Vishay AN817 Data Sheet

查询AN817供应商

AN817 Vishay Siliconix

A Discrete Approach to Battery Charging for Cellular Phones

Guy Moxey and Michael Speed

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

All portable cordless appliances must receive power from an

significant advantages. However, in an integrated package

external source, whether it’s a wall cube adapter, car charger,

both components operate in a highly dissipative manner,

or docking station. This external source will then charge, in a

making the choice of package a critical decision.

predetermined fashion, the equipment’s internal battery.

The LITTLE FOOT Plus Schottky diodes come in a variety of

In the case of a portable phone, the power management

packages, with a range of r DS(on) values. Just as important to

system will incorporate charging control circuitry to regulate

the performance of the charger as r DS(on) values are the

the voltage supplied to the battery from the external charger.

thermal ratings of the packages. From the table below we can

External charging equipment—whether wall cubes or

see the choices of Rth JA values available in today’s

chargers that utilize car cigarette lighters—will supply a

industry-standard surface-mount packages.

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,

Device Rth JA (C/W) Typical

controlled via the onboard power management ASIC or

system microprocessor. Integration of these two discrete SO-8 —Si4833DY

power components into a single power package, such as the TSSOP-8 — Si6923DQ Chip FET
, reduces size and simplifies the assembly.

TSOP-6 — Si3853DV 1206-8 Chip FET — Si5853DC

Charger

LITTLE FOOT Plus

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

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 FIGURE 1. LITTLE FOOT Plus
Schottky—The Integrated constant-voltage mode. For constant-voltage operation, the

Solution of MOSFET and Schottky in One Power controller will terminate the MOSFET linear operation and Package

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 Regardless of the charging device selected, the designer is to prevent reverse current flow through the MOSFET’s body still bound by space, cost, and efficiency considerations. drain diode when the external power source is unplugged or There is therefore an obvious desire to increase levels of unpowered. Using separate MOSFETs and Schottkys rather integration and reduce the component count and board size. than an integrated package consumes valuable board space.

Document Number: 71395 www.vishay.com 22-Jan-01