LT3431 Просмотр технического описания (PDF) - Linear Technology

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LT3431

High Voltage, 3A, 500kHz Step-Down Switching Regulator

Linear Technology 

APPLICATIO S I FOR ATIO

LT3431

TO FREQUENCY

SHIFTING

ERROR

AMPLIFIER

1.4V

Q1

1.2V

R3

R4

1k

2k

BUFFER

Q2

TO SYNC CIRCUIT

LT3431

VSW

L1

OUTPUT

5V

R1

FB

+

C1

R2

VC GND

3431 F02

Figure 2. Frequency and Current Limit Foldback

will increase 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 5µH to 33µH. 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 LT3431 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 LT3431 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.

VOUT USING

47µF CERAMIC

OUTPUT

CAPACITOR

10mV/DIV

VOUT USING

100µF, 0.08Ω

TANTALUM

OUTPUT

CAPACITOR

VIN = 12V

VOUT = 5V

L = 10µH

1µs/DIV

3431 F03

Figure 3. LT3431 Output Ripple Voltage Waveforms.

Ceramic vs Tantalum Output Capacitors

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

sn3431 3431fs

9

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