LTC4090(RevA) View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC4090 Datasheet PDF : 28 Pages
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LTC4090/LTC4090-5
OPERATION
500
IIN
100
400
80
IIN
500
IIN
400
ILOAD
ILOAD
ILOAD
300
60
300
IBAT = ICHG
200
40
200
IBAT = ICL = IOUT
100
0
0
4090 F02a
IBAT
(CHARGING)
100 200 300 400 500
ILOAD(mA)
IBAT
(IDEAL DIODE)
20
0
0
4090 F02b
IBAT
(CHARGING)
20 40 60 80 100
ILOAD(mA)
IBAT
(IDEAL DIODE)
100
0
0
4090 F02c
IBAT
(CHARGING)
100 200 300 400 500
ILOAD (mA)
IBAT
(IDEAL DIODE)
(a) High Power Mode/Full Charge
RPROG = 100k and RCLPROG = 2k
(a) Low Power Mode/Full Charge
RPROG = 100k and RCLPROG = 2k
(a) High Power Mode with
ICL = 500mA and ICHG = 250mA
RPROG = 100k and RCLPROG = 2k
Figure 2. Input and Battery Currents as a Function of Load Current
The oscillator reduces the switch regulator’s operating
frequency when the voltage at the HVOUT pin is low (be-
low 2.95V). This frequency foldback helps to control the
output current during start-up and overload.
The high voltage regulator contains a power good com-
parator which trips when the HVOUT pin is at 2.8V. The PG
output is an open-collector transistor that is off when the
output is in regulation, allowing an external resistor to pull
the PG pin high. Power good is valid when the switching
regulator is enabled and HVIN is above 3.6V.
Ideal Diode From BAT to OUT
The LTC4090/LTC4090-5 have 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 LTC4090/LTC4090-5
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 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 desired input current increases beyond the pro-
grammed input current limit additional current will be drawn
from the battery via the internal ideal diode. Furthermore,
if power to IN (USB VBUS) or HVIN (high voltage input) 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 approximately 215mΩ.
If this is sufficient for the application then no external
components are necessary. However if more conductance
is needed, an external P-channel MOSFET can be added
from BAT to OUT. The GATE pin of the LTC4090/LTC4090-5
drives the gate of the external PFET for automatic ideal
diode control. The source of the external MOSFET should
be connected to OUT and the drain should be connected
to BAT. In order to help protect the external MOSFET in
overcurrent situations, it should be placed in close thermal
contact to the LTC4090/LTC4090-5.
Burst Mode is a registered trademark of Linear Technology Corporation
4090fa
14
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