of
integration and reduce the component count and board size.
Document Number: 71395
22-Jan-01
To this end, moving away from a separate Schottky diode and
MOSFET to the single package integration of both devices, as
in the Vishay Siliconix LITTLE FOOT Plus^TM, may have
significant advantages. However, in an integrated package
both components operate in a highly dissipative manner,
making the choice of package a critical decision.
The LITTLE FOOT Plus Schottky diodes come in a variety of
packages, with a range of r_DS(on) values. Just as important to
the performance of the charger as r_DS(on) values are the
thermal ratings of the packages. From the table below we can
see the choices of R_thJA values available in today’s
industry-standard surface-mount packages.
TABLE 1.
LITTLE FOOT Plus PACKAGE OPTIONS
Device
SO-8 —Si4833DY
TSSOP-8 — Si6923DQ
TSOP-6 — Si3853DV
1206-8 ChipFET — Si5853DC
R_thJA (_C/W) Typical
90
115
130
90
POWER DISSIPATION ISSUES
To select the correct part in the smallest package, the power
dissipated by the two power devices must be examined. In the
case of the charger switch, there are two modes of operation
to consider.
In the first phase of charging, constant current is used and the
MOSFET is operated in the linear mode. In this mode the
device is effectively a variable resistor used to regulate the
battery charging current.
Once the battery has charged to the predetermined 4.1-V
level, the system voltage loop will begin to reduce the charging
current in order to maintain the desired float voltage, hence the
constant-voltage mode. For constant-voltage operation, the
controller will terminate the MOSFET linear operation and
revert to a pulse width modulation (PWM) mode. The
MOSFET is driven as a fully-saturated (Ohmic) switch.
The Schottky diode is always required in series with the switch
to prevent reverse current flow through the MOSFET’s body
drain diode when the external power source is unplugged or
unpowered. Using separate MOSFETs and Schottkys rather
than an integrated package consumes valuable board space.
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