LT1934IS6 Ver la hoja de datos (PDF) - Linear Technology

Número de pieza

componentes Descripción

Fabricante

LT1934IS6

Micropower Step-Down Switching Regulators in ThinSOT and DFN

Linear Technology 

LT1934/LT1934-1

APPLICATIONS INFORMATION

when used with ceramic capacitors will be lower and may

be acceptable.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT1934. A ceramic

input capacitor combined with trace or cable inductance

forms a high quality (under damped) tank circuit. If the

LT1934 circuit is plugged into a live supply, the input volt-

age can ring to twice its nominal value, possibly exceeding

the LT1934’s rating. This situation is easily avoided; see

the Hot Plugging Safely section.

Catch Diode

A 0.5A Schottky diode is recommended for the catch

diode, D1. The diode must have a reverse voltage rating

equal to or greater than the maximum input voltage. The

ON Semiconductor MBR0540 is a good choice; it is rated

for 0.5A forward current and a maximum reverse voltage

of 40V.

Schottky diodes with lower reverse voltage ratings usu-

ally have a lower forward drop and may result in higher

efficiency with moderate to high load currents. However,

these diodes also have higher leakage currents. This leakage

current mimics a load current at the output and can raise

the quiescent current of the LT1934 circuit, especially at

elevated temperatures.

BOOST Pin Considerations

Capacitor C3 and diode D2 are used to generate a boost

voltage that is higher than the input voltage. In most cases

a 0.1μF capacitor and fast switching diode (such as the

1N4148 or 1N914) will work well. Figure 2 shows two

ways to arrange the boost circuit. The BOOST pin must

be more than 2.5V above the SW pin for best efficiency.

For outputs of 3.3V and above, the standard circuit (Fig-

ure 2a) is best. For outputs between 2.8V and 3V, use a

0.22μF capacitor and a small Schottky diode (such as the

BAT-54). For lower output voltages the boost diode can be

tied to the input (Figure 2b). The circuit in Figure 2a is more

efficient because the BOOST pin current comes from a lower

voltage source. You must also be sure that the maximum

voltage rating of the BOOST pin is not exceeded.

The minimum operating voltage of an LT1934 applica-

tion is limited by the undervoltage lockout (~3V) and by

D2

BOOST

C3

LT1934

VIN

VIN

SW

GND

VBOOST – VSW VOUT

MAX VBOOST VIN + VOUT

(2a)

D2

VOUT

BOOST

C3

LT1934

VIN

VIN

SW

VOUT

GND

VBOOST – VSW VIN

MAX VBOOST 2VIN

(2b)

1934 F02

Figure 2. Two Circuits for Generating the Boost Voltage

the maximum duty cycle as outlined above. For proper

start-up, the minimum input voltage is also limited by the

boost circuit. If the input voltage is ramped slowly, or the

LT1934 is turned on with its SHDN pin when the output

is already in regulation, then the boost capacitor may not

be fully charged. Because the boost capacitor is charged

with the energy stored in the inductor, the circuit will rely

on some minimum load current to get the boost circuit

running properly. This minimum load will depend on input

and output voltages, and on the arrangement of the boost

circuit. The minimum load generally goes to zero once the

circuit has started. Figure 3 shows a plot of minimum load

to start and to run as a function of input voltage. In many

cases the discharged output capacitor will present a load

to the switcher which will allow it to start. The plots show

the worst-case situation where VIN is ramping very slowly.

Use a Schottky diode (such as the BAT-54) for the lowest

start-up voltage.

At light loads, the inductor current becomes discontinu-

ous and the effective duty cycle can be very high. This

reduces the minimum input voltage to approximately

300mV above VOUT. At higher load currents, the inductor

current is continuous and the duty cycle is limited by the

1934fe

11

Share Link: