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LT3570

3570fa

APPLICATIONS INFORMATION

Boost Diode Selection

A Schottky diode is recommended for use with the LT3570

inverter/boost regulator. The Microsemi UPS120 is a very

good choice. Where the input to output voltage differen-

tial exceeds 20V, use the UPS140 (a 40V diode). These

diodes are rated to handle an average forward current of

1A. For applications where the average forward current

of the diode is less than 0.5A, use an ON Semiconductor

MBR0520L diode.

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 MMSD914LT1) will work well. Fig-

ure 2 shows three ways to arrange the boost circuit. The

BOOST pin must be more than 2.5V above the SW pin for

full efﬁciency. For outputs of 3.3V and higher, the standard

circuit (Figure 2a) 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 2b). The circuit in Figure 2a is more

efﬁcient because the BOOST pin current comes from a

lower voltage source. Finally, as shown in Figure 2c, 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, be sure that the maximum voltage at the BOOST

pin is less than 60V and the voltage difference between

the BOOST and SW2 pins is less than 25V.

The minimum operating voltage of an LT3570 application

is limited by the undervoltage lockout (2.5V) and by the

maximum duty cycle. The boost circuit also limits the

minimum input voltage for proper start-up. If the input

voltage ramps slowly, or the LT3570 turns on when the

output is already in regulation, the boost capacitor may

not be fully charged. Because the boost capacitor charges

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 current generally goes to zero

once the circuit has started. Even without an output load

current, in many cases the discharged output capacitor will

present a load to the switcher that will allow it to start.

Switcher Frequency Compensation

The LT3570 uses current mode control to regulate the

output. This simpliﬁes loop compensation. In particular, the

LT3570 does not depend on the ESR of the output capaci-

tor for stability so you are free to use ceramic capacitors

to achieve low output ripple and small circuit size.

To compensate the feedback loop of the LT3570, a series

resistor-capacitor network should be connected from

the VC pin to GND. For most applications, a capacitor in

the range of 500pF to 4.7nF will sufﬁce. A good starting

value for the compensation capacitor, CC, is 1nF. The

VIN

BOOST

GND

LT3570

(2a)

VIN

BOOST

GND

LT3570

(2b)

VIN

BOOST

GND

LT3570

(2c)

VEXT

3570 F02

Figure 2. Boost Pin Conﬁgurations

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