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LT3430

sn3430 3430is

with high input voltage. High frequency pickup will in-

crease and the protection accorded by frequency and

current foldback will decrease.

CHOOSING THE INDUCTOR

For most applications, the output inductor will fall into the

range of 5

µH to 47µH. Lower values are chosen to reduce

physical size of the inductor. Higher values allow more

output current because they reduce peak current seen by

the LT3430 switch, which has a 3A limit. Higher values

also reduce output ripple voltage.

When choosing an inductor you will need to consider

output ripple voltage, maximum load current, peak induc-

tor current and fault current in the inductor. In addition,

other factors such as core and copper losses, allowable

component height, EMI, saturation and cost should also

be considered. The following procedure is suggested as a

way of handling these somewhat complicated and con-

flicting requirements.

Output Ripple Voltage

Figure 3 shows a comparison of output ripple voltage for

the LT3430 using either a tantalum or ceramic output

capacitor. It can be seen from Figure 3 that output ripple

voltage can be significantly reduced by using the ceramic

output capacitor; the significant decrease in output ripple

voltage is due to the very low ESR of ceramic capacitors.

–

1.2V

BUFFER

VSW

GND

TO SYNC CIRCUIT

3430 F02

TO FREQUENCY

SHIFTING

OUTPUT

ERROR

AMPLIFIER

1.4V

LT3430

Figure 2. Frequency and Current Limit Foldback

APPLICATIO S I FOR ATIO

Output ripple voltage is determined by ripple current

(ILP-P) through the inductor and the high frequency

impedance of the output capacitor. At high frequencies,

the impedance of the tantalum capacitor is dominated by

its effective series resistance (ESR).

Tantalum Output Capacitor

The typical method for reducing output ripple voltage

when using a tantalum output capacitor is to increase the

inductor value (to reduce the ripple current in the induc-

tor). The following equations will help in choosing the

required inductor value to achieve a desirable output

ripple voltage level. If output ripple voltage is of less

Figure 3. LT3430 Output Ripple Voltage Waveforms.

Ceramic vs Tantalum Output Capacitors

20mV/DIV

VOUT USING

100

µF CERAMIC

OUTPUT

CAPACITOR

VOUT USING

100

µF, 0.08Ω

TANTALUM

OUTPUT

CAPACITOR

VIN = 40V

µs/DIV

3430 F03

VOUT = 5V

L = 22

µH

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