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LP38856 Datasheet(PDF) 10 Page - Texas Instruments

Part No. LP38856
Description  3A Fast-Response High-Accuracy LDO Linear Regulator with Enable
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
Logo TI1 - Texas Instruments

LP38856 Datasheet(HTML) 10 Page - Texas Instruments

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Bias Capacitor
The capacitor on the bias pin must be at least 1 µF. It can be any good quality capacitor (ceramic is
The input voltage (VIN) 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 VOUT + VDO, and no higher than whatever value is
used for VBIAS.
The bias voltage (VBIAS) 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.
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 become 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.
There is no requirement for the order that VIN or VBIAS are applied or removed. However, the output voltage
cannot be specified until both VIN and VBIAS are within the range of specified operating values.
If used in a dual-supply system where the regulator load is returned to a negative supply, the output pin must be
diode clamped to ground. A Schottky diode is recommend for this diode clamp.
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 VIN is abruptly taken low and COUT 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 VBIAS is below the UVLO threshold.
When VBIAS 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 VBIAS 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 RDS(ON) of the pass element and the output to input
voltage differential. This condition is outside the specified operating range and should be avoided.
The Enable pin (EN) provides a mechanism to enable, or disable, the regulator output stage. The Enable pin has
an internal pull-up, through a typical 180 k
Ω resistor, to VBIAS.
If the Enable pin is actively driven, pulling the Enable pin above the VEN threshold of 1.25V (typical) will turn the
regulator output on, while pulling the Enable pin below the VEN threshold will turn the regulator output off. There
is approximately 100 mV of hysteresis built into the Enable threshold provide noise immunity.
If the Enable function is not needed this pin should be left open, or connected directly to VBIAS. If the Enable pin
is left open, stray capacitance on this pin must be minimized, otherwise the output turn-on will be delayed while
the stray capacitance is charged through the internal resistance (rEN).
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Copyright © 2006–2011, Texas Instruments Incorporated
Product Folder Links: LP38856

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