Linear Technology LT1374CFE User manual

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
IN
, 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
IN
of
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.
0
20
40
60
80
0123
Load (Amps)
Tem perature (Degrees C)
4
Figure 2. Die Temperature vs Output
Current, V
IN
= 12V
QUICK START PROCEDURE
Refer to Figure 3 for proper measure-
ment equipment setup and follow the
procedure below:
1.
Connect input power to the Vin and
GND terminals. V
IN
should be be-
tween 5V and 25V. The power source
should be capable of delivering
25W.
NOTE:
5V output operation requires
a minimum V
IN
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 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
IN
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”.
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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 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
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