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

Diode Selection

The catch diode (D1 from Figure 1) 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 limit.

If transient input voltages exceed 40V, use a Schottky

diode with a reverse voltage rating of 45V or higher. If

the maximum transient input voltage is under 40V, use a

Schottky diode with a reverse voltage rating greater than

the maximum input voltage. Table 3 lists several Schottky

diodes and their manufacturers:

Table 3. Schottky Diodes

MANUFACTURER/

VR

PART NUMBER

(V)

On Semiconductor

MBRM140

40

MicroSemi

UPS140

40

Diodes Inc.

DFLS140L

40

1N5819HW

40

IAVE

VF at 1A

(A)

(mV)

1

550

1

450

1

550

1

450

Short and Reverse Protection

Provided the inductors are chosen to not go deep into

their saturation region at the maximum ILIMIT current the

LT3509 will tolerate a short circuit on one or both outputs.

The excess current in the inductor will be detected by the

DA comparator and the frequency will reduced until the

valley current is below the limit. This shouldn’t affect the

other channel unless the channel that is shorted is also

providing the boost supply to the BD pin. In this case the

voltage drop of the other switch will increase and lower

the efficiency. This could eventually cause the part to reach

the thermal shutdown limit. One other important feature

of the part that needs to be considered is that there is a

parasitic diode in parallel with the power switch. In normal

operation this is reverse biased but it could conduct if the

load can be powered from an alternate source when the

LT3509 has no input. This may occur in battery charging

applications or in battery backup systems where a bat-

tery or some other supply is diode ORed with one of the

LT3509 regulated outputs. If the SW pin is at more than

about 4V the VIN pin can attain sufficient voltage for LT3509

control circuitry to power-up to the quiescent bias level

and up to 2mA could be drawn from the backup supply.

This can be minimized if some discrete FETs or open-drain

buffers are used to pull down the RUN/SS pins. Of course

the gates need to be driven from the standby or battery

backed supply. If there is the possibility of a short circuit

at the input or just other parallel circuits connected to VIN

it would be best to add a protection diode in series with

VIN. This will also protect against a reversed input polarity.

These concepts are illustrated in Figure 12.

D2

VIN

VIN

BD

CIN

BOOST

LT3509

RUN/SS1 SW

RUN/SS2

DA

GND

CBOOST

L1

D1

COUT

VOUT

SLEEP

3509 F12

Figure 12. Reverse Bias Protection

3509fc

18

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