LTC3252

7

3252f

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

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

4.7

µFoutputcapacitorona1.5Voutput.Theripplevoltage

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.

OPERATIO

(Refer to Simplified Block Diagram)

Figure 1. Conventional Buck Input Noise

Figure 2. LTC3252 Input Noise