Linear LT3995 Demo Manual

Description
Demonstration circuit 1832A is a monolithic step-down
DC/DC switching regulator featuring the LT3995. The
LT3995 is a compact, high efficiency monolithic step-down
switching regulator that consumes only 2.7μA of quiescent
current. The demo circuit is designed for 3.3V, 3A output
from a 4.3V to 60V input. The switching frequency can be
programmed either via oscillator resistor or external clock
up to 2MHz. To synchronize to an external clock, move
JP2 to SYNC and apply the external clock to the SYNC
turret. The RT resistor (R5) should be chosen to set the
LT3995 internal switching frequency at least 20% below
the lowest synchronization input frequency.
Low ripple Burst Mode
®
ciency at the light load while keeping the output ripple
below 15mV. The SYNC pin on the demo board is grounded
by default by setting JP2 at RT FREQ for low ripple
Burst Mode operation. Figure 1 shows the demo board
efficiency at 12V input voltage.
The LT3995 is in shutdown when the EN pin is low and
active when the pin is high. The threshold of the EN pin
is accurate at 1.02V with 60mV of hysteresis. Users can
populate R7 and R8 to provide a programmable under
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60
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0 0.5 1
LOAD CURRENT (A)
Figure 1. LT3995 Efficiency vs Load Current,
12V to 3.3V Efficiency
IN OUT
operation increases the effi-
1.5 2 2.5 3
dc1832a F01
DEMO MANUAL DC1832A
60V, 3A, 2MHz Step-Down
Switching Regulator with
2.7µA Quiescent Current
voltage lockout. A low dropout voltage of 500mV is main-
tained when the input voltage drops below the programmed
output voltage. During a short-circuit fault, the LT3995
has current limit foldback to limit the power dissipation.
The demo board has an EMI filter installed. The EMI
performance of the demo board at 12V input voltage is
shown on Figure 2. The limit in Figure 2 is EN55022 Class
B. The figure shows the circuit passes the test with a wide
margin. To use the EMI filter, the input should be tied to
VEMI not V .
IN
The LT3995 data sheet gives a complete description of
the part, operation and application information. The data
sheet must be read in conjunction with this demo manual
for demo circuit 1832A. The LT3995 is assembled in a 16-
lead plastic MSOP package with an exposed pad for low
thermal resistance. Proper board layout is essential for
both proper operation and maximum thermal performance.
See the data sheet sections for details.
Design files for this circuit board are available at
http://www.linear.com/demo
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
70
TRACE 1 PASS
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40
30
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10
0
–10
–20
–30
START 150kHz
RES BW (CISPR) 9kHz
Figure 2. LT3995 Demo Board EMI Performance
(Switching Frequency = 300kHz, V
LT3995
VBW 62kHz STOP 30.00 MHz
SWEEP 717ms (6637pts)
dc1832a F02
= 12V)
IN
dc1832af
1