LTC3252_15 Datasheet, PDF (7/12 Pages) Linear Technology – Dual, Low Noise, Inductorless Step-Down DC/DC Converter

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OPERATIO (Refer to Simplified Block Diagram)
Unlike conventional buck converters, the LTC3252’s inter-
nal oscillator is designed to produce a clock pulse whose
period is random on a cycle-by-cycle basis but fixed
between 1MHz and 1.6MHz. This has the benefit of spread-
ing the switching noise over a range of frequencies, thus
significantly reducing the peak noise. Figures 1 and 2
show how the spread spectrum feature of the LTC3252
significantly reduces the peak harmonic noise and virtu-
ally elliminates harmonics compared to a conventional
buck converter.
Spread spectrum operation is always enabled but is most
effective when the LTC3252’s outputs are out of Burst
Mode operation and the oscillator is running continuously
(see the Low Current Burst Mode Operation section).
Low Current Burst Mode Operation
To improve efficiency at low output currents, a Burst Mode
operation function is included in the LTC3252. An output
current sense is used to detect when the required output
current of both outputs drop below an internally set
LTC3252
threshold (30mA typ). When this occurs, the part shuts
down the internal oscillator and goes into a low current
operating state. The LTC3252 will remain in the low
current operating state until either output has dropped
enough to require another burst of current. The LTC3252
resumes continuous operation when the load on one or
both outputs exceeds the internally set threshold. Unlike
traditional charge pumps where the burst current is highly
dependant on many factors (i.e., supply, switch strength,
capacitor selection, etc.), the LTC3252’s burst current is
set by the burst threshold and hysteresis. This means that
the output ripple voltage in Burst Mode operation is
relatively consistent and is typically about 12mV with a
4.7µF output capacitor on a 1.5V output. The ripple voltage
amplitude is a direct function of the output capacitor size.
Burst Mode operation ripple voltage does increase slightly
at lower output voltages due to the increase in loop gain.
Users can counteract output voltage ripple increase through
the use of a slightly larger output capacitor. See Recom-
mended Output Capacitance guidelines of Figure 3.
Figure 1. Conventional Buck Input Noise
Figure 2. LTC3252 Input Noise
3252f
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