LTC4011 datasheet(13/24 Pages) LINER

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LTC4011

4011p

APPLICATIO S I FOR ATIO

External DC Source

The external DC power source should be connected to the

charging system and the VCC pin through either a power

diode or P-channel MOSFET. This prevents catastrophic

system damage in the event of an input short to ground or

reverse-voltage polarity at the DC input. The LTC4011

automatically senses when this input is present. The open-

circuit voltage of the DC source should be between 4.5V

and 34V, depending on the number of cells being charged.

In order to avoid low dropout operation, ensure 100%

capacity at charge termination, and allow reliable detec-

tion of battery insertion, removal or overvoltage, the

following equation can be used to determine the minimum

full-load voltage that should be provided by the external

DC power source.

DCIN(MIN) = (n • 2V) + 0.3V

where n is the number of series cells in the battery pack.

The LTC4011 will properly charge over a wide range of

DCIN and BAT voltage combinations. Operating the

LTC4011 in low dropout or with DCIN much greater than

BAT will force the PWM frequency to be much less than

550kHz. The LTC4011 disables charging and sets a fault if

a large DCIN to BAT differential would cause generation of

audible noise.

PowerPath Control

Proper PowerPath control is an important consideration

when fast charging nickel cells. This control ensures that

the system load remains powered at all times, but that

normal system operation and associated load transients

do not adversely affect fast charge termination. For high

efficiency and low dropout applications, the LTC4011 can

provide gate drive from the INFET pin directly to an input

P-channel MOSFET.

The battery should also be connected to the raw system

supply by a switch that selects the battery for system power

only if an external DC source is not present. Again, for

applications requiring higher efficiency, a P-channel

MOSFET with its gate driven from the DC input can be used

to perform this switching function (see Figure 8). Gate

voltage clamping may be necessary on an external PMOS

transistor used in this manner at higher input voltages.

Alternatively, a diode can be used in place of this FET.

Battery Chemistry Selection

The desired battery chemistry is selected by programming

the CHEM pin to the proper voltage. If it is wired to GND,

a set of parameters specific to charging NiMH cells is

selected. When CHEM is left floating or connected to VRT,

charging is optimized for NiCd cells. The various charging

parameters are detailed in Table 2.

Programming Charge Current

Charge current is programmed using the following

equation:

SENSE

PROG

100

RSENSE is an external resistor connected between the

SENSE and BAT pins. A 1% resistor with a low temperature

coefficient and sufficient power dissipation capability to

avoid self-heating effects is recommended.

Programming Maximum Charge Times

Connecting the appropriate resistor between the TIMER

pin and GND programs the maximum duration of various

charging states. To some degree, the value should reflect

how closely the programmed charge current matches the

1C rate of targeted battery packs. The maximum fast

charge period is determined by the following equation:

Hours

TIMER

MAX

()

•

–

Ω =

30 10 6

Some typical timing values are detailed in Table 3. RTIMER

should not be less than 15k. The actual time limits used by

the LTC4011 have a resolution of approximately ±30

seconds in addition to the tolerances given the Electrical

Characteristics table. The maximum time period is ap-

proximately 4.3 hours.

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