TPS79850-Q1_15 Datasheet, PDF (12/25 Pages) Texas Instruments – TPS798xx-Q1 50 mA, 3 V to 50 V, Micropower, Low-Dropout Linear Regulator

TPS79801-Q1, TPS79850-Q1
SLVS822E – MARCH 2009 – REVISED SEPTEMBER 2015
www.ti.com
Feature Description (continued)
The FB pin of the adjustable device can be pulled above or below ground by as much as 7 V without damaging
the device. If the input is left open or grounded, the FB pin behaves as an open circuit when pulled below
ground, or as a large resistor (typically, 100 kΩ) in series with a diode when pulled above ground. If the input is
powered by a voltage source, pulling the FB pin below the reference voltage increases the output voltage. This
configuration causes the output to go to a unregulated high voltage. Pulling the FB pin above the reference
voltage turns off all output current.
In situations where the FB pin is connected to a resistor divider that would pull the FB pin above its 7-V clamp
voltage if the output is pulled high, the FB pin input current must be limited to less than 5 mA. For example, a
resistor divider provides a regulated 1.5-V output from the 1.275-V reference when the output is forced to 28 V.
The top resistor of the resistor divider must be chosen to limit the current into the FB pin to less than 5 mA when
the FB pin is at 7 V. The 21-V difference between the OUT and FB pins divided by the 5-mA maximum current
into the FB pin yields a minimum top resistor value of 5.8 kΩ.
In circuits where a backup battery is required, several different input/output conditions can occur. The output
voltage may be held up while the input is either pulled to ground, pulled to some intermediate voltage, or is left
open. The rise in reverse output current above 7 V occurs from the breakdown of the 7-V clamp on the FB pin.
With a resistor divider on the regulator output, this current is reduced, depending on the size of the resistor
divider.
When the IN pin of the TPS798xx-Q1 is forced below the OUT pin, or the OUT pin is pulled above the IN pin,
input current typically drops to less than 0.6 mA. This scenario can occur if the input of the TPS798xx-Q1 is
connected to a discharged (low voltage) battery and the output is held up by either a backup battery or a second
regulator circuit. The state of the EN pin has no effect on the reverse output current when the output is pulled
above the input.
7.4 Device Functional Modes
7.4.1 Low-Voltage Tracking
At low input voltages, the regulator drops out of regulation and the output voltage tracks input minus a voltage
based on the load current and switch resistance. This allows for a smaller input capacitor and can possibly
eliminate the need of using a boost convertor during cold-crank conditions.
12
Submit Documentation Feedback
Copyright © 2009–2015, Texas Instruments Incorporated
Product Folder Links: TPS79801-Q1 TPS79850-Q1