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LT3464

3464f

Capacitor Selection

The small size and low ESR of ceramic capacitors makes

them suitable for LT3464 applications. X5R and X7R types

are recommended because they retain their capacitance

over wider voltage and temperature ranges than other

types such as Y5V or Z5U. A 1

µF input capacitor and a

0.22

µF to 0.47µF output capacitor are sufficient for most

LT3464 applications. Always use a capacitor with a suffi-

cient voltage rating. Table 2 shows a list of several capaci-

tor manufacturers. Consult the manufacturers for more

detailed information and for their entire selection of related

parts.

Table 2. Recommended Ceramic Capacitor Manufacturers

MANUFACTURER

PHONE

URL

Taiyo Yuden

408-573-4150

www.t-yuden.com

AVX

843-448-9411

www.avxcorp.com

Murata

814-237-1431

www.murata.com

Kemet

408-986-0424

www.kemet.com

Output Voltage Ripple

Using low ESR capacitors will help minimize the output

ripple voltage, but proper selection of the inductor and the

output capacitor also plays a big role. The LT3464 pro-

vides energy to the load in bursts by ramping up the

inductor current, then delivering that current to the load.

If too large an inductor value or too small a capacitor value

is used, the output ripple voltage will increase because the

capacitor will be slightly overcharged each burst cycle. To

reduce this effect, a larger output capacitor may be used.

The LT3464 also includes an on-chip phase-lead capacitor

between the CAP pin and the FB pin to greatly reduce

ripple; however, certain applications can benefit from

additional capacitance in parallel with the integrated ca-

pacitor, which may be added externally between the CAP

and FB pins. Typical effective values range from 4.7pF to

20pF. Since the FB pin sits at a low voltage, be sure the

chosen capacitor has a sufficient voltage rating.

Setting Output Voltage and the

Auxiliary Reference Input

The LT3464 is equipped with both an internal 1.25V

reference and an auxiliary reference input. This allows the

user to select between using the built-in reference, and

supplying an external reference voltage. The voltage at the

CTRL pin can be adjusted while the chip is operating to

alter the output voltage of the LT3464 for purposes such

as display dimming or contrast adjustment. To use the

internal 1.25V reference, the CTRL pin must be held higher

than 1.5V, which can be done by tying it to VIN. When the

CTRL pin is held between 0V and 1.2V the LT3464 will

regulate the output such that the FB pin voltage is equal to

the CTRL pin voltage.

To set the output voltage, select the values of R1 and R2

according to the following equation (see Figure 4).

OUT

REF









–

Where VREF =1.25V if the internal reference is used, or

VREF = VCTRL if VCTRL is between 0V and 1.2V.

Choosing a Feedback Node

The top of the feedback divider may be connected to the

OUT pin or to the CAP pin (see Figure 4). Regulating the

OUT pin eliminates the output offset resulting from the

voltage drop across the output disconnect. However, in

the case of a short-circuit fault at the OUT pin, the LT3464

will switch continuously because the FB pin is low. While

operating in this open-loop condition, the rising voltage at

the CAP pin is limited only by the current limit of the output

disconnect. Given worst-case parameters this voltage

may reach 25V. When the short-circuit is removed, the

OUT pin will bounce up to the voltage on the CAP pin,

potentially exceeding the set output voltage until the

capacitor voltages fall back into regulation. While this is

harmless to the LT3464, this should be considered in the

context of the external circuitry if short-circuit events are

expected.

Regulating the CAP pin ensures that the voltage on the

OUT pin never exceeds the set output voltage after a short-

circuit event. However, this setup does not compensate

for the voltage drop across the output disconnect, result-

ing in an output voltage that is slightly lower than the

voltage set by the resistor divider. The next section dis-

cusses how to compensate for this drop.

APPLICATIO S I FOR ATIO

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