LTC4010 datasheet(14/24 Pages) LINER

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LTC4010

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External DC Source

The external DC power source should be connected to the

charging system and the VCC pin through a power diode

acting as an input rectifier. This prevents catastrophic

system damage in the event of an input short to ground

or reverse-voltage polarity at the DC input. The LTC4010

automatically senses when this input drives the VCC pin

above BAT. The open-circuit voltage of the DC source

should be between 5.5V and 34V, depending on the num-

ber of cells being charged. In order to avoid low dropout

operation, ensure 100% capacity at charge termination,

and allow reliable detection of battery insertion, removal

or overvoltage, the following equation can be used to

determine the minimum full-load voltage that should be

produced at VCC when the external DC power source is

connected.

VCC(MIN) = (n • 2V) + 0.3V

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

The LTC4010 will properly charge over a wide range of VCC

and BAT voltage combinations. Operating the LTC4010 in

low dropout or with VCC much greater than BAT will force

the PWM frequency to be much less than 550kHz. The

LTC4010 disables charging and sets a fault if a large VCC to

BAT differential would cause generation of audible noise.

Load Control

Proper load current 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. The input

protecton detailed in the previous paragraph is an integral

part of the necessary load control.

The battery should also be connected to the raw system

supply by some rectifying means, thus forming a switch

that selects the battery for system power only if an external

DC source is not present.

Battery Chemistry Selection

The desired battery chemistry is selected by program-

ming the CHEM pin to the proper voltage. If it is wired

to GND, a set of parameters specific to charging NiMH

applicaTions inForMaTion

cells is selected. When CHEM is left floating, 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

SENSEandBATpins.A1%resistorwithalowtemperature

coefficient and sufficient power dissipation capability to

avoid self-heating effects is recommended. Charge rate

should be between approximately C/2 and 2C.

Inductor Value Selection

Formanyapplications,10µHrepresentsanoptimumvalue

for the inductor the PWM uses to generate charge current.

For applications with IPROG of 1.5A or greater running

from an external DC source of 15V or less, values between

5µH and 7.5µH can often be selected. For wider operating

conditions the following equation can be used as a guide

for selecting the minimum inductor value.

L > 6.5 • 10–6 • VDCIN • RSENSE, L ≥ 4.7µH

Actualpartselectionshouldaccountforbothmanufacturing

tolerance and temperature coefficient to ensure this mini-

mum. A good initial selection can be made by multiplying

the calculated minimum by 1.4 and rounding up or down

to the nearest standard inductance value.

Ultimately, there is no substitute for bench evaluation of

the selected inductor in the target application, which can

also be affected by other environmental factors such as

ambient operating temperature. Using inductor values

lower than recommended by the equation shown above

can result in a fault condition at the start of precharge or

top-off charge.

Programming Maximum Charge Times

Connecting the appropriate resistor between the TIMER

pin and GND programs the maximum duration of various

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