Datasheet LT1307, LT1307B (Analog Devices) - 10

LT1307/LT1307B
U U W U APPLICATIO S I FOR ATIO
away from 455kHz. Figure 8 shows the noise spectrum of To eliminate the low frequency noise of Figure 6, the the converter with the load increased to 20mA. The LT1307 can be replaced with the LT1307B. Figure 9 LT1307 shifts out of Burst Mode operation, eliminating details the spectral noise at the output of Figure 1’s circuit low frequency ripple. Spectral energy is present only at using an LT1307B at 5mA load. Although spectral energy the switching fundamental and its harmonics. Noise is present at 333kHz due to alternate pulse skipping, all voltage measures – 5dBmV Burst Mode operation spectral components are gone. RMS or 560µVRMS at the 575kHz switching frequency, and is below – 60dBmV Alternate pulse skipping can be eliminated by increasing RMS for all other frequencies in the range. By combining Burst inductance. Mode with fixed frequency operation, the LT1307 keeps noise away from 455kHz.
FREQUENCY COMPENSATION
Obtaining proper values for the frequency compensation 0 RBW = 100Hz network is largely an empirical, iterative procedure, since ) –10 variations in input and output voltage, topology, capacitor RMS –20 value and ESR, and inductance make a simple formula –30 elusive. As an example, consider the case of a 1.25V to –40 3.3V boost converter supplying 50mA. To determine –50 optimum compensation, the circuit is built and a transient –60 –70 load is applied to the circuit. Figure 10 shows the setup. –80 OUTPUT NOISE VOLTAGE (dBmV –90 10µH MBR0520L VOUT –100255 455 655 FREQUENCY (kHz) VIN SW 1M 66Ω 1307 F08 SHDN 1µF LT1307
Figure 8. With Converter Delivering 20mA, Low Frequency
VC FB 3300Ω
Sidebands Disappear. Noise is Present Only at the 575kHz
1.25V GND
Switching Frequency
10µF* R 590k C 50Ω 0 1307 • F10 ) –10 *CERAMIC RMS –20
Figure 10. Boost Converter with Simulated Load
–30 –40 Figure 11a details transient response without compensa- –50 tion components. Although the output ripple voltage at a –60 1mA load is low, allowing the error amplifier to operate –70 wideband results in excessive ripple at a 50mA load. Some –80 OUTPUT VOLTAGE NOISE (dBmV kind of loop stabilizing network is obviously required. A –90 100k/22nF series RC is connected to the V –100 C pin, resulting 205 455 705 in the response pictured in Figure 11b. The output settles FREQUENCY (kHz) LT1307 • F09 in about 7ms to 8ms. This may be acceptable, but we can do better. Reducing C to 2nF gives Figure 11c’s response.
Figure 9. LT1307B at 5mA Load Shows No Audio Components or Sidebands About Switching Frequency, 333kHz
This is clearly in the right direction. After another order of
Fundamental Amplitude is –10dBmV, or 316
µ
VRMS
magnitude reduction, Figure 11d’s response shows some 1307fa 10