LT1956 データシートの表示（PDF） - Linear Technology

部品番号

コンポーネント説明

メーカー

LT1956

High Voltage, 1.5A, 500kHz Step-Down Switching Regulators

Linear Technology 

APPLICATIO S I FOR ATIO

LT1956

TO FREQUENCY

SHIFTING

ERROR

AMPLIFIER

1.4V

1.2V

R3

1k

Q1

R4

2k

BUFFER

Q2

TO SYNC CIRCUIT

LT1956/LT1956-5

VSW

L1

OUTPUT

5V

R1

FB

+

C1

R2

VC GND

1956 F02

Figure 2. Frequency and Current Limit Foldback

with high input voltage. High frequency pickup will in-

crease and the protection accorded by frequency and

current foldback will decrease.

10mV/DIV

VOUT USING

22µF CERAMIC

OUTPUT

CAPACITOR

CHOOSING THE INDUCTOR

For most applications, the output inductor will fall into the

range of 5µH to 30µH. 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 LT1956 switch, which has a 1.5A 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 LT1956 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.

10mV/DIV

VOUT USING

100µF, 0.08Ω

TANTALUM

OUTPUT

CAPACITOR

VIN = 12V

VOUT = 5V

L = 15µH

1µs/DIV

1956 F03

Figure 3. LT1956 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 importance,

the subsequent suggestions in Peak Inductor and Fault

Current and EMI will additionally help in the selection of

the inductor value.

1956f

9

Share Link: