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Fabricante

LT1940

Dual Monolithic 1.4A, 1.1MHz Step-Down Switching Regulator

Linear Technology 

LT1940/LT1940L

APPLICATIO S I FOR ATIO

Table 2. Low-ESR Surface Mount Capacitors

Vendor

Type

Series

Taiyo Yuden

Ceramic X5R, X7R

AVX

Ceramic X5R, X7R

Tantalum

TPS

Kemet

Tantalum

Ta Organic

Al Organic

T491,T494,T495

T520

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.

D2

D2

BOOST

C3

LT1940

VIN

VIN

SW

GND

VOUT

BOOST

C3

LT1940

VIN

VIN

SW

GND

VOUT

VIN2 > 3V

VBOOST – VSW ≅ VOUT

MAX VBOOST ≅ VIN + VOUT

(3a)

D2

BOOST

C3

LT1940

VIN

VIN

SW

GND

VBOOST – VSW ≅ VIN2

MAX VBOOST ≅ VIN2 + VIN

MINIMUM VALUE FOR VIN2 = 3V

(3c)

VOUT

1940 F03

VIN2

>VIN + 3V

VIN

VBOOST – VSW ≅ VIN

MAX VBOOST ≅ 2VIN

(3b)

D2

BOOST

LT1940

VIN

SW

GND

MAX VBOOST – VSW ≅ VIN2

MAX VBOOST ≅ VIN2

MINIMUM VALUE FOR VIN2 = VIN + 3V

(3d)

VOUT

1940 F03

Figure 3. Generating the Boost Voltage

1940fa

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