LT3991-5_15 Datasheet, PDF (14/24 Pages) Linear Technology – 55V, 1.2A Step-Down Regulator with 2.8A Quiescent Current

LT3991/LT3991-3.3/LT3991-5
Applications Information
Table 4. Schottky Diodes. The Reverse Current Values Listed Are
Estimates Based Off of Typical Curves for Reverse Current
vs Reverse Voltage at 25°C.
I_R at V_R =
V_R I_AVE V_F at 1A V_F at 2A 20V 25°C
PART NUMBER (V) (A)
(mV)
(mV)
(µA)
On Semiconductor
MBR0520L
20 0.5
30
MBR0540
40 0.5
620
0.4
MBRM120E
20
1
530
595
0.5
MBRM140
40
1
550
20
Diodes Inc.
B0530W
30 0.5
15
B0540W
40 0.5
620
1
B120
20
1
500
1.1
B130
30
1
500
1.1
B140
40
1
500
1.1
B150
50
1
700
0.4
B220
20
2
500
20
B230
30
2
500
0.6
B140HB
40
1
1
DFLS240L
40
2
500
4
DFLS140
40 1.1
510
1
DFLS160
60
1
500
2.5
DFLS2100
100 2
770
860
0.01
B240
40
2
500
0.45
Central Semiconductor
CMSH1 - 40M 40
1
500
CMSH1 - 60M 60
1
700
CMSH1 - 40ML 40
1
400
CMSH2 - 40M 40
2
550
CMSH2 - 60M 60
2
700
CMSH2 - 40L 40
2
400
CMSH2 - 40
40
2
500
CMSH2 - 60M 60
2
700
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 LT3991 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 LT3991 due to their piezoelectric nature.
When in Burst Mode operation, the LT3991’s switching
frequency depends on the load current, and at very light
loads the LT3991 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3991
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 LT3991. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3991 circuit is plugged into a
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT3991’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 0.47μF capacitor will work well. Figure 3 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  3a)
is best. For outputs between 2.8V and 3V, use a 1μF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
3991fa
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