LTC3251_15 Datasheet, PDF (9/16 Pages) Linear Technology – 500mA High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converter

U
OPERATIO (Refer to Block Diagram)
4.7µH
V_IN
IN SW
10µF
FB
COMP
GND
10nH*
22µF
V_OUT
1µF
*10nH = 1cm OF PCB TRACE
3251 F01a
LTC3251/
LTC3251-1.2/LTC3251-1.5
V_IN
IN OUT
1µF _LTC3251
FB
C1⁺ C2⁺
1µF
C1^– C2^–
GND
10nH*
10µF
V_OUT
1µF
1µF
*10nH = 1cm OF PCB TRACE
3251 F02a
Figure 1a. Conventional Buck Switching Converter
Figure 2a. LTC3251 Buck Converter
–40
–50
–60
–70
–80
–90
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F01b
Figure 1b. Conventional Buck Converter Output Noise
Spectrum with 22µF Output Capacitor (I_O = 500mA)
–40
–50
–60
–70
–80
–90
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F01c
Figure 1c. Conventional Buck Converter Input Noise
Spectrum with 10µF Input Capacitor (I_O = 500mA)
–40
–50
–60
–70
–80
–90
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F02b
Figure 2b. LTC3251 Output Noise Spectrum
with 10µF Output Capacitor (I_O = 500mA)
–40
–50
–60
–70
–80
–90
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F02c
Figure 2c. LTC3251 Input Noise Spectrum
with 1µF Input Capacitor (I_O = 500mA)
Unlike conventional buck converters, the LTC3251’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 2b and 2c
are the input and output noise spectrums for the LTC3251
of Figure 2a with V_IN = 3.6V, V_OUT = 1.5V and I_OUT =
500mA. Note the significant reduction in peak output
noise (>20dBm) with only 1/2 the output capacitance and
the virtual elimination of input harmonics with only 1/10
the input capacitance. Spread spectrum operation is used
exclusively in “continuous” mode and for output currents
greater than about 50mA in Burst Mode operation.
Low Current Burst Mode Operation
To improve efficiency at low output currents, a Burst Mode
function is included in the LTC3251 family of parts. An
output current sense is used to detect when the required
output current drops below an internally set threshold
(50mA typ). When this occurs, the part shuts down the
internal oscillator and goes into a low current operating
state. The part will remain in the low current operating
state until the output voltage has dropped enough to
require another burst of current. When the output current
exceeds 50mA, the part will operate in continuous mode.
Unlike traditional charge pumps, where the burst current
is dependant on many factors (i.e., supply, switch strength,
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