40V, 5A, 2MHz Step-Down Switching Regulator
Description
DEMO MANUAL DC1848A
LT3976
with 3.3µA Quiescent Current
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 stepdown 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 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 R
resistor (R5) should be chosen
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
voltage
lockout. A low dropout voltage of 500mV is main-
R8 to provide a programmable under
tained when the input voltage drops below the programmed
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 performance 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
, not VIN. V
EMI
EMI
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.
performance summary
PARAMETER CONDITIONS VALUE
Minimum Input Voltage 4.3V
Maximum Input Voltage 40V
Output Voltage V
Switching Frequency R
Guaranteed Maximum Output Current I
Typical Efficiency V
Typical Output Voltage Ripple V
OUT
OUT
Specifications are at TA = 25°C
VIN = 4.3V ~ 40V 3.3V ± 3%
= 130k 400kHz ± 20%
T
VIN = 4.3V ~ 40V 5A
= 12V, I
IN
= 12V, I
IN
= 5A 82.1%
OUT
= 5A 20mV
OUT
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DEMO MANUAL DC1848A
performance summary
87
86
85
84
83
EFFICIENCY (%)
82
81
80
0
Figure 1. LT3976 Efficiency vs Load Current Figure 2. LT3976 Demo Board EMI Performance,
20.5
LOAD CURRENT (A)
VIN = 12V
= 3.3V
V
OUT
= 400kHz
f
S
DC1848A F01
51.5 4.53 3.52.5 41
Quick start proceDure
Demonstration circuit 1848A is easy to set up to evaluate the performance of the LT3976. Refer to Figure 3 for
proper measurement equipment setup and follow the
procedure below:
Switching Frequency 400kHz
4. With power off, connect load from V
to GND.
OUT
5. Turn on the power at the input.
6. Check for the proper output voltage (3.3V).
NOTE: When measuring the input or output voltage ripple,
care must be taken to avoid a long ground lead on the
oscilloscope probe. Measure the input or output voltage
ripple by touching the probe tip directly across the V
or V
and GND terminals. See Figure 4 for the proper
OUT
IN
scope technique.
1. Place JP1 on the ON position.
2. Place JP2 on the RT FREQ position.
3. With power off, connect the input power supply to V
IN
and GND. Make sure that the input voltage does not
exceed 40V.
NOTE: If there is no output, temporarily disconnect the
load to make sure that the load is not set too high or
is shorted.
7. Once the proper output voltage is established, adjust
the load within the operating ranges and observe the
output voltage regulation, ripple voltage, efficiency and
other parameters.
8. An external clock
SYNC function is used (JP2 on the SYNC posi-
when
tion). Please make sure that the SYNC frequency is at
can be added to the SYNC terminal
least 20% higher than the set switching frequency. See
the data sheet section Synchronization.
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