LT1307CS8 datasheet(10/20 Pages) LINER

10 / 20 page

LT1307/LT1307B

APPLICATIONS INFORMATION

To eliminate the low frequency noise of Figure 6, the

LT1307 can be replaced with the LT1307B. Figure 9

details the spectral noise at the output of Figure 1’s circuit

using an LT1307B at 5mA load. Although spectral energy

is present at 333kHz due to alternate pulse skipping, all

Burst Mode operation spectral components are gone.

Alternate pulse skipping can be eliminated by increasing

inductance.

FREQUENCY COMPENSATION

Obtaining proper values for the frequency compensation

network is largely an empirical, iterative procedure, since

variations in input and output voltage, topology, capacitor

value and ESR, and inductance make a simple formula

elusive. As an example, consider the case of a 1.25V to

3.3V boost converter supplying 50mA. To determine

optimum compensation, the circuit is built and a transient

load is applied to the circuit. Figure 10 shows the setup.

away from 455kHz. Figure 8 shows the noise spectrum of

the converter with the load increased to 20mA. The

LT1307 shifts out of Burst Mode operation, eliminating

low frequency ripple. Spectral energy is present only at

the switching fundamental and its harmonics. Noise

voltage measures – 5dBmVRMS or 560µVRMS at the

575kHz switching frequency, and is below – 60dBmVRMS

for all other frequencies in the range. By combining Burst

Mode with fixed frequency operation, the LT1307 keeps

noise away from 455kHz.

µH

MBR0520L

1307 • F10

590k

3300

Ω

µF

1.25V

µF*

Ω

VOUT

Ω

*CERAMIC

VIN

LT1307

SHDN

GND

Figure 10. Boost Converter with Simulated Load

Figure 11a details transient response without compensa-

tion components. Although the output ripple voltage at a

1mA load is low, allowing the error amplifier to operate

wideband results in excessive ripple at a 50mA load. Some

kind of loop stabilizing network is obviously required. A

100k/22nF series RC is connected to the VC pin, resulting

in the response pictured in Figure 11b. The output settles

in about 7ms to 8ms. This may be acceptable, but we can

do better. Reducing C to 2nF gives Figure 11c’s response.

This is clearly in the right direction. After another order of

magnitude reduction, Figure 11d’s response shows some

FREQUENCY (kHz)

205

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

LT1307 • F09

455

705

Figure 9. LT1307B at 5mA Load Shows No Audio Components

or Sidebands About Switching Frequency, 333kHz

Fundamental Amplitude is –10dBmV, or 316

µVRMS

FREQUENCY (kHz)

255

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

455

1307 F08

655

RBW = 100Hz

Figure 8. With Converter Delivering 20mA, Low Frequency

Sidebands Disappear. Noise is Present Only at the 575kHz

Switching Frequency

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