Datasheet

LP38858
SNVS462D –OCTOBER 2006–REVISED APRIL 2013 www.ti.com
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 values
is used for V
BIAS
.
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. The bias voltage must be in the range of 3.0V to 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 V
BIAS
rises above the UVLO
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 ensured 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.
When V
BIAS
is above the UVLO 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 to 1000 μF in this manner will not damage the device as
the current will decay rapidly. However, continuous reverse current should be avoided.
SOFT-START
The LP38858 incorporates a Soft-Start function that reduces the start-up current surge into the output capacitor
(C
OUT
) by allowing V
OUT
to rise slowly to the final value. This is accomplished by controlling V
REF
at the SS pin.
The soft-start timing capacitor (C
SS
) is internally held to ground until V
BIAS
rises above the Under-Voltage Lock-
Out threshold (ULVO).
V
REF
will rise at an RC rate defined by the internal resistance of the SS pin (r
SS
), and the external capacitor
connected to the SS pin. This allows the output voltage to rise in a controlled manner until steady-state
regulation is achieved. Typically, five time constants are recommended to assure that the output voltage is
sufficiently close to the final steady-state value. During the soft-start time the output current can rise to the built-in
current limit.
10 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LP38858