3509 データシートの表示（PDF） - Linear Technology

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3509

Dual 36V, 700mA Step-Down Regulator

Linear Technology 

LT3509

APPLICATIONS INFORMATION

Boost Pin Considerations

Figure 7 through Figure 9 show several ways to arrange

the boost circuit. The BOOST pin must be more than 2V

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.

BD

VIN

VIN

BOOST

LT3509

SW

CIN

GND DA

CBOOST

L1

D1

VOUT

COUT

VBOOST – VSW VOUT

MAX VBOOST VIN  VOUT

VOUT r3V

3509 F07

Figure 7. BD Tied to Regulated 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:

L

=

(VOUT

+

VF)

•

2.1MHz

fSW

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 overcurrent

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.

BD

BOOST

VIN

VIN

LT3509

SW

CIN

GND DA

CBOOST

L1

D1

VBOOST – VSW VIN

MAX VBOOST 2VIN

Figure 8. Supplied from VIN

VOUT

COUT

3509 F08

VBD

BD

BOOST

VIN

VIN

LT3509

SW

CIN

GND DA

CBOOST

L1

D1

VOUT

COUT

VBOOST – VSW VBD

MAX VBOOST VIN  VBD

VBD r3V

3509 F09

Figure 9. Separate Boost Supply

3509fc

13

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