Data Sheet

LT4423

analog.com

Rev. 0

9 of 18

STATUS is an open-drain output intended to indicate power path conditions. STATUS is designed for connection to

a power supply through a pull-up resistor. STATUS pulls low whenever the internal power path is disabled. STATUS

may pull low for the following reasons. First, driving SHDN low pulls STATUS low. Second, when the power path is

reverse biased (the OUT voltage exceeds the IN voltage by approximately 20mV), STATUS also pulls low. When the

power path is enabled, the STATUS pull-down is removed.

APPLICATIONS INFORMATION

The LT4423 is intended for power path control applications, including diode ORing of multiple power supplies, load

sharing between multiple batteries or power sources, automatic switch-over from a primary to an auxiliary power

source, charging of multiple batteries from a single charger, or acting as a high-side load switch.

USB PD 3.0 and Back-Up Battery Supply

The front-page schematic demonstrates the LT4423 in an application with USB PD (Power Delivery) 3.0 and a backup

lithium-ion battery cell. The input voltage from the USB 3.0 connector can range from 5V to 20V, all easily handled

by the LT4423. On the other hand, the single lithium-ion cell can be down at 3V to 3.6V, which is easily handled by the

LT4423. The application has a logic control signal on SHDN for the primary power path. When this signal switches

low, the primary power path from the USB is disabled, and the backup battery takes over the delivery of power to

the load. The common output automatically biases to the higher of the two input voltages.

The configuration of STATUS in this application allows the LED to indicate that the USB power path is active, ensuring

that the higher power consumption associated with the LED occurs when the USB power source is present. Even if

the USB power source is disconnected without a SHDN control signal coming first, the application seamlessly

switches to the backup power source. Note that the LED and STATUS functions are optional and easily removed for

simplicity by leaving STATUS open.

The LT4423 SHDN rising threshold is 1.6V (maximum). To obtain the fastest turn-on response time, it is

recommended that SHDN swing to at least the minimum device operating voltage of 1.9V for strong overdrive of the

SHDN

threshold. To obtain the fastest turn-off of the power path, pulling SHDN well below the minimum 1V

threshold (or close to GND) is also recommended. SHDN should never be left open. If logical on/off control is not

required, automatic turn-on is achieved by connecting SHDN to IN.

Automatic Power Path Control

The automatic switchover from a battery to a wall adapter or other power source is shown in Figure 13. Initially, with

only the battery applied, the load is powered purely through the LT4423 from the battery. If the wall adapter is

applied and is greater than the battery voltage, the OUT voltage rises above the IN voltage. The LT4423 senses this

increase, disables the IN to OUT connection, and electrically isolates the battery from the power path. Now, only the

wall adapter powers the load, and the battery remains protected.

With STATUS connected, as shown in Figure 13, the LED indicator turns on with the wall adapter present. When the

wall adapter is not present and the battery powers the load, STATUS pulls high, and the LED indicator turns off. This

configuration allows the higher power consumption of the LED to be supplied by the wall adapter when it is

connected.