DN390 - Tiny Monolithic Step-Down Regulators Operate with Wide Input Range

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Tiny Monolithic Step-Down Regulators Operate with
Wide Input Range – Design Note 390
Kevin Huang
Introduction
Automotive batteries, industrial power supplies, distributed supplies and wall transformers are all sources of
wide-ranging high voltage inputs. The easiest way to step
down these sources is with a high voltage monolithic
step-down regulator that can directly accept a wide input
range and produce a well-regulated output. The LT® 3493
accepts inputs from 3.6V to 36V and LT3481 accepts
inputs from 3.6V to 34V. Both provide excellent line
and load regulation and dynamic response. The LT3481
offers a high efficiency solution over a wide load range and
keeps the output ripple low during Burst Mode® operation
while the LT3493 provides a tiny solution with minimal
external components. The LT3493 operates at 750kHz
and the LT3481 has adjustable frequency from 300kHz
to 2.8MHz. High frequency operation enables the use of
small, low cost inductors and ceramic capacitors.
Low Ripple and High Efficiency Solution over Wide
Load Range
The LT3481 is available in a 10-pin MSOP or a 3mm ×
3mm DFN package with an integrated 3.8A power switch
and external compensation for design flexibility. The
switching frequency can be programmed from 300kHz
to 2.8MHz by using a resistor tied from the RT pin to
ground. Figure 1 shows the LT3481 producing 3.3V at 2A
from an input of 4.5V to 34V. Figure 2 shows the circuit
efficiency at 12V input.
The high side bootstrapping boost diode is integrated
into the IC to minimize solution size and cost. When the
output voltage is at least 2.8V, the anode of the boost
diode can be connected to output. For output voltages
lower than 2.5V, the boost diode can be tied to the input.
For systems that rely on a well-regulated power source,
the LT3481 provides a power good flag that signals when
VOUT reaches 90% of the programmed output voltage. A
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
VOUT
3.3V
2A
VIN
4.5V TO
34V
0.47µF
220pF
VC
4.7µF
LT3481
EFFICIENCY (%)
BD
BOOST
4.7µH
SW
RT
30k
PG
BIAS
GND
60.4k
FB
90
10000
80
1000
70
100
60
10
1.0
50
VIN = 12V
VOUT = 3.3V
L = 4.7µH
F = 800kHz
324k
40
200k
22µF
DN390 TA01
Figure 1. 800kHz LT3481 DC/DC Converter
Delivers 2A at 3.3V Output
30
0.0001
0.001
0.01
0.1
LOAD CURRENT (A)
1
POWER LOSS (mW)
VIN
RUN/SS
OFF ON
05/06/390
The LT3481 offers low ripple Burst Mode operation that
maintains high efficiency at light load while keeping the
output voltage ripple below 15mVP-P. During Burst Mode
operation, the LT3481 delivers single cycle bursts of current to the output capacitor followed by sleep periods when
the output power is delivered to the load by the output
capacitor. Between bursts, all circuitry associated with
controlling the output switch is shut down reducing the
input supply current to 50µA. Figure 3 shows the inductor
current and output voltage ripple under single pulse Burst
Mode operation from 12V input to 3.3V output. As the load
current decreases to a no load condition, the percentage
of time that the LT3481 operates in sleep mode increases
and the average input current is greatly reduced resulting
in high efficiency. The LT3481 has a very low shutdown
current (less than 1µA) which significantly extends battery life in applications that spend long periods of time
in sleep or shutdown mode.
0.1
0.0
10
DN390 F02
Figure 2. Efficiency vs Load Current for Figure 1 Circuit
VIN
4.2V TO 36V
IL
0.5A/DIV
VIN
0.1µF 10µH
LT3493
ON OFF
SW
SHDN
32.4k
VSW
5V/DIV
GND
1µF
VOUT
3.3V
1.2A, VIN > 12V
0.95A, VIN > 5V
BOOST
22pF
FB
10µF
10k
DN390 F05
VOUT
10mV/DIV
5µs/DIV
Figure 5. LT3493 Wide Input Range
DC/DC Converter Application to 3.3V
DN390 F03
VIN = 12V; FIGURE 1 APPLICATION
ILOAD = 10mA
Figure 3. LT3481 Burst Mode Operation
at 10mA Load Current
resistor and capacitor on the RUN/SS pin programs the
LT3481’s soft-start, reducing maximum inrush current
during start-up. Figure 4 shows the circuit and start-up
waveform.
12V
0V
100k
LT3481
RUN/SS
0.1µF
GND
ESR electrolytic capacitor at the input is recommended
to damp the overshoot voltage. Refer to AN88 for details.
The SHDN pin can be driven through an external RC filter
to soft-start the LT3493.
Additional Features of LT3481 and LT3493
During short circuit, both parts offer cycle-by-cycle current limit and frequency foldback which decreases the
switching frequency when the output is low. The low frequency allows the inductor current to safely discharge.
Conclusion
The wide input ranges, small size and robust design of
the LT3493 and LT3481 make them an excellent choice
for a wide variety of step-down applications. Their high
input voltage, high power switch capability and excellent
package thermal conductivity add to their versatility.
IL
500mA/DIV
VOUT
2V/DIV
VIN = 12V
VOUT = 3.3V
fS = 800kHz
DN390 F04
Figure 4. Soft-Start of the LT3481
Small Solution Size
The LT3493 includes an internal 1.75A power switch in
a tiny 6-pin DFN package (2mm × 3mm). The current
mode control circuit with its internal loop compensation
eliminates external compensation components, minimizing component count and reducing the PC board space
to less than 50mm2. The LT3493’s reference voltage is
0.78V, making it suitable for applications with low output
voltage. Figure 5 shows an application of the LT3493
switching at 750kHz. This circuit generates 3.3V from an
input of 4.2V to 36V. In applications where the circuit is
plugged into a live input source through long leads, a high
Figure 6. LT3493 Demo Board
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2759
www.linear.com
Linear Technology Corporation
dn390f LT/TP 0506 409K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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