Datasheet

9
LTC3251/
LTC3251-1.2/LTC3251-1.5
32511215fb
Figure 1a. Conventional Buck Switching Converter
Figure 1b. Conventional Buck Converter Output Noise
Spectrum with 22µF Output Capacitor (I
O
= 500mA)
Figure 1c. Conventional Buck Converter Input Noise
Spectrum with 10µF Input Capacitor (I
O
= 500mA)
Figure 2a. LTC3251 Buck Converter
Figure 2b. LTC3251 Output Noise Spectrum
with 10µF Output Capacitor (I
O
= 500mA)
Figure 2c. LTC3251 Input Noise Spectrum
with 1µF Input Capacitor (I
O
= 500mA)
OPERATIO
U
(Refer to Block Diagram)
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,
SWIN
COMP
22µF
3251 F01a
V
OUT
4.7µH
FB
V
IN
GND
10µF 1µF
10nH*
*10nH = 1cm OF PCB TRACE
OUTIN
C1
+
C1
–
C2
+
C2
–
10µF
3251 F02a
V
OUT
LTC3251
FB
V
IN
GND
1µF
1µF1µF
1µF
10nH*
*10nH = 1cm OF PCB TRACE
–40
–50
NOISE (dBm)
–60
–70
–80
–90
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F01b
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
NOISE (dBm)
–40
–50
–60
–70
–80
–90
3251 F02b
NOISE (dBm)
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
–40
–50
–60
–70
–80
–90
3251 F01c
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
NOISE (dBm)
–40
–50
–60
–70
–80
–90
3251 F02c