LT1307CS8 datasheet(9/20 Pages) LINER

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LT1307/LT1307B

APPLICATIONS INFORMATION

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 – 55dBmVRMS. 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 100

µH. 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 VC pin loop compensation has been

made more conservative (increased C, decreased R).

Figure 5. Increasing L to 100

µH, Along with RC = 36k,

CC = 20nF and COUT = 10µF, Low Noise Performance of

LT1307B Can Be Realized at Light Loads of 5mA to 10mA

10mA

5mA

VIN = 1.25V

1ms/DIV

1307 F05

VOUT = 3.3V

20mA/DIV

VOUT

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 IQ falls from 1mA to 50µA by

pulling down VC 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 60

µW 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)

–10

–20

–30

–40

–50

–60

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 23dBmVRMS

or 14mVRMS

FREQUENCY (kHz)

255

–20

–25

–30

–35

–40

–45

–50

–55

–60

–65

–70

455

1307 F07

655

RBW = 100Hz

Figure 7. Span Centered at 455kHz Shows – 55dBmVRMS

(1.8

µVRMS) at 455kHz. Burst Mode Creates Sidebands 5.1kHz

Apart Around the Switching Frequency Fundamental of 575kHz

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