LT3509 datasheet(13/24 Pages) LINER

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LT3509

3509f

APPLICATIONS INFORMATION

Figure 7. BD Tied to Regulated Output

COUT

VOUT

CBOOST

VBOOST –VSW VOUT

MAX VBOOST VIN VOUT

VOUT 3V

VIN

3509 F07

LT3509

GND

VIN

BOOST

CIN

Boost Pin Considerations

Figure 7 through Figure 9 show several ways to arrange

the boost circuit. The BOOST pin must be more than 2.0V

above the SW pin for full efﬁciency. For outputs of 3.3V

and higher, the standard circuit Figure 7 is best. For lower

output voltages, the boost diode can be tied to the input

Figure 8. The circuit in Figure 7 is more efﬁcient because

the boost pin current comes from a lower voltage source.

Finally, as shown in Figure 9, the BD pin 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 SW pins is less than 30V.

Inductor Selection and Maximum Output Current

A good ﬁrst choice for the inductor value is:

MHz

OUT

() •

where VF is the voltage drop of the catch diode (~0.5V)

and L is in μH.

The inductor’s RMS current rating must be greater than the

maximum load current and its saturation current should

be at least 30% higher. For highest efﬁciency, the series

resistance (DCR) should be less than 0.15Ω. Table 2 lists

several vendors and types that are suitable.

The current in the inductor is a triangle wave with an average

value equal to the load current. The peak switch current

is equal to the output current plus half the peak-to-peak

inductor ripple current. The LT3509 limits its switch current

in order to protect itself and the system from over-current

faults. Therefore, the maximum output current that the

LT3509 will deliver depends on the switch current limit,

the inductor value and the input and output voltages.

COUT

VOUT

CBOOST

VBOOST –VSW VIN

MAX VBOOST 2VIN

CIN

VIN

3509 F08

LT3509

GND

VIN

BOOST

Figure 8. Supplied from VIN

COUT

VOUT

CBOOST

VBOOST –VSW VBD

MAX VBOOST VIN VBD

VBD 3V

CIN

VIN

VBD

3509 F09

LT3509

GND

VIN

BOOST

Figure 9. Separate Boost Supply

LT3509 Datasheet(PDF) 13 Page - Linear Technology

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