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LTC4085

4085fc

Programming Current Limit

The formula for input current limit is:

CLPROG

⎛

⎝⎜

⎞

⎠⎟

1000

•

where VCLPROG is the CLPROG pin voltage and RCLPROG

is the total resistance from the CLPROG pin to ground.

For example, if typical 500mA current limit is required,

calculate:

CLPROG ==

500

1000

•

In USB applications, the minimum value for RCLPROG

should be 2.1k. This will prevent the application current

from exceeding 500mA due to LTC4085 tolerances and

quiescent currents. A 2.1k CLPROG resistor will give

a typical current limit of 476mA in high power mode

(HPWR = 1) or 95mA in low power mode (HPWR = 0).

VCLPROG will track the input current according to the fol-

lowing equation:

CLPROG

• 1000

For best stability over temperature and time, 1% metal

ﬁlm resistors are recommended.

Ideal Diode from BAT to OUT

The LTC4085 has an internal ideal diode as well as a

controller for an optional external ideal diode. If a battery

is the only power supply available or if the load current

exceeds the programmed input current limit, then the

battery will automatically deliver power to the load via an

ideal diode circuit between the BAT and OUT pins. The

ideal diode circuit (along with the recommended 4.7μF

capacitor on the OUT pin) allows the LTC4085 to handle

large transient loads and wall adapter or USB VBUS con-

nect/disconnect scenarios without the need for large bulk

capacitors. The ideal diode responds within a few micro-

seconds and prevents the OUT pin voltage from dropping

signiﬁcantly below the BAT pin voltage. A comparison of

the I-V curve of the ideal diode and a Schottky diode can

be seen in Figure 3.

If the input current increases beyond the programmed

input current limit additional current will be drawn from

the battery via the internal ideal diode. Furthermore, if

power to IN (USB VBUS) or OUT (external wall adapter) is

removed, then all of the application power will be provided

by the battery via the ideal diode. A 4.7μF capacitor at OUT

is sufficient to keep a transition from input power to battery

power from causing significant output voltage droop. The

ideal diode consists of a precision amplifier that enables a

large P-channel MOSFET transistor whenever the voltage

at OUT is approximately 20mV (VFWD) below the voltage at

BAT. The resistance of the internal ideal diode is approxi-

mately 200m

Ω. If this is sufficient for the application then

no external components are necessary. However, if more

conductance is needed, an external PFET can be added from

BAT to OUT. The GATE pin of the LTC4085 drives the gate

of the PFET for automatic ideal diode control. The source

of the external PFET should be connected to OUT and the

drain should be connected to BAT. In order to help protect

the external PFET in overcurrent situations, it should be

placed in close thermal contact to the LTC4085.

FORWARD VOLTAGE (V)

(BAT-OUT)

SCHOTTKY

DIODE

SLOPE: 1/RDIO(ON)

IMAX

VFWD

4085 F03

Figure 3. LTC4085 Schottky Diode vs Forward Voltage Drop

OPERATION

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