LT3050MPMSE-3.3 Ver la hoja de datos (PDF) - Linear Technology

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LT3050MPMSE-3.3

100mA, Linear Regulator with Precision Current Limit and Diagnostic Outputs

Linear Technology 

LT3050 Series

APPLICATIONS INFORMATION

Table 1 shows 1% resistor divider values for some common

output voltages with a resistor divider current of 5μA.

Table 1. Output Voltage Resistor Divider Valves

VOUT (V)

1.2

R1 (kΩ)

118

1.5

121

1.8

124

2.5

115

3

124

3.3

124

5

115

R2 (kΩ)

118

182

249

365

499

562

845

Bypass Capacitance, Output Voltage Noise and

Transient Response

The LT3050 regulator provides low output voltage noise

over the 10Hz to 100kHz bandwidth while operating at

full load with the addition of a reference bypass capacitor

(CREF/BYP) from the REF/BYP pin to GND. A good quality,

low leakage capacitor is recommended. This capacitor will

bypass the internal reference of the regulator, providing a

low frequency noise pole. With the use of 10nF for CREF/BYP,

the output voltage noise decreases to as low as 30μVRMS

when the output voltage is set for 0.6V. For higher output

voltages (generated by using a feedback resistor divider),

the output voltage noise gains up accordingly when using

CREF/BYP by itself.

To lower the output voltage noise for higher output voltages,

include a feedforward capacitor (CFF) from VOUT to the

ADJ pin. A good quality, low leakage capacitor is recom-

mended. This capacitor will bypass the error amplifier of

the regulator, providing a low frequency noise pole. With

the use of 10nF for both CFF and CREF/BYP, output voltage

noise decreases to 30μVRMS when the output voltage is

set to 5V by a 5μA feedback resistor divider. If the current

in the feedback resistor divider is doubled, CFF must also

be doubled to achieve equivalent noise performance.

Higher values of output voltage noise are often measured if

care is not exercised with regard to circuit layout and test-

ing. Crosstalk from nearby traces induces unwanted noise

onto the LT3050’s output. Power supply ripple rejection

must also be considered. The LT3050 regulator does not

have unlimited power supply rejection and passes a small

portion of the input noise through to the output.

18

Using a feedforward capacitor (CFF) from VOUT to the ADJ

pin has the added benefit of improving transient response

for output voltages greater than 0.6V. With no feedforward

capacitor, the settling time will increase as the output

voltage is raised above 0.6V. Use the equation in Figure 2

to determine the minimum value of CFF to achieve a

transient response that is similar to 0.6V output voltage

performance regardless of the chosen output voltage

(See Figure 3 and Transient Response in the Typical Perf-

ormance Characteristics section).

During start-up, the internal reference will soft-start if a

reference bypass capacitor is present. Regulator start-

up time is directly proportional to the size of the bypass

capacitor, slowing to 6ms with a 10nF bypass capacitor

(See Start-up Time vs REF/BYP Capacitor in the Typical

Performance Characteristics section). The reference by-

pass capacitor is actively pulled low during shutdown to

reset the internal reference.

IN

OUT

VIN

LT3050-5

VOUT

CFF

COUT

SHDN ADJ

GND REF/BYP

CREF/BYP

3050 F02

( ) CFF

10nF •

10μA

IFB

DIVIDER

( ) IFB

DIVIDER

=

VOUT

R1+R2

10μA FOR FIXED VOUT

Figure 2. Feedforward Capacitor for Fast Transient Response

VOUT = 5V

COUT = 10μF

0

IFB-DIVIDER = 10μA

100pF

1nF

10nF

LOAD CURRENT

100mA/DIV

100μs/DIV

3050 F03

Figure 3. Transient Response vs Feedforward Capacitor

3050fa

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