Linear Technology DC1848A Manual

40V, 5A, 2MHz Step-Down Switching Regulator
Description
Demonstration circuit 1848A is a monolithic step-down DC/
DC switching regulator featuring the LT3976. The LT3976
is a compact, high efficiency, high speed monolithic step-
down switching regulator that consumes only 3.3μA of
quiescent current. The demo circuit is designed for 3.3V,
5A output from a 4.3V to 40V input. The switching fre-
quency 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 R
T
to set the LT3976 internal switching frequency at least
20% below the lowest synchronization input frequency.
Low ripple Burst Mode operation increases the efficiency
®
at the light load while keeping the output ripple below
15mV. The SYNC pin on the demo board is grounded by
default for low ripple burst mode operation.
The LT3976 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
voltage lockout. A low dropout voltage of 500mV is main-
tained when the input voltage drops below the programmed
performance summary
PARAMETER
Minimum Input Voltage
Maximum Input Voltage
Output Voltage V
OUT
Switching Frequency
Guaranteed Maximum Output Current I
Typical Efficiency
Typical Output Voltage Ripple
resistor (R5) should be chosen
Specifications are at T
CONDITIONS
V
IN
R
V
OUT IN
V
IN
V
IN
DEMO MANUAL DC1848A
with 3.3µA Quiescent Current
output voltage. During a short circuit fault, the LT3976
has current limit foldback to limit the power dissipation.
The demo board has an EMI filter installed. The EMI per-
formance of the demo board 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 V
pad is on the back of the demo board.
The LT3976 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 1848A. The LT3976 is assembled in a
16-lead plastic MSOP and 24-lead 5mm × 3mm QFN
packages 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.
= 25°C
A
= 4.3V ~ 40V
= 130k
T
= 4.3V ~ 40V
= 12V, I = 5A
OUT
= 12V, I = 5A
OUT
LT3976
, not V . V
EMI IN EMI
VALUE
4.3V
40V
3.3V ± 3%
400kHz ± 20%
5A
82.1%
20mV
dc1848af
1