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參數(shù)資料
| 型號： | CLM2950C |
| 廠商： | CALOGIC LLC |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | 150mA Low Dropout Voltage Regulators |
| 中文描述： | 150mA的低壓差穩(wěn)壓器 |
| 文件頁數(shù)： | 5/10頁 |
| 文件大?。?/td> | 154K |
| 代理商： | CLM2950C |
APPLICATION HINTS
EXTERNAL CAPACITORS
The stability of the CLM2950/CLM2951 requires a 1.0F or
greater capacitor between output and ground. Oscillation
could occur without this capacitor. Most types of tantalum
or aluminum electrolytic works fine here. For operations
below -25
oC a solid tantalum is recommended since the
many aluminum types have electrolytes that freeze at about
-30
oC.
The ESR of about 5
or less and resonant
frequency about 500kHz are the most important parameters
in the value of the capacitor. The capacitors value can be
increased without limit.
At lower values of output current, less output capacitance is
required for stability. For currents below 10mA the value of
the capacitor can be reduced to 0.5
F and 0.15F for 1mA.
More output capacitance is needed for the 8-pin version at
voltages below 5V since it runs the error amplifier at lower
gain.
At worst case 5F or greater must be used for the condition
of 150mA load at 1.23V output.
The CLM2950, unlike other low dropout regulators, will
remain stable and in regulation with no load in addition to
the internal voltage divider.
The feature is especially
important in applications like CMOS RAM keep-alive
circuits. When setting the output voltage of the CLM2951
version with external resistors, a minimum load of 1
A is
recommended.
If there is more than 10 inches of wire between the input
and the AC filter capacitor or if a battery is used as the input
then a 1
A tantalum or aluminum electrolytic capacitor
should be placed from the input to the ground.
Instability can occur if there is a stray capacitance to the
CLM2951 feedback terminal (pin 7). This could cause more
problems when using a higher value of external resistors to
set the output voltage. This problem can be fixed by adding
a 100pF capacitor between output and feedback and
increasing the output capacitor to at least 3.3
F.
ERROR DETECTION COMPARATOR OUTPUT
The Comparator produces a logic low output whenever the
CLM2951 output falls out of regulation by more than around
5%.
This is around 60mV offset divided by the 1.235
reference voltage. This trip level remains 5% below normal
regardless of the programmed output voltage of the regulator.
Figure 1 shows the timing diagram depicting the ERROR
signal and the regulator output voltage as the CLM2951 input
is ramped up and down. The ERROR signal becomes low at
around 1.3V input, and goes high around 5V input (input
voltage at which VOUT = 4.75). Since the CLM2951’s dropout
voltage is load dependent, the input voltage trip point (around
5V) will vary with the load current. The output voltage trip
point (approx. 4.75V) does not vary with load.
The error comparator has an open-collector output which
requires an external pull-up resistor.
Depending on the
system requirements the resistor may be returned to 5V
output or other supply voltage. In determining the value of
this resistor, note that the output is rated to sink 400
A, this
value adds to battery drain in a low battery condition.
Suggested values range from 100K to 1M
. If the output is
unused this resistor is not required.
PROGRAMMING THE OUTPUT VOLTAGE OF CLM2951
The CLM2951 may be pin-strapped for 5V using its internal
voltage divider by tying Pin 1 (output) to Pin 2 (sense) and Pin
7 (feedback) to Pin 6 (5V Tap). Also, it may be programmed
for any output voltage between its 1.235V reference and its
30V maximum rating. As seen in Figure 2, an external pair of
resistors is required.
Refer to the below equation for the programming of the output
voltage:
VOUT = VREF
× (1 + R1\R2) + IFBR1
The VREF is 1.235 and IFB is the feedback bias current,
nominally -20nA. The minimum recommended load current of
1
A forces an upper limit of 1.2M on value of R2. If no load
is presented the IFB produces an error of typically 2% in VOUT
which may be eliminated at room temperature by trimming R1.
To improve the accuracy choose the value of R2 = 100k this
reduces the error by 0.17% and increases the resistor
program current by 12
A. Since the CLM2951 typically draws
60
A at no load with Pin 2 open-circuited this is a small price
to pay.
REDUCING OUTPUT NOISE
It may be an advantage to reduce the AC noise present at the
output.
One way is to reduce the regulator bandwidth by
increasing the size of the output capacitor. This is the only
way that noise can be reduced on CLM2950 but is relatively
inefficient, as increasing the capacitor from 1
F to 220F only
decreases the noise from 430
V to 160Vrms for a 100kHz
bandwidth at 5V output.
Noise could also be reduced fourfold by a bypass capacitor
across R1, since it reduces the high frequency gain from 4 to
unity.
CBYPASS
1/2πR1 × 200Hz
or choose 0.01
F. When doing this, the output capacitor must
be increased to 3.3
F to maintain stability. These changes
reduce the output noise from 430
V to 100Vrms for a
100kHz bandwidth at 5V output. With the bypass capacitor
added, noise no longer scales with output voltage so that
improvements are more dramatic at higher voltages.
CLM2950 / CLM2951
CORPORATION
CALOGIC CORPORATION, 237 Whitney Place, Fremont, California 94539, Telephone: 510-656-2900, FAX: 510-651-3025
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