Linear Technology LT1374CFE Quick Start Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 474

DESCRIPTION

4A, 500KHZ STEP DOWN SWITCHING REGULATOR

LT1374CFE

Demonstration circuit 474 is a step­down 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. 95

0. 9

Ef ficiency

0. 85

0. 8
012345

Load Cu rrent

Figure 1. Efficiency, 10V VIN, 5V V

OUT

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

of

IN

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.

80
60
40
20

0

Temper at ur e (Degrees C)

0123

Load (Amps)

4

Figure 2. Die Temperature vs Output

Current, V

= 12V

IN

Figure 1 shows efficiency versus load
for a typical application. The best
efficiency is realized with a load

QUICK START PROCEDURE

Refer to Figure 3 for proper measure­ment equipment setup and follow the
procedure below:

1.

Connect input power to the Vin and
GND terminals. V

should be be-

IN

tween 5V and 25V. The power source
should be capable of delivering
25W.

NOTE:

5V output operation requires

a minimum VIN of 6V

2.

Connect the output load to the Vout
and GND terminals.

3.

Select desired output voltage using
the voltage selecting jumper

NOTE:

Choose 3.3V operation by mov­ing the voltage selection jumper to
the 3.3V position. Demonstration
circuit 474 is optimized for 5V op­eration, and when operating at 3.3V
out, high V

and light load some

IN

instability may arise. Increasing
the output capacitance removes this
instability. Use a 330µF, 4V, low
ESR cap such as the “Poscap” or
“Oscon”.

1

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 474

4A, 500KHZ STEP DOWN SWITCHING REGULATOR

4.

To turn the circuit off, ground the
terminal labeled SHDN

5.

To test the synchronization feature
of the demo board apply a sync sig­nal to the sync terminal and note
the shift in the operating fre­quency. 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 cur­rent. 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 impor­tant. The scope ground lead and
sheath should be removed and the out­put ripple measured by placing the
scope probe directly across the out­put Vout and GND pins as illustrated
in Figure 4.

–

+

–

LOAD

+

+

–

+

–

–

+

+

–

5V TO 25V

SUPPLY

25W

Figure 3. Proper Measurement Equipment Setup

2

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 474

4A, 500KHZ STEP DOWN SWITCHING REGULATOR

VIN

GND

Figure 4. Scope Probe Placement for Measuring Output Ripple

3