LT3592IMSE datasheet(13/24 Pages) LINEAR

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LT3592

3592fc

at the LT3592 and to force this very high frequency switch-

ing current into a tight local loop, minimizing EMI. A 1μF

capacitor is capable of this task, but only if it is placed

close to the LT3592 and catch diode (see the PCB layout

section). A second precaution regarding the ceramic input

capacitor concerns the maximum input voltage rating of

the LT3592. A ceramic input capacitor combined with trace

or cable inductance forms a high quality (underdamped)

tank circuit. If the LT3592 circuit is plugged into a live

supply, the input voltage can ring to twice its nominal

value, possibly exceeding the LT3592’s voltage rating.

This situation can easily be avoided, as discussed in the

Hot Plugging Safety section. For more details, see Linear

Technology Application Note 88.

Output Capacitor

For most 2.2MHz LED applications, a 3.3μF or higher

output capacitor is sufﬁcient for stable operation. A

900kHz application should use a 4.7μF or higher output

capacitor. 400kHz applications require a 22μF or higher

output capacitor. The minimum recommended values

should provide an acceptable (if somewhat underdamped)

transient response, but larger values can always be used

when extra damping is required or desired.

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 stabilizes the LT3592’s

control loop. Because the LT3592 operates at a high fre-

quency,minimaloutputcapacitanceisnecessary.Inaddition,

the control loop operates well with or without the presence

of signiﬁcant output capacitor equivalent series resistance

(ESR). Ceramic capacitors, which achieve very low output

ripple and small circuit size, are therefore an option.

APPLICATIONS INFORMATION

You can estimate output ripple with the following

equation:

RIPPLE =

ΔILP−P

8• ƒ • C

OUT

where ΔILP-P is the peak-to-peak ripple current in the in-

ductor. The RMS content of this ripple is very low, so the

RMS current rating of the output capacitor is usually not

a concern. It can be estimated with the formula:

C(RMS) =

ΔIL

The low ESR and small size of ceramic capacitors make

them the preferred type for LT3592 applications. Not all

ceramic capacitors are the same, though. Many of the

higher value ceramic capacitors use poor dielectrics with

high temperature and voltage coefﬁcients. In particular,

Y5V and Z5U types lose a large fraction of their capacitance

with applied voltage and at temperature extremes.

Because loop stability and transient response depend on

the value of COUT, this loss may be unacceptable. Use X7R

and X5R types. Table 3 lists several capacitor vendors.

Figure 4 shows the transient response of the LT3592 when

switching between DIM and BRIGHT current levels with

two output capacitor choices. The output load is two series

Luxeon K2 Red LEDs, the DIM current is 50mA and the

BRIGHT current is 500mA, and the circuit is running at

900kHz. The upper photo shows the recommended 4.7μF

value. The second photo shows the improved response

resulting from a larger output capacitor.

Table 3. Capacitor Vendor Information

SUPPLIER

PHONE

FAX

WEBSITE

AVX

(803) 448-9411

(803) 448-1943

www.avxcorp.com

Sanyo

(619) 661-6322

(619) 661-1055

www.sanyovideo.com

Taiyo Yuden

(408) 573-4150

(408) 573-4159

www.t-yuden.com

TDK

(847) 803-6100

(847) 803-6296

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