1307BI View Datasheet(PDF) - Linear Technology
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1307BI Datasheet PDF : 20 Pages
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LT1307/LT1307B
APPLICATIO S I FOR ATIO
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.
0
RBW = 100Hz
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
255
455
FREQUENCY (kHz)
655
1307 F08
Figure 8. With Converter Delivering 20mA, Low Frequency
Sidebands Disappear. Noise is Present Only at the 575kHz
Switching Frequency
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
205
455
FREQUENCY (kHz)
705
LT1307 • F09
Figure 9. LT1307B at 5mA Load Shows No Audio Components
or Sidebands About Switching Frequency, 333kHz
Fundamental Amplitude is –10dBmV, or 316µVRMS
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.
1µF
1.25V
10µH MBR0520L
VIN
SW
SHDN
LT1307
VC
FB
GND
R
C
1M
3300Ω
10µF*
590k
VOUT
66Ω
50Ω
*CERAMIC
1307 • F10
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
1307fa
10
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