Datasheet

10
LT1307/LT1307B
1307fa
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 –5dBmV
RMS
or 560µV
RMS
at the
575kHz switching frequency, and is below –60dBmV
RMS
for all other frequencies in the range. By combining Burst
Mode with fixed frequency operation, the LT1307 keeps
noise away from 455kHz.
V
IN
V
OUT
FB
1307 • F10
GND
SW
1µF
10µH
MBR0520L
C
R
590k
1M
50Ω
66Ω
3300Ω
1.25V
V
C
10µF*
*CERAMIC
SHDN
LT1307
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 V
C
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
OUTPUT VOLTAGE NOISE (dBmV
RMS
)
0
–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 316µV
RMS
FREQUENCY (kHz)
255
OUTPUT NOISE VOLTAGE (dBmV
RMS
)
0
–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
APPLICATIO S I FOR ATIO
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