powerZONE Products for the week of September 1, 2003
Linear Technology Says . . .
LTC4006: Low Pin Count Li-Ion Battery Charger IC Needs
No Microprocessor
Linear Technology Corporation introduced the LTC4006, a 2- to 4-cell Li-Ion battery charger with up to 4A charge capability that includes essential I/O status signals required for constructing a simple autonomous high power battery charger system. The LTC4006 is housed in a narrow 16-pin SSOP package and offers output signals for end-of-charge and wall adapter detection, as well as charge current value indicator for gas-gauging. The IC's synchronous DC/DC step-down operation allows up to 96% operating efficiency from supplies from 6V to 28V supplies. Moreover, the LTC4006 can operate at up to 98% duty factor, which permits 0.5V dropout operation and allows charging from low voltage wall adapters. To improve safety during charging, the LTC4006 features a battery temperature monitor as well as overvoltage and overcurrent protection. End applications include memory backup systems, portable medical and automotive diagnostics, GPS, as well as notepad and notebook PCs.
The LTC4006 features a quasi-constant frequency, constant off-time architecture
that does not generate audible noise even when using ceramic capacitors.
The IC is offered in three versions for 8.4V (2-cell), 12.6V (3-cell) and
16.8V (4-cell) float voltages with ±0.8% accuracy. Charging current
is programmable with a resistor to ±4% typical accuracy. For gas
gauging, a proprietary feature represents the charging current as a voltage
to a microcontroller or an A/D converter. A resistor sets total charge time
from 1 to 3 hours. The LTC4006 includes automatic recharge and trickle charging.
analogZONE Says...
Linear released an earlier Li-Ion battery charger, the LTC4007, in April 2003. I didn't write it up because I thought it was just too complex to get readily adopted. It had indicator outputs like "low battery" -- which is like, of course, that's why it's charging -- and input current limiting -- which seemed only to focus on the attributes of an LTC Patent. In the LTC4006, the company has now combined the ACP (ac adapter present) and Shutdown pins into one and has reduced the package to an SSOP-16 from an SSOP-24, as well as reducing the price a bit. The part is now available in a version suitable for 2-cell applications as well as the previous 3- or 4-cell applications. The simplifications and package reduction will now put the part in a prime position in the market.
I'm tougher on chargers than I used to be because I am suffering from the results of a really cutesy charger in a laptop. The vendor (not LTC) managed to complicate things so much that it ignored a couple of the basics and this particular laptop goes into a condition where it stubbornly refuses to charge to more than 45% of capacity. So far I have managed to fool it into fully charging, a process that requires considerable patience.
The LTC4006 operates from a supply from 6 V up to 28 V and includes the vitally important protection against over-temperature (using an external NTC thermistor) in addition to over-voltage and over-current protection. There are three versions for 8.4 V, 12.6 V and 16.8 V with an accuracy of ±0.8%. Current accuracy of the IC itself (not the current sense resistor) is ±5% and the parts can drive over 4 A out. The LTC4006 is a synchronous current-mode PWM buck controller with an oscillator running at a nominal 300 kHz, dropping to 25 kHz in drop out; the maximum duty cycle is 99%.
Unique to Linear, if the cell voltages fall below 2.5 V at the start of the charge cycle (deeply discharged) the charger will trickle charge at a rate of 10% of the maximum programmed current. If the cell remains deeply discharged for more than 25% of the charge time the sequence is terminated and a high put on the charge pin. The charge time is set by an external resistor, which would be typically set at 2 hours.
With a dropout voltage of about 0.5 V the parts offer efficiencies in the high 90s%. Linear will find a lot of sockets with these parts in portable products and standalone chargers.
The LTC4006 is in production in TSSOP-16 priced at $3.60 in 1000-piece lots.