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

In any step-down switcher the duty cycle when operating

in continuous, or fixed frequency, mode is dependent

on the step-down ratio. This is because for a constant

average load current the decay of the inductor current

when the switch is off must match the increase in inductor

current when the switch is on. The can be estimated by

the following formula:

DC

=

VOUT + VF

VIN − VSW + VF

where:

DC = Duty Cycle (Fraction of Cycle when Switch is On)

VOUT = Output Voltage

VIN = Input Voltage

VF = Catch Diode Forward Voltage

VSW = Switch Voltage Drop

Note: This formula neglects switching and inductor losses

so in practice the duty cycle may be slightly higher.

It is clear from this equation that the duty cycle will approach

100% as the input voltage is reduced and become smaller

as the input voltage increases. There are practical limits to

the minimum and maximum duty cycles for continuous

operation due to the switch minimum off and on times.

These are independent of operating frequency so it is clear

that range of usable duty cycle is inverserly proportional to

frequency. Therefore at higher frequency the input voltage

range (for constant frequency operation) will narrow.

The minimum duty cycle is given by:

DCMIN = fSW • tON(MIN)

where:

fSW = Switching Frequency

tON(MIN) = Switch Minimum On-Time

The minimum on time increases with increasing tempera-

ture so the value for the maximum operating temperature

should be used. See the Minimum On-Time vs Temperature

graph in the Typical Performance Characteristics.

The maximum input voltage for this duty cycle is given by:

VIN(MAX)

=

VOUT + VF

DCMIN

−

VF

+

VSW

Above this voltage the only way the LT3509 can maintain

regulation is to skip cycles so the effective frequency will

reduce. This will cause an increase in ripple and the switch-

ing noise will shift to a lower frequency. This calculation

will in practice drive the maximum switching frequency

for a desired step-down ratio.

VOUT

100mV/DIV

(AC COUPLED)

IL

0.5A/DIV

TIME 1μs/DIV

3509 F03

Figure 3. Continuous Mode

VOUT

100mV/DIV

(AC COUPLED)

IL

0.5A/DIV

TIME 1μs/DIV

3509 F04

Figure 4. Pulse Skipping

3509fc

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