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

or from RC. This model works well as long as the inductor

current ripple is not too low (ΔIRIPPLE > 5% IOUT) and the

loop crossover frequency is less than fSW/5. An optional

phase lead capacitor (CPL) across the feedback divider

may improve the transient response.

1.1S

VIN

COMP-

NODE

LT3509

260μS

CPL

R1

RC 75k

95pF

1.73M

VREF = 0.8V

R2

VOUT

COUT

3509 F10

Figure 10. Small-Signal Equivalent Circuit

Output Capacitor Selection

The output capacitor filters the inductor current to generate

an output with low voltage ripple. It also stores energy in

order to satisfy transient loads and stabilize the LT3509’s

control loop. Because the LT3509 operates at a high

frequency, minimal output capacitance is necessary. In

addition, the control loop operates well with or without

the presence of output capacitor series resistance (ESR).

Ceramic capacitors, which achieve very low output ripple

and small circuit size, are therefore an option.

You can estimate output ripple with the following

equations.

For ceramic capacitors where low capacitance value is

more significant than ESR:

VRIPPLE = ΔIL / (8 • fSW • COUT )

For electrolytic capacitors where ESR is high relative to

capacitive reactance:

VRIPPLE = ΔIL • ESR

where ΔIL is the peak-to-peak ripple current in the inductor.

The RMS content of this ripple is very low so the RMS

current rating of the output capacitor is usually not of

concern. It can be estimated with the formula:

IC(RMS) = ΔIL / 12

Another constraint on the output capacitor is that it must

have greater energy storage than the inductor; if the stored

energy in the inductor transfers to the output, the resulting

voltage step should be small compared to the regulation

voltage. For a 5% overshoot, this requirement indicates:

COUT > 10 •L •(ILIM / VOUT)2

The low ESR and small size of ceramic capacitors make

them the preferred type for LT3509 applications. Not all

ceramic capacitors are the same, however. Many of the

higher value capacitors use poor dielectrics with high

temperature and voltage coefficients. In particular, Y5V

3509fc

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