REG102 datasheet(11/20 Pages) TI

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REG102

SBVS024F

www.ti.com

FIGURE 6. Transient and DC Dropout.

350

300

250

200

150

100

150

200

250

OUT (mA)

DROPOUT VOLTAGE vs I

OUT

0mA to I

OUT Transient

DROPOUT VOLTAGE

The REG102 uses an N-channel DMOS as the pass element.

When (VIN – VOUT) is less than the drop-out voltage (VDROP),

the DMOS pass device behaves like a resistor; therefore, for

low values of (VIN – VOUT), the regulator input-to-output

resistance is the RdsON of the DMOS pass element (typically

600m

Ω). For static (DC) loads, the REG102 typically main-

tains regulation down to a (VIN – VOUT) voltage drop of 150mV

at full rated output current. In Figure 6, the bottom line (DC

dropout) shows the minimum VIN to VOUT voltage drop re-

quired to prevent dropout under DC load conditions.

For large step changes in load current, the REG102 requires

a larger voltage drop across it to avoid degraded transient

response. The boundary of this transient drop-out region is

shown as the top line in Figure 6 and values of VIN to VOUT

voltage drop above this line insure normal transient re-

sponse.

case conditions (full-scale load change with (VIN – VOUT)

voltage drop close to DC dropout levels), the REG102 can

take several hundred microseconds to re-enter the specified

window of regulation.

TRANSIENT RESPONSE

The REG102 response to transient line and load conditions

improves at lower output voltages. The addition of a capacitor

(nominal value 0.47

µF) from the output pin to ground can

improve the transient response. In the adjustable version, the

addition of a capacitor, CFB (nominal value 10nF), from the

output to the adjust pin can also improve the transient

response.

THERMAL PROTECTION

Power dissipated within the REG102 can cause the junction

temperature to rise. The REG102 has thermal shutdown

circuitry that protects the regulator from damage which dis-

ables the output when the junction temperature reaches

approximately 160

°C, allowing the device to cool. When the

junction temperature cools to approximately 140

°C, the out-

put circuitry is again enabled. Depending on various condi-

tions, the thermal protection circuit can cycle on and off. This

limits the dissipation of the regulator, but can have an

undesirable effect on the load.

Any tendency to activate the thermal protection circuit indi-

cates excessive power dissipation or an inadequate heat

sink. For reliable operation, junction temperature must be

limited to 125

°C, maximum. To estimate the margin of safety

in a complete design (including heat sink), increase the

ambient temperature until the thermal protection is triggered;

use worst-case loads and signal conditions. For good reliabil-

ity, thermal protection should trigger more than 35

°C above

the maximum expected ambient condition of the application.

This produces a worst-case junction temperature of 125

°C at

the highest expected ambient temperature and worst-case

load.

The internal protection circuitry of the REG102 is designed to

protect against overload conditions and is not intended to

replace proper heat sinking. Continuously running the REG102

into thermal shutdown will degrade reliability.

In the transient dropout region between DC and Transient,

transient response recovery time increases. The time required

to recover from a load transient is a function of both the

magnitude and rate of the step change in load current and the

available headroom VIN to VOUT voltage drop. Under worst-

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