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LT3507

3507f

Electrolytic capacitors are also an option. The ESRs of

most aluminum electrolytic capacitors are too large to

deliver low output ripple. Tantalum, as well as newer,

lower-ESR organic electrolytic capacitors intended for

power supply use are suitable. Chose a capacitor with a

low enough ESR for the required output ripple. Because

the volume of the capacitor determines its ESR, both the

size and the value will be larger than a ceramic capacitor

that would give similar ripple performance. One beneﬁt

is that the larger capacitance may give better transient

response for large changes in load current. Table 2 lists

several capacitor vendors.

Table 2. Low ESR Surface Mount Capacitors

VENDOR

TYPE

SERIES

Taiyo-Yuden

Ceramic

AVX

Ceramic

Tantalum

TPS

Kemet

Tantalum

Tantalum Organic

Aluminum Organic

T491,T494,T495

T520

A700

Sanyo

Tantalum or

Aluminum Organic

POSCAP

Panasonic

Aluminum Organic

SP CAP

TDK

Ceramic

Diode Selection

The catch diode (D1 from Figure 2) conducts current only

during switch off time. Average forward current in normal

operation can be calculated from:

ID(AVG) =

IOUT VIN –VOUT

()

VIN

The only reason to consider a diode with a larger current

rating than necessary for nominal operation is for the

worst-case condition of shorted output. The diode current

will then increase to the typical peak switch current. Peak

reverse voltage is equal to the regulator input voltage.

Use a diode with a reverse voltage rating greater than the

input voltage. The programmable OVLO can protect the

diode from excessive reverse voltage by shutting down

the regulator if the input voltage exceeds the maximum

rating of the diode. Table 3 lists several Schottky diodes

and their manufacturers.

Table 3. Schottky Diodes

PART NUMBER

(V)

IAVE

(A)

VF AT 1A

(mV)

VF AT 2A

(mV)

On Semiconductor

MBRM120E

530

595

MBRM140

550

Diodes Inc

B120

500

B140

500

B220

500

B240

500

DFLS140L

550

DFLS240L

550

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 small ceramic capacitor and fast switching diode (such

as the CMDSH-3 or MMSD914LT1) will work well. The

capacitor value is a function of the switching frequency,

peak current, duty cycle and boost voltage; in general a

value of (0.1μF • 1MHz/fSW) works 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 ef-

ﬁciency. 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 BAT54).

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

this case, the 3.3V output cannot be set to track the 1.8V

output (see Output Voltage Tracking).

In any case, be sure that the maximum voltage at the

BOOST pin is less than 55V and the voltage difference

between the BOOST and SW pins is less than 25V.

The minimum operating voltage of an LT3507 applica-

tion is limited by the internal undervoltage lockout (4V

for Channel 1, 3V for Channels 2 and 3) and by the

APPLICATIONS INFORMATION

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