Datasheet

LT1940/LT1940L
11
1940fa
Table 2. Low-ESR Surface Mount Capacitors
Vendor Type Series
Taiyo Yuden Ceramic X5R, X7R
AVX Ceramic X5R, X7R
Tantalum TPS
Kemet Tantalum T491,T494,T495
Ta Organic T520
Al Organic A700
Sanyo Ta or Al Organic POSCAP
Panasonic Al Organic SP CAP
TDK Ceramic X5R, X7R
Catch Diode
Use a 1A Schottky diode for the catch diode (D1 in
Figure 2). The diode must have a reverse voltage rating
greater than the maximum input voltage. The ON Semi-
conductor MBRM120LT3 (20V) and MBRM130LT3 (30V)
are good choices; they have a tiny package with good
thermal properties. Many vendors have surface mount
versions of the 1N5817 (20V) and 1N5818 (30V) 1A
Schottky diodes such as the Microsemi UPS120 that are
suitable.
Boost Pin Considerations
The capacitor and diode tied to the BOOST pin generate a
voltage that is higher than the input voltage. In most cases
a 0.1µF capacitor and fast switching diode (such as the
CMDSH-3 or FMMD914) will work well. Figure 3 shows
three ways to arrange the boost circuit. The BOOST pin
must be more than 2.5V above the SW pin for full effi-
ciency. For outputs of 3.3V and higher the standard circuit
(Figure 3a) is best. For outputs between 2.8V and 3.3V,
use a small Schottky diode (such as the BAT-54). For lower
output voltages the boost diode can be tied to the input
(Figure␣ 3b). The circuit in Figure 3a is more efficient
because the BOOST pin current comes from a lower
voltage source. Finally, as shown in Figure 3c, the anode
of the boost diode can be tied to another source that is at
least 3V. For example, if you are generating 3.3V and 1.8V
and the 3.3V is on whenever the 1.8V is on, the 1.8V boost
diode can be connected to the 3.3V output. In any case,
you must also be sure that the maximum voltage at the
BOOST pin is less than the maximum specified in the
Absolute Maximum Ratings section.
Figure 3. Generating the Boost Voltage
V
IN
BOOST
GND
SW
V
IN
LT1940
(3a)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
OUT
MAX V
BOOST
≅ V
IN
+ V
OUT
V
IN
BOOST
GND
SW
V
IN
LT1940
(3b)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
IN
MAX V
BOOST
≅ 2V
IN
V
IN
BOOST
GND
SW
V
IN
LT1940
(3d)
1940 F03
V
OUT
MAX V
BOOST
– V
SW
≅ V
IN2
MAX V
BOOST
≅ V
IN2
MINIMUM VALUE FOR V
IN2
=
V
IN
+ 3V
V
IN2
>V
IN
+ 3V
D2
V
IN
BOOST
GND
SW
V
IN
LT1940
(3c)
1940 F03
V
OUT
V
BOOST
– V
SW
≅ V
IN2
MAX V
BOOST
≅ V
IN2
+ V
IN
MINIMUM VALUE FOR V
IN2
=
3V
D2
V
IN2
> 3V
C3
APPLICATIO S I FOR ATIO
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