LTC4085EDE-3 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC4085EDE-3
Linear Technology
'LTC4085EDE-3' PDF : 24 Pages View PDF
LTC4085-3/LTC4085-4
OPERATION
Programming Current Limit
The formula for input current limit is:
ICL
=
⎛
⎝⎜
1000
RCLPROG
•
⎞
VCLPROG ⎠⎟
=
1000V
RCLPROG
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:
RCLPROG
=
1V
500mA
•
1000
=
2k
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:
IIN
= VCLPROG
RCLPROG
• 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 con-
troller 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 connect/disconnect
scenarios without the need for large bulk capacitors. The
ideal diode responds within a few microseconds and pre-
vents 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 200mΩ. 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 over-current situations, it should be
placed in close thermal contact to the LTC4085.
IMAX
SLOPE: 1/RDIO(ON)
SCHOTTKY
DIODE
VFWD
FORWARD VOLTAGE (V)
(BAT-OUT)
408534 F03
Figure 3. LTC4085 Schottky Diode vs Forward Voltage Drop
408534fb
15
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