5A Low-Voltage Low-Dropout Regulator

LM39500/39501/39502

HTC

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Minimum Load Current

The LM39500/1/2 regulator is specified between finite loads.

If the output current is too small, leakage currents dominate and the output voltage rises.

A 10mA minimum load current is necessary for proper regulation.

Transient Response and 3.3V to 2.5V Conversion

The LM39500/1/2 has excellent transient response to varia-tions in input voltage and load current. The device has

been designed to respond quickly to load current variations and input voltage variations. Large output capacitors are

not required to obtain this performance. A standard 47F output capacitor, preferably tantalum, is all that is required.

Larger values help to improve performance even further.

By virtue of its low-dropout voltage, this device does not saturate into dropout as readily as similar NPN-based de-

signs. When converting from 3.3V to 2.5V, the NPN-based regulators are already operating in dropout, with typical

dropout requirements of 1.2V or greater. To convert down to 2.5V without operating in dropout, NPN-based regulators

require an input voltage of 3.7V at the very least. The LM39500/1/2 regulator will provide excellent performance with

an input as low as 3.0V. This gives the PNP-based regulators a distinct advantage over older, NPN-based linear

regulators does not have the headroom to dothis conversion.

Error Flag

The LM39501 version features an error flag circuit which monitors the output voltage and signals an error condition

when the voltage drops 5% below the nominal output voltage. The error flag is an open-collector output that can sink

10mA during a fault condition.

Low output voltage can be caused by a number of problems, including an overcurrent fault (device in current limit) or

low input voltage. The flag is inoperative during overtemperature shutdown.

Enable Input

The LM39501 version features an enable input for on/off control of the device. Its shutdown state draws “zero” current

(only microamperes of leakage). The enable input is TTL/ CMOS compatible for simple logic interface, but can be

connected to up to 20V.

Adjustable Regulator Design

The LM39502 allows programming the output voltage any-where between 1.25V and the 16V maximum operating

rating of the family. Two resistors are used. Resistors can be quite large, up to 1MΩ, because of the very high input

impedance and low bias current of the sense comparator: The resistor values are calculated by: R1=R2(Vout/1.250-1)

Where VO is the desired output voltage. Figure 1 shows component definition. Applications with widely varying load

currents may scale the resistors to draw the minimum load current required for proper operation.