Linear Technology LT1374CFE Manuel utilisateur

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 474

4A, 500KHZ STEP DOWN SWITCHING REGULATOR

LT1374CFE

DESCRIPTION

Demonstration circuit 474 is a step-

down buck regulator using the

LT1374CFE 500kHz-switching regulator.

The high switching frequency allows

the use of small inductors and filter

components, making this circuit ideal

for space conscious systems.

0.8

0.85

0.9

0.95

012345

Load Current

Efficiency

Figure 1. Efficiency, 10V V

, 5V V

OUT

Figure 1 shows efficiency versus load

for a typical application. The best

efficiency is realized with a load

current between 700mA and 2 amps.

Full load efficiency is 87%. The

maximum allowed die temperature is

125°C. Figure 2 shows die temperature

vs. output load current with V

12V. At full load the die temperature

rise is 70°C indicating that the demo

is suitable for ambient temperatures

of up to 125°C – 70°C = 55°C.

0123

Load (Amps)

Tem perature (Degrees C)

Figure 2. Die Temperature vs Output

Current, V

= 12V

QUICK START PROCEDURE

Refer to Figure 3 for proper measure-

ment equipment setup and follow the

procedure below:

Connect input power to the Vin and

GND terminals. V

should be be-

tween 5V and 25V. The power source

should be capable of delivering

25W.

NOTE:

5V output operation requires

a minimum V

of 6V

Connect the output load to the Vout

and GND terminals.

Select desired output voltage using

the voltage selecting jumper

NOTE:

Choose 3.3V operation by mov-

ing the voltage selection jumper to

the 3.3V position. Demonstration

circuit 474 is optimized for 5V op-

eration, and when operating at 3.3V

out, high V

and light load some

instability may arise. Increasing

the output capacitance removes this

instability. Use a 330µF, 4V, low

ESR cap such as the “Poscap” or

“Oscon”.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 474

4A, 500KHZ STEP DOWN SWITCHING REGULATOR

To turn the circuit off, ground the

terminal labeled SHDN

To test the synchronization feature

of the demo board apply a sync sig-

nal to the sync terminal and note

the shift in the operating fre-

quency. The Sync signal should have

an amplitude greater than 2.2V,

duty cycle of between 10% and 90%

and a frequency of 580kHz–1000kHz

MEASUREMENT NOTES

Voltage measurements

The correct way to measure voltage is

at the terminal of the demo board.

Attempts to measure output voltage at

the load will result in a too low

output voltage due to wiring loss.

This is especially true at high cur-

rent. Needless to say any efficiency

calculation should be made using demo

circuit terminal voltages as opposed

to load or power source terminal

voltages.

Ripple Voltage Measurements

The ceramic capacitors used in this

demo board have almost no ESR and are

extremely effective filters. In order

to correctly measure output ripple

voltage correct technique is impor-

tant. The scope ground lead and

sheath should be removed and the out-

put ripple measured by placing the

scope probe directly across the out-

put Vout and GND pins as illustrated

in Figure 4.

5V TO 25V

SUPPLY

25W

–

–+

–

LOAD

Figure 3. Proper Measurement Equipment Setup

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 474

4A, 500KHZ STEP DOWN SWITCHING REGULATOR

VIN

GND

Figure 4. Scope Probe Placement for Measuring Output Ripple

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