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3509

Dual 36V, 700mA Step-Down Regulator

Linear Technology 

LT3509

APPLICATIONS INFORMATION

When the switch is off, the potential across the inductor is

the output voltage plus the catch diode forward voltage. This

gives the peak-to-peak ripple current in the inductor:

ΔIL

=

(1–

DC)

VOUT + VF

L • fSW

where:

DC = Duty Cycle

fSW = Switching Frequency

L = Inductor Value

VF = Diode Forward Voltage

The peak inductor and switch current is:

ISWPK

=

ILPK

=IOUT

+

ΔIL

2

To maintain output regulation, this peak current must be

less than the LT3509’s switch current limit ILIM. This is

dependent on duty cycle due to the slope compensation.

For ILIM is at least 1.4A at low duty cycles and decreases

linearly to 1.0A at DC = 0.8.

The theoretical minimum inductance can now be calcu-

lated as:

LMIN

=

1– DCMIN

f

•

VOUT + VF

ILIM –IOUT

where DCMIN is the minimum duty cycle called for by the

application i.e.:

DCMIN

=

VOUT(MAX) +

VIN(MIN) – VSW

VF

+ VF

There is a limit to the actual minimum duty cycle imposed

by the minimum on-time of the switch. For a robust design

it is important that inductor that will not saturate when

the switch is at its minimum on-time, the input voltage

is at maximum and the output is short circuited. In this

case the full input voltage, less the drop in the switch, will

appear across the inductor. This doesn’t require an actual

short, just starting into a capacitive load will provide the

same conditions. The Diode current sensing scheme will

ensure that the switch will not turn-on if the inductor

current is above the DA current limit threshold, which has

a maximum of 1.1A. The peak current under short-circuit

conditions can then be calculated from:

IPEAK

=

VIN

•

tON(MIN)

L

+

1.1A

The inductor should have a saturation current greater than

this value. For safe operation with high input voltages this

can often mean using a physically larger inductor as higher

value inductors often have lower saturation currents for

a given core size. As a general rule the saturation current

should be at least 1.8A to be short-circuit proof. However, it’s

generally better to use an inductor larger than the minimum

value. For robust operation at input voltages greater than

30V, use an inductor with a value of 4.2μH or greater, and

a saturation current rating of 1.8A or higher. The minimum

inductor has large ripple currents which increase core

losses and require large output capacitors to keep output

voltage ripple low. Select an inductor greater than LMIN

that keeps the ripple current below 30% of ILIM.

3509fc

14

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