LT1940/LT1940L
U
W U U
APPLICATIO S I FOR ATIO
Table 2. Low-ESR Surface Mount Capacitors
Boost Pin Considerations
Vendor
Taiyo Yuden
AVX
Type
Series
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,
useasmallSchottkydiode(suchastheBAT-54).Forlower
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.
Ceramic X5R, X7R
Ceramic X5R, X7R
Tantalum
TPS
Kemet
Tantalum
Ta Organic
Al Organic
T491,T494,T495
T520
A700
Sanyo
Ta or Al Organic
Al Organic
POSCAP
SP CAP
Panasonic
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.
D2
D2
C3
C3
BOOST
LT1940
BOOST
LT1940
V
IN
V
V
IN
V
OUT
V
SW
V
SW
OUT
IN
IN
GND
GND
V
– V
BOOST
V
IN
V
– V
BOOST
V
BOOST
SW
OUT
BOOST
SW
IN
IN
MAX V
V
+ V
MAX V
2V
OUT
(3a)
(3b)
D2
D2
V
IN2
V
> 3V
IN2
>V + 3V
IN
BOOST
LT1940
BOOST
LT1940
C3
V
IN
V
V
IN
V
OUT
V
SW
V
SW
OUT
IN
IN
GND
GND
1940 F03
1940 F03
V
– V
BOOST
V
MAX V
MAX V
– V
SW
V
IN2
V
IN2
BOOST
SW
IN2
BOOST
BOOST
MAX V
V
+ V
IN2 IN
MINIMUM VALUE FOR V = 3V
MINIMUM VALUE FOR V = V + 3V
IN2
IN2 IN
(3c)
(3d)
Figure 3. Generating the Boost Voltage
1940fa
11
Linear [ Linear ]
