LT3971A/LT3971A-5
16
3971af
APPLICATIONS INFORMATION
An additional consideration is reverse leakage current.
When the catch diode is reversed biased, any leakage
current will appear as load current. When operating under
light load conditions, the low supply current consumed
by the LT3971A will be optimized by using a catch diode
with minimum reverse leakage current. Low leakage
Schottky diodes often have larger forward voltage drops
at a given current, so a trade-off can exist between low
load and high load efficiency. Often Schottky diodes with
larger reverse bias ratings will have less leakage at a given
output voltage than a diode with a smaller reverse bias
rating. Therefore, superior leakage performance can be
achieved at the expense of diode size. Table 4 lists several
Schottky diodes and their manufacturers.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3971A due to their piezoelectric
nature. When in Burst Mode operation, the LT3971A’s
switching frequency depends on the load current, and
at very light loads the LT3971A can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT3971A operates at a lower current limit during
Burst Mode operation, 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 LT3971A. As
previously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3971A circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the LT3971A’s rating.
This situation is easily avoided (see the Hot Plugging
Safely section).
BOOST and BD Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age that is higher than the input voltage. In most cases
a 0.47F capacitor will work well. Figure 4 shows three
ways to arrange the boost circuit. The BOOST pin must
be more than 2.3V above the SW pin for best efficiency.
For outputs of 3V and above, the standard circuit (Figure 4a)
is best. For outputs between 2.8V and 3V, use a 1F boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BOOST pin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1F boost
capacitor (Figure 4b). For output voltages below 2.5V,
the boost diode can be tied to the input (Figure 4c), or to
another external supply greater than 2.8V. However, the
circuit in Figure 4a is more efficient because the BOOST pin
current comes from a lower voltage source. You must also
be sure that the maximum voltage ratings of the BOOST
and BD pins are not exceeded.
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
GND
LT3971A
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
D2
GND
LT3971A
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
GND
LT3971A
3971A FO4
(4a) For V
OUT
> 2.8V
(4b) For 2.5V < V
OUT
< 2.8V
(4c) For V
OUT
< 2.5V; V
IN(MAX)
= 27V
Figure 4. Three Circuits for Generating the Boost Voltage
