LP38851S-ADJ Datasheet, PDF (14/29 Pages) Texas Instruments – LP38851 800 mA Fast-Response High-Accuracy Adjustable LDO Linear Regulator with Enable and Soft-Start

LP38851
SNVS492C – JUNE 2007 – REVISED APRIL 2013
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Please refer to Application Note AN-1378 (SNVA112) for additional information on how resistor tolerances affect
the calculated V_OUT value.
INPUT VOLTAGE
The input voltage (V_IN) is the high current external voltage rail that will be regulated down to a lower voltage,
which is applied to the load. The input voltage must be at least V_OUT + V_DO, and no higher than whatever value is
used for V_BIAS.
For applications where V_BIAS is higher than 4.5V, V_IN must be no greater than 4.5V, otherwise output voltage
accuracy may be affected.
BIAS VOLTAGE
The bias voltage (V_BIAS) is a low current external voltage rail required to bias the control circuitry and provide
gate drive for the N-FET pass transistor. When V_OUT is set to 1.20V, or less, V_BIAS may be anywhere in the
operating range of 3.0V to 5.5V. If V_OUT is set higher than 1.20V , V_BIAS must be between 4.5V and 5.5V to
ensure proper operation of the device.
UNDER VOLTAGE LOCKOUT
The bias voltage is monitored by a circuit which prevents the device from functioning when the bias voltage is
below the Under-Voltage Lock-Out (UVLO) threshold of approximately 2.45V.
As the bias voltage rises above the UVLO threshold the device control circuitry becomes active. There is
approximately 150 mV of hysteresis built into the UVLO threshold to provide noise immunity.
When the bias voltage is between the UVLO threshold and the Minimum Operating Rating value of 3.0V the
device will be functional, but the operating parameters will not be within the specified limits.
SUPPLY SEQUENCING
There is no requirement for the order that V_IN or V_BIAS are applied or removed.
One practical limitation is that the Soft-Start circuit starts charging C_SS when both V_BIAS rises above the UVLO
threshold and the Enable pin is above the V_EN(ON) threshold. If the application of V_IN is delayed beyond this point
the benefits of Soft-Start will be compromised.
In any case, the output voltage cannot be specified until both V_IN and V_BIAS are within the range of specified
operating values.
If used in a dual-supply system where the regulator output load is returned to a negative supply, the output pin
must be diode clamped to ground. A Schottky diode is recommended for this diode clamp.
REVERSE VOLTAGE
A reverse voltage condition will exist when the voltage at the output pin is higher than the voltage at the input pin.
Typically this will happen when V_IN is abruptly taken low and C_OUT continues to hold a sufficient charge such that
the input to output voltage becomes reversed.
The NMOS pass element, by design, contains no body diode. This means that, as long as the gate of the pass
element is not driven, there will not be any reverse current flow through the pass element during a reverse
voltage event. The gate of the pass element is not driven when V_BIAS is below the UVLO threshold, or when the
Enable pin is held low.
When V_BIAS is above the UVLO threshold, and the Enable pin is above the V_EN(ON) threshold, the control circuitry
is active and will attempt to regulate the output voltage. Since the input voltage is less than the output voltage the
control circuit will drive the gate of the pass element to the full V_BIAS potential when the output voltage begins to
fall. In this condition, reverse current will flow from the output pin to the input pin , limited only by the R_DS(ON) of
the pass element and the output to input voltage differential. Discharging an output capacitor up 1000 µF in this
manner will not damage the device as the current will rapidly decay. However, continuous reverse current should
be avoided.
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