CLM2950 데이터 시트보기 (PDF) - Calogic, LLC

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CLM2950

150mA Low Dropout Voltage Regulators

Calogic, LLC 

CLM2950 / CLM2951

CORPORATION

APPLICATION HINTS

EXTERNAL CAPACITORS

• The stability of the CLM2950/CLM2951 requires a 1.0µF 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 -25oC a solid tantalum is recommended since the

many aluminum types have electrolytes that freeze at about

-30oC. The ESR of about 5Ω 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 0.5µF and 0.15µF 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 5µF 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 1µA 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 1µA 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 3.3µF.

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 400µA, this

value adds to battery drain in a low battery condition.

Suggested values range from 100K to 1MΩ. 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

1µA forces an upper limit of 1.2MΩ 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 12µA. Since the CLM2951 typically draws

60µA 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 1µF to 220µF only

decreases the noise from 430µV to 160µVrms 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 0.01µF. When doing this, the output capacitor must

be increased to 3.3µF to maintain stability. These changes

reduce the output noise from 430µV to 100µVrms 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.

CALOGIC CORPORATION, 237 Whitney Place, Fremont, California 94539, Telephone: 510-656-2900, FAX: 510-651-3025

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