LT3991EMSE-3.3#PBF Linear Technology, LT3991EMSE-3.3#PBF Datasheet - Page 14

• Manufacturer Part Number: LT3991EMSE-3.3#PBF
• Description: IC, DC-DC CONV, 2MHz, MSOP10
• Manufacturer: Linear Technology

Specifications of LT3991EMSE-3.3#PBF

• Primary Input Voltage: 55V
• No. Of Outputs: 1
• Output Voltage: 3.3V
• Output Current: 1.2A
• No. Of Pins: 10
• Operating Temperature Range: -40°C To +125°C
• Peak Reflow Compatible (260 C): Yes
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
LT3991/LT3991-3.3/LT3991-5
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.
PART NUMBER
On Semiconductor
MBR0520L
MBR0540
MBRM120E
MBRM140
Diodes Inc.
B0530W
B0540W
B120
B130
B140
B150
B220
B230
B140HB
DFLS240L
DFLS140
DFLS160
DFLS2100
B240
Central Semiconductor
CMSH1 - 40M
CMSH1 - 60M
CMSH1 - 40ML
CMSH2 - 40M
CMSH2 - 60M
CMSH2 - 40L
CMSH2 - 40
CMSH2 - 60M
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
14
100
(V)
V
20
40
20
40
30
40
40
40
60
40
40
60
20
30
40
50
20
30
40
40
40
60
40
40
60
R
I
(A)
0.5
0.5
0.5
0.5
1.1
AVE
1
1
1
1
1
1
2
2
1
2
1
1
1
2
2
2
2
2
2
1
2
V
F
(mV)
620
530
550
620
500
500
500
700
510
500
770
500
700
400
at 1A
V
F
(mV)
595
500
500
500
860
500
550
700
400
500
700
at 2A
I
20V 25°C
R
at V
(µA)
0.01
0.45
0.4
0.5
1.1
1.1
1.1
0.4
0.6
2.5
30
20
15
20
1
1
4
1
R
=
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