connected from the output to the adjust pin will reduce both
the output noise and the peak error from a load transient.
Figure 7 shows improved output noise performance for two
capacitor combinations.
For large step changes in load current, the REG104 requires
a larger voltage drop across it to avoid degraded transient
response. The boundary of this transient dropout region is
shown as the top line in Figure 8. Values of VIN to VOUT
voltage drop above this line insure normal transient re-
sponse.
10.0
In the transient dropout region between DC and Transient,
transient response recovery time increases. The time re-
quired 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-case conditions (full-scale load change with VIN to
1.0
VOUT voltage drop close to DC dropout levels), the REG104
can take several hundred microseconds to re-enter the
specified window of regulation.
COUT = 0, CFB = 0
COUT = 0, CFB = 0.01µF
COUT = 10µF, CFB = 0.01µF
0.1
TRANSIENT RESPONSE
10
100
1000
10000
100000
The REG104 response to transient line and load conditions
improves at lower output voltages. The addition of a capaci-
tor (nominal value 10nF) from the output pin to ground may
improve the transient response. In the adjustable version, the
addition of a capacitor, CFB (nominal value 10nF), from the
output to the adjust pin will also improve the transient
response.
Frequency
FIGURE 7. Output Noise Density on Adjustable Versions.
The REG104 utilizes an internal charge pump to develop an
internal supply voltage sufficient to drive the gate of the
DMOS pass element above VIN. The charge-pump switching
noise (nominal switching frequency = 2MHz) is not measur-
able at the output of the regulator.
THERMAL PROTECTION
Power dissipated within the REG104 will cause the junction
temperature to rise. The REG104 has thermal shutdown
circuitry that protects the regulator from damage. The ther-
mal protection circuitry disables the output when the junc-
tion temperature reaches approximately 150°C, allowing
the device to cool. When the junction temperature cools to
approximately 130°C, the output circuitry is again enabled.
Depending on various conditions, the thermal protection
circuit may cycle on and off. This limits the dissipation of the
regulator, but may have an undesirable effect on the load.
DROP-OUT VOLTAGE
The REG104 uses an N-channel DMOS as the pass ele-
ment. When the input voltage is within a few hundred
millivolts of the output voltage, the DMOS device behaves
like a resistor. Therefore, for low values of VIN to VOUT, the
regulator’s input-to-output resistance is the RdsON of the
DMOS pass element (typically 230mΩ). For static (DC)
loads, the REG104 will typically maintain regulation down to
VIN to VOUT voltage drop of 230mV at full rated output current.
Any tendency to activate the thermal protection circuit indi-
cates excessive power dissipation or an inadequate heat
sink. For reliable operation, junction temperature should 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 reli-
ability, thermal protection should trigger more than 35°C
above the maximum expected ambient condition of your
application. This produces a worst-case junction temperature
of 125°C at the highest expected ambient temperature and
worst-case load.
In Figure 8, the bottom line (DC dropout) shows the minimum
VIN to VOUT voltage drop required to prevent dropout under
DC load conditions.
REG104 –3.3 at 25°C
250
DC
Transient
200
150
100
50
The internal protection circuitry of the REG104 has been
designed to protect against overload conditions. It was not
intended to replace proper heat sinking. Continuously run-
ning the REG104 into thermal shutdown will degrade reliabil-
ity.
0
0
100
200
300
400
500
IOUT (mA)
FIGURE 8. Transient and DC Dropout.
10
REG104
SBVS025G
BB [ BURR-BROWN CORPORATION ]
