LP38511TJ-ADJ Datasheet, PDF (9/21 Pages) Texas Instruments – LP38511-ADJ 800mA Fast-Transient Response Adjustable Low-Dropout Linear Voltage Regulator

LP38511-ADJ
www.ti.com
SNVS545D – JANUARY 2009 – REVISED APRIL 2013
APPLICATION INFORMATION
EXTERNAL CAPACITORS
Like any low-dropout regulator, external capacitors are required to assure stability. These capacitors must be
correctly selected for proper performance.
Input Capacitor
A ceramic input capacitor of at least 10 µF is required. For general usage across all load currents and operating
conditions, a 10 µF ceramic input capacitor will provide satisfactory performance.
Output Capacitor
A ceramic capacitor with a minimum value of 10 µF is required at the output pin for loop stability. It must be
located less than 1 cm from the device and connected directly to the output and ground pin using traces which
have no other currents flowing through them. As long as the minimum of 10 µF ceramic is met, there is no
limitation on any additional capacitance.
X7R and X5R dielectric ceramic capacitors are strongly recommended, as they typically maintain a capacitance
range within ±20% of nominal over full operating ratings of temperature and voltage. Of course, they are typically
larger and more costly than Z5U/Y5U types for a given voltage and capacitance.
Z5U and Y5V dielectric ceramics are not recommended as the capacitance will drop severely with applied
voltage. A typical Z5U or Y5V capacitor can lose 60% of its rated capacitance with half of the rated voltage
applied to it. The Z5U and Y5V also exhibit a severe temperature effect, losing more than 50% of nominal
capacitance at high and low limits of the temperature range.
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. A less common condition is when an alternate voltage source is
connected to the output.
There are two possible paths for current to flow from the output pin back to the input during a reverse voltage
condition.
While V_IN is high enough to keep the control circuity alive, and the Enable pin is above the V_EN(ON) threshold, the
control circuitry will attempt to regulate the output voltage. Since the input voltage is less than the programmed
output voltage, the control circuit will drive the gate of the pass element to the full on condition 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 rapidly decay. However, continuous
reverse current should be avoided. When the Enable is low this condition will be prevented.
The internal PFET pass element in the LP38511-ADJ has an inherent parasitic diode. During normal operation,
the input voltage is higher than the output voltage and the parasitic diode is reverse biased. However, if the
output voltage to input voltage differential is more than 500 mV (typical) the parasitic diode becomes forward
biased and current flows from the output pin to the input pin through the diode. The current in the parasitic diode
should be limited to less than 1A continuous and 5A peak.
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 protective clamp.
SHORT-CIRCUIT PROTECTION
The LP38511-ADJ is short circuit protected, and in the event of a peak over-current condition the short-circuit
control loop will rapidly drive the output PMOS pass element off. Once the power pass element shuts down, the
control loop will rapidly cycle the output on and off until the average power dissipation causes the thermal
shutdown circuit to respond to servo the on/off cycling to a lower frequency. Please refer to the POWER
DISSIPATION/HEAT-SINKING section for power dissipation calculations.
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