LT3651 Просмотр технического описания (PDF) - Linear Technology

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LT3651

Monolithic 4A Wide Input 1 Cell Li-Ion Battery Charger

Linear Technology 

LT3651-4.1/LT3651-4.2

OPERATION

setting the maximum charge current. Manipulation of

maximum charge current is possible through the RNG/SS

and ILIM pins (see the RNG/SS: Dynamic Charge Current

Adjust, RNG/SS: Soft-Start and ILIM Control sections).

If the voltage on the BAT pin (VBAT) is below VBAT(PRE),

A7 initiates the precondition mode. During the pre-

condition interval, the charger continues to operate in

constant current mode, but the ITH clamp is reduced to

0.15V reducing charge current to 15% of the maximum

programmed value.

As VBAT approaches the float voltage (VFLOAT) the voltage

error amp V-EA takes control of ITH and the charger transi-

tions into constant voltage (CV) mode. As this occurs, the

ITH voltage falls from the limit clamp and charge current is

reduced from the maximum value. When the ITH voltage

falls below 0.1V, A8 signals C/10. If the charger is config-

ured for C/10 termination the charge cycle is terminated.

Once the charge cycle is terminated, the CHRG status

pin becomes high impedance and the charger enters low

current standby mode.

The LT3651 contains an internal charge cycle timer that

terminates a successful charge cycle after a programmed

amount of time. This timer is typically programmed to

achieve end-of-cycle in three hours, but can be configured

for any amount of time by setting an appropriate timing

capacitor value (CTIMER). When timer termination is used,

the charge cycle does not terminate after C/10 is achieved.

Because the CHRG status pin responds to the C/10 current

level, the IC will indicate a fully charged battery status,

but the charger will continue to source low currents. At

the programmed end of the cycle time the charge cycle

stops and the part enters standby mode. If the battery

did not achieve at least 97.5% of the full float voltage at

the end-of-cycle, charging is deemed unsuccessful and

another full-timer cycle is initiated.

Use of the timer function also enables bad battery detec-

tion. This fault condition is achieved if the battery does

not respond to preconditioning and the charger remains

in (or enters) precondition mode after one-eighth of the

programmed charge cycle time. A bad battery fault halts

the charging cycle, the CHRG status pin goes high imped-

ance and the FAULT pin is pulled low.

When the LT3651 terminates a charging cycle, whether

through C/10 detection or by reaching timer end-of-cycle,

the average current mode analog loop remains active but

the internal float voltage reference is reduced by 2.5%.

Because the voltage on a successfully charged battery is

at the full float voltage, the voltage error amp detects an

overvoltage condition and rails low. When the voltage error

amp output drops below 0.3V, the IC enters standby mode,

where most of the internal circuitry is disabled and the

VIN bias current is reduced to <100µA. When the voltage

on the BAT pin drops below the reduced float reference

level, the output of the voltage error amp will climb, at

which point the IC comes out of standby mode and a new

charging cycle is initiated.

The system current limit allows charge current to be

reduced in order to maintain a constant input current.

Input current is measured via a resistor (RCL) that is

placed between the CLP and CLN pins. Power is applied

through this resistor and is used to supply both VIN of the

chip and other system loads. An offset produced on the

inputs of A12 sets the threshold. When that threshold is

achieved, ITH is reduced, lowering the charge current thus

maintaining the maximum input current.

50µA of current is sourced from ILIM to a resistor (RILIM)

that is placed from that pin to ground. The voltage on ILIM

determines the regulating voltage across RCL. 1V on ILIM

corresponds to 95mV across RCL. The ILIM pin clamps

internally to 1V maximum.

If the junction temperature of the die becomes excessive,

A10 activates decreasing ITH and reduces charge current.

This reduces on-chip power dissipation to safe levels but

continues charging.

For more information www.linear.com/LT3651-4.1

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