Datasheet

9
LT1307/LT1307B
1307fa
quite evident, as is this particular device’s 575kHz switch-
ing frequency (nominal switching frequency is 600kHz).
Note, however, the absence of significant energy at 455kHz.
Figure 7’s plot reduces the frequency span from 255kHz to
655kHz with a 455kHz center. Burst Mode low frequency
ripple creates sidebands around the 575kHz switching
fundamental. These sidebands have low signal amplitude
at 455kHz, measuring –55dBmV
RMS
. As load current is
further reduced, the Burst Mode frequency decreases.
This spaces the sidebands around the switching fre-
quency closer together, moving spectral energy further
Trace B. Similarly, Figure 4 details the two circuits with a
load step from 5mA to 45mA with a 1.5V input.
The LT1307B also can be used in lower current applica-
tions where a clean, low ripple output is needed. Figure 5
details transient response of a single cell to 3.3V con-
verter, using an inductor value of 100µH. This high induc-
tance minimizes ripple current, allowing the LT1307B to
regulate without skipping cycles. As the load current is
stepped from 5mA to 10mA, the output voltage responds
cleanly. Note that the V
C
pin loop compensation has been
made more conservative (increased C, decreased R).
Figure 5. Increasing L to 100µH, Along with R
C
= 36k,
C
C
= 20nF and C
OUT
= 10µF, Low Noise Performance of
LT1307B Can Be Realized at Light Loads of 5mA to 10mA
10mA
5mA
I
L
V
IN
= 1.25V 1ms/DIV 1307 F05
V
OUT
= 3.3V
I
L
20mA/DIV
V
OUT
100mV/DIV
AC COUPLED
At light loads, the LT1307B will begin to skip alternate
cycles. The load point at which this occurs can be de-
creased by increasing the inductor value. However, output
ripple will continue to be significantly less than the LT1307
output ripple. Further, the LT1307B can be forced into
micropower mode, where I
Q
falls from 1mA to 50µA by
pulling down V
C
to 0.3V or less externally.
DC/DC CONVERTER NOISE CONSIDERATIONS
Switching regulator noise is a significant concern in many
communications systems. The LT1307 is designed to
keep noise energy out of the sensitive 455kHz band at all
load levels while consuming only 60µW to 100µW at no
load. At light load levels, the device is in Burst Mode,
causing low frequency ripple to appear at the output.
Figure 6 details spectral noise directly at the output of
Figure 1’s circuit in a 1kHz to 1MHz bandwidth. The
converter supplies a 5mA load from a 1.25V input. The
Burst Mode fundamental at 5.1kHz and its harmonics are
FREQUENCY (kHz)
1
OUTPUT NOISE VOLTAGE (dBmV
RMS
)
40
30
20
10
0
–10
–20
–30
–40
–50
–60
10 100 1000
1307 F06
RBW = 100Hz
Figure 6. Spectral Noise Plot of 3.3V Converter Delivering
5mA Load. Burst Mode Fundamental at 5.1kHz is 23dBmV
RMS
or 14mV
RMS
FREQUENCY (kHz)
255
OUTPUT NOISE VOLTAGE (dBmV
RMS
)
–20
–25
–30
–35
–40
–45
–50
–55
–60
–65
–70
455
1307 F07
655
RBW = 100Hz
Figure 7. Span Centered at 455kHz Shows –55dBmV
RMS
(1.8µV
RMS
) at 455kHz. Burst Mode Creates Sidebands 5.1kHz
Apart Around the Switching Frequency Fundamental of 575kHz
APPLICATIO S I FOR ATIO
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