DC1167A - Demo Manual

DEMO CIRCUIT 1167
LT3685
QUICK START GUIDE
LT3685EMSE
2A, 38V Step-down Switching Regulator
with SYNC Function
DESCRIPTION
Demonstration circuit 1167 is a monolithic step-dow n
DC/DC sw itching regulator featuring LT3685. The LT3685
can be synchronized over a 250 KH z to 2 M H z range. The
demo board is designed for 5V output from a 6.3V to
38V input w ith transient up to 60V. The w ide input range
of the LT3685 allow s a variety of input sources. The typical sources are automotive batteries, w all adaptors and
industrial supplies.
start. In this mode, the R U N /SS pin is driven through an
external R C filter to create a voltage ramp on this pin. The
soft start function reduces the input current surge during
start-up.
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 LT3685’s
integrated boost diode reduces the parts count. The
R U N /SS pin can be used to set the part in micropow er
shutdow n mode,reducing the supply current to less than
1uA. The R U N /SS pin can also be used to program soft
N ote:It is best to ground the SYN C pin if the SYN C function is not being used.
The LT3685 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 1167.
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 Regulator (TA = 25oC )
PAR AM ETER FO R BU CK R EG U LATO 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
6.3V
38V
5V +/- 4%
2A
550kH z
QUICK START PROCEDURE
Demonstration circuit 1167 is easy to set up to evaluate
the performance of the LT3685. R efer 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 G N D.
3.
Turn on the pow er at the input.
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.
O nce the proper output voltage is established, adjust
the load w ithin the operating range and observe the
1
LT3685
output voltage regulation,ripple voltage,efficiency and
other parameters.
6.
An external clock can be added to the SYN C pin w hen
SYN C function is used. See synchronization section in
the datasheet for details.
A
A
+
+
+
V
+ _
_
_
V
_
Figure 1. Proper M easurem ent Equipm ent Setup
GND
VIN
Figure 2. M easuring Input or Output Ripple
2
+
Load
_
LT3685
3
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