LP38691DTX-5.0 Datasheet, PDF (12/27 Pages) Texas Instruments – LP38693 500mA Low Dropout CMOS Linear Regulators Stable with Ceramic Output Capacitors

LP38691, LP38693
SNVS321K – JANUARY 2005 – REVISED APRIL 2013
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TANTALUM
Solid Tantalum capacitors have good temperature stability: a high quality Tantalum will typically show a
capacitance value that varies less than 10-15% across the full temperature range of -40°C to +125°C. ESR will
vary only about 2X going from the high to low temperature limits.
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.
1. While V_IN is high enough to keep the control circuity alive, and the Enable pin (LP38693 only) is above the
V_EN(ON) threshold, the control circuitry will attempt to regulate the output voltage. If 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. 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 pin is low this condition will be prevented.
2. The internal PFET pass element 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, when V_IN is below the
value where the control circuity is alive, or the Enable pin is low (LP38693 only), and the output voltage is
more than 500 mV (typical) above the input voltage 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 to limit the negative voltage transition. A Schottky diode is
recommended for this protective clamp.
PCB LAYOUT
Good PC layout practices must be used or instability can be induced because of ground loops and voltage drops.
The input and output capacitors must be directly connected to the input, output, and ground pins of the regulator
using traces which do not have other currents flowing in them (Kelvin connect).
The best way to do this is to lay out C_IN and C_OUT near the device with short traces to the V_IN, V_OUT, and ground
pins. The regulator ground pin should be connected to the external circuit ground so that the regulator and its
capacitors have a "single point ground".
It should be noted that stability problems have been seen in applications where "vias" to an internal ground plane
were used at the ground points of the IC and the input and output capacitors. This was caused by varying ground
potentials at these nodes resulting from current flowing through the ground plane. Using a single point ground
technique for the regulator and it’s capacitors fixed the problem. Since high current flows through the traces
going into V_IN and coming from V_OUT, Kelvin connect the capacitor leads to these pins so there is no voltage drop
in series with the input and output capacitors.
WSON MOUNTING
The NGG0006A (No Pullback) 6-Lead WSON package requires specific mounting techniques which are detailed
in the TI AN-1187 Application Report. Referring to the section PCB Design Recommendations (Page 5), it should
be noted that the pad style which should be used with the WSON package is the NSMD (non-solder mask
defined) type. Additionally, it is recommended the PCB terminal pads to be 0.2 mm longer than the package
pads to create a solder fillet to improve reliability and inspection.
The input current is split between two V_IN pins, 1 and 6. The two V_IN pins must be connected together to ensure
that the device can meet all specifications at the rated current.
The thermal dissipation of the WSON package is directly related to the printed circuit board construction and the
amount of additional copper area connected to the DAP.
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