Datasheet

REG102
11
SBVS024F
www.ti.com
FIGURE 6. Transient and DC Dropout.
350
300
250
200
150
100
50
0
Dropout Voltage (mV)
0 50 100 150 200
DC
250
I
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 (V
IN
– V
OUT
) is less than the drop-out voltage (V
DROP
),
the DMOS pass device behaves like a resistor; therefore, for
low values of (V
IN
– V
OUT
), the regulator input-to-output
resistance is the Rds
ON
of the DMOS pass element (typically
600mΩ). For static (DC) loads, the REG102 typically main-
tains regulation down to a (V
IN
– V
OUT
) voltage drop of 150mV
at full rated output current. In Figure 6, the bottom line (DC
dropout) shows the minimum V
IN
to V
OUT
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 V
IN
to V
OUT
voltage drop above this line insure normal transient re-
sponse.
case conditions (full-scale load change with (V
IN
– V
OUT
)
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 0.47µF) from the output pin to ground can
improve the transient response. In the adjustable version, the
addition of a capacitor, C
FB
(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 V
IN
to V
OUT
voltage drop. Under worst-