LT3973 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LT3973
Linear Technology
'LT3973' PDF : 24 Pages View PDF
LT3973/LT3973-3.3/LT3973-5
APPLICATIONS INFORMATION
Table 3. Recommended Ceramic Capacitor Vendors
MANUFACTURER
WEBSITE
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3973 due to their piezoelectric nature.
When in Burst Mode operation, the LT3973’s switching
frequency depends on the load current, and at very light
loads the LT3973 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3973
operates at a lower current limit during Burst Mode op-
eration, the noise is typically very quiet to a casual ear. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3973. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3973 circuit is plugged into a
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT3973’s rating. This situa-
tion is easily avoided (see the Hot Plugging Safely section).
greatly reduced resulting in high efficiency even at very
low loads. Note that during Burst Mode operation, the
switching frequency will be lower than the programmed
switching frequency. See Figure 3.
At higher output loads (above 90mA for the front page
application) the LT3973 will be running at the frequency
programmed by the RT resistor, and will be operating in
standard PWM mode. The transition between PWM and
low ripple Burst Mode is seamless, and will not disturb
the output voltage.
VSW
5V/DIV
IL
200mA/DIV
VOUT
10mV/DIV
5µs/DIV
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
ILOAD = 15mA
f = 600kHz
3973 F02
Figure 2. Burst Mode Operation
700
FRONT PAGE APPLICATION
600
500
400
Low Ripple Burst Mode Operation
To enhance efficiency at light loads, the LT3973 operates
in low ripple Burst Mode operation which keeps the output
capacitor charged to the proper voltage while minimizing
the input quiescent current. During Burst Mode opera-
tion, the LT3973 delivers single cycle bursts of current
to the output capacitor followed by sleep periods where
the output power is delivered to the load by the output
capacitor. Because the LT3973 delivers power to the output
with single, low current pulses, the output ripple is kept
below 10mV for a typical application. See Figure 2.
As the load current decreases towards a no load condi-
tion, the percentage of time that the LT3973 operates in
sleep mode increases and the average input current is
300
200
100
0
0 100 200 300 400 500 600 700
LOAD CURRENT (mA)
3973 F03
Figure 3. Switching Frequency in Burst Mode Operation
BOOST and BD Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see the
Block Diagram) are used to generate a boost voltage that
is higher than the input voltage. In most cases a 0.47µF
capacitor will work well. Figure 4 shows two ways to
arrange the boost circuit. The BOOST pin must be more
3973fb
For more information www.linear.com/LT3973
15
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