LTC1759 datasheet(22/28 Pages) LINER

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LTC1759

When wake-up is asserted to the current DAC block, the

delta-sigma is then fixed at a value equal to 80mA, inde-

pendent of the ILIMIT setting.

Note:

The external resistor connected to the ISET pin must be

multiplied by 0.8 to compensate for the 80% maximum

duty cycle of the sigma-delta modulator. Note also that the

80% duty cycle converts the rise/fall time mismatches to

a small gain error, rather than a nonlinearity. The parasitic

OPERATIO

capacitance at the ISET pin should be minimized to keep

these errors small.

The Battery Monitor Block (PWR_FAIL)

Two internal resistor dividers compare the BAT2 terminal

to the DCIN terminal. When BAT2 is above 89% of the

voltage at DCIN the PWR_FAIL bit is set to 1. A small

amount of proportional hysteresis, ~ 2%, is used for noise

immunity. The PWR_FAIL bit is set low if AC_PRESENT

is low.

APPLICATIONS INFORMATION

Adapter Limiting

An important feature of the LTC1759 is the ability to

automatically adjust charging current to a level which

avoids overloading the wall adapter. This allows the prod-

uct to operate at the same time that batteries are being

charged without complex load management algorithms.

Additionally, batteries will automatically be charged at the

maximum possible rate of which the adapter is capable.

This feature is created by sensing total adapter output

current and adjusting charging current downward if a

preset adapter current limit is exceeded. True analog

control is used, with closed loop feedback ensuring that

adapter load current remains within limits. Amplifier CL1

in Figure 9 senses the voltage across RS4, connected

between the CLP and CLN pins. When this voltage exceeds

92mV, the amplifier will override programmed charging

current to limit adapter current to 92mV/RS4. A lowpass

filter formed by 500

Ω and 1µF is required to eliminate

switching noise. If the current limit is not used, both CLP

and CLN pins should be connected to VCC.

Setting Input Current Limit

To set the input current limit, you need to know the

minimum wall adapter current rating. Subtract 5% for the

input current limit tolerance and use that current to deter-

mine the resistor value.

RS4 = 92mV/ILIMIT

ILIMIT =

Adapter Min Current – (Adapter Min Current • 5%)

Table 7. Common RS4 Resistor Values

ADAPTER

RS4 VALUE*

RS4 POWER

RATING (A)

(

Ω) 1%

DISSIPATION (W)

RATING (W)

1.5

0.06

0.135

0.25

1.8

0.05

0.162

0.25

0.045

0.18

0.25

2.3

0.039

0.206

0.25

2.5

0.036

0.225

0.5

2.7

0.033

0.241

0.5

0.03

0.27

0.5

* Values shown above are rounded to nearest standard value.

As is often the case, the wall adapter will usually have at

least a +10% current limit margin and many times one can

simply set the adapter current limit value to the actual

adapter rating (see Table 7).

Figure 9. Adapter Current Limiting

92mV

–

500

Ω

CLP

CLN

VCC

1759 F09

LTC1759

µF

RS4*

CIN

VIN

CL1

AC ADAPTER

INPUT

*RS4 =

92mV

ADAPTER CURRENT LIMIT

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