Datasheet LT1307, LT1307B (Analog Devices) - 9

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