Application Notes

LMS5258 is a linear regulator designed to be used with a low

ESR, low cost ceramic capacitors.

EXTERNAL CAPACITORS

The LMS5258 regulator requires an output capacitor to

maintain stability. The capacitor must be at least 1µF or

greater. The capacitor can be low-ESR ceramic chip capaci-

tor, however for improved capacitance over temperature,

tantalum capacitors can be used.

A 1µF input capacitor is recommended when the supply

capacitance is more than 10 inches away from the device, or

when the supply is a battery.

X7R dielectric ceramic capacitors are recommended be-

cause of their temperature performance. X7R-type capaci-

tors change capacitance by 15% over their operating tem-

perature range and are the most stable type of ceramic

capacitors. Z5U and Y5V dielectric capacitors change value

by as much 50% and 60% respectively over their operating

temperature range. To use a ceramic chip capacitor with Y5V

dielectric, the value must be much higher than an X7R

ceramic or a tantalum capacitor to ensure the same mini-

mum capacitance value over the operating temperature

range. Tantalum capacitors have a very stable dielectric

(10% over their operating temperature range) and can also

be used with this device.

ENABLE/SHUTDOWN

The LMS5258 has an active high enable pin that allows the

to the Shutdown pin will cause the output to turn off, in this

state current consumed by the regulator goes nearly to zero.

The enable/shutdown pin can’t be left floating; a floating

enable pin may cause an indeterminate state on the output.

ACTIVE SHUTDOWN

The LMS5258 designed with a N-channel MOSFET that acts

as a shutdown clamp. The N-channel turns on when the

device is disabled to allow the output capacitor and load to

discharge.

POWER GOOD

The power good output is an open-drain output. It is de-

signed essentially to work as a power-on reset generator

once the regulated voltage was up and/ or a fault condition.

When a fault condition and an undervoltage detection occur,

the output of the power good pin goes low. The power good

output comes back up once the output has reached 97% of

its nominal value and 1ms to 5ms delay has passed , see

timing diagram.

The LMS5258 internal circuit monitors overcurrent, tempera-

ture and falling output voltage. If one of these conditions is

flaged that indicates a fault condition.

The flaged condition output is fed into an onchip delay circuit

that drives the open drain output transistor.

TRANSIENT RESPONSE

The LMS5258 implements a unique output stage to dramati-

cally improve transient response recovery time. The output is

a totem-pole configuration with a P-channel MOSFET pass

device and a N-channel MOSFET clamp. The N-channel

clamp is a significantly smaller device that prevents the

output voltage from overshooting when a heavy load is

removed. This feature helps to speed up the transient re-

sponse by significantly decreasing transient response recov-

ery time during the transition from heavy load to light load.

THERMAL BEHAVIOR

The LMS5258 regulator has internal thermal shutdown to

protect the device from over heating. Under all operating

conditions, the maximum junction temperature of the

LMS5258 must be below 125˚C. Maximum power dissipation

can be calculated based on the output current and the

voltage drop across the part. The maximum power dissipa-

tion is

P

θ

JA

θ

for the LMS5258 in the SOT23-5 package. T

mum ambient temperature T

temperature of the die, 125˚C.

When operating the LMS5258 at room temperature, the

maximum power dissipation is 425mW.

The actual power dissipated by the regulator is

P

Substituting P

for the operating condition that are critical to the application

will give the maximum operating conditions for the regulator

circuit. To prevent the device from entering thermal shut-

down, maximum power dissipation cannot be exceeded.

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