DC1273A - Demo Manual

DEMO C IR C U IT 127 3
Q U IC K S TA R T GLT1938
U IDE
L T 19 3 8 EDD
2.2A , 25 V S te p -d o w n S w itc h in g R e g u la to r
DESCRIPTION
Demonstration circuit 1273 is a monolithic step-dow n
DC/DC sw itching regulator featuring LT1938. The demo
board is designed for 3.3V output from a 4.5V to 25V
input. The w ide input range of the LT1938 allow s a variety of input sources. The typical sources are automotive
batteries, w all adaptors and industrial supplies. The programmed 800 kHz sw itching frequency allow s the use of
small, low cost inductor and ceramic capacitors, resulting in low , predictable output ripple. The current mode
control scheme creates fast transient response and good
loop stability. The gate drive of the internal sw itch is
boosted to a voltage that is higher than the Vin to ensure
saturation of the sw itch. The LT1938’s integrated boost
diode reduces the parts count. The RUN/SS pin can be
used to set the part in micropow er shutdow n mode, re-
ducing the supply current to less than 1uA. The RUN/SS
pin can also be used to program soft start. In this mode,
the RUN/SS pin is driven through an external RC filter to
create a voltage ramp on this pin. The soft start function
reduces the input current surge during start-up.
The LT1938 datasheet gives a complete description of
the part,operation and application information. The datasheet must be read in conjunction w ith this quick start
guide for demo circuit 1273.
Design files for this circuit board are available. Call
the LTC factory.
,LTC and LT are registered trademarks ofLinear Technology Corporation. ThinSOT and
Pow erPath are trademarks ofLinear Technology Corporation.
Perform ance Sum m ary for Step-dow n Sw itching R egulator ( TA = 25oC )
PAR AM ETER FO R BU CK R EG ULATO R
M inimum Input Voltage
M aximum Input Voltage
O utput Voltage VOUT
M aximum O utput Current
Typical Sw itching Frequency
CO N DITIO N
VALU E
4.5V
25V
3.3V +/- 4%
2.2A
800kH z
Q U ICK STA RT PROCEDU RE
Demonstration circuit 1273 is easy to set up to evaluate
the performance of the LT1938. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below :
NOTE. W hen measuring the input or output voltage ripple,care must be
taken to avoid a long ground lead on the oscilloscope probe. M easure the
input or output voltage ripple by touching the probe tip directly across the
Vin or Vout and GND terminals. See Figure 2 for proper scope probe
technique.
1. Place JP1 on the R U N position:
2. W ith pow er off,connect the input pow er supply to Vin
and GND.
3. Turn on the pow er at the input.
NOTE. M ake sure that the input voltage does not exceed the maximum
input voltage.
4. Check for the proper output voltage.
NOTE. Ifthere is no output,temporarily disconnect the load to make sure
that the load is not set too high.
5. Once the proper output voltages is established, adjust
the load w ithin the operating range and observe the
output voltage regulation, ripple voltage, efficiency
and other parameters.
1
LT1938
_
A
+
V
_
+
+
V
+ _
_
A
+
_
Figure 1. Proper M easurem ent Equipm ent Setup
GND
VIN
Figure 2. M easuring Input or Output R ipple
2
_
Load
+
LT1938
3
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