Datasheet

LT3973/LT3973-3.3/LT3973-5
15
3973fa
applicaTions inFormaTion
Figure 2. Burst Mode Operation
Figure 3. Switching Frequency in Burst Mode Operation
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 situation
is easily avoided (see the Hot Plugging Safely section).
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 capaci-
tor. 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 condition,
the percentage of time that the LT3973 operates in sleep
mode increases and the average input current is 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 R
T
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.
V
OUT
10mV/DIV
V
SW
5V/DIV
I
L
200mA/DIV
5µs/DIV
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 5V
I
LOAD
= 15mA
f = 600kHz
3973 F02
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
LOAD CURRENT (mA)
0
0
SWITCHING FREQUENCY (kHz)
100
300
400
500
300
500 600
700
700
3973 F03
200
100 200 400
600
FRONT PAGE APPLICATION