DN373 - ThinSOT Micropower Buck Regulator Has Low Output Ripple

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ThinSOT Micropower Buck Regulator Has Low Output Ripple
Design Note 373
Keith Szolusha
Introduction
High voltage monolithic step-down converters simplify
circuit design and save space by integrating the highside power switch into the device. In most cases, the
switch is an n-type transistor (NMOS or NPN) with a
boot-strapped drive stage, requiring an external boost
diode and capacitor as well as the main catch diode,
complicating the applications circuit.
If the load is light, the IC alternates between micropower
and switching states to keep the output in regulation
(Figure 3a). Hysteretic mode allows the IC to provide
single switch-cycle bursts for the lowest possible lightload output voltage ripple (<20mV peak-to-peak from
12V to 3.3V at zero load.) During continuous switching
mode (Figure 3b) at higher current levels, the output
voltage ripple is even smaller (<10mV peak-to-peak).
The LT®3470 is a 40V step-down converter with the
power switch, catch diode and boost diode integrated
in a tiny ThinSOT™ package. The boosted NPN power
stage provides high voltage capability, high power
density and high switching speed without the cost and
space of external diodes.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks and ThinSOT is a trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
In continuous mode, the difference between the top and
bottom current comparator levels is about 150mA. Since
the switch only turns on when the catch diode current
falls below threshold, switching frequency decreases,
keeping switch current under control during start-up
or short-circuit conditions.
10/05/373_conv
2.2μF
50V
CERAMIC
X5R
VIN BOOST
SHDN VSW
BIAS
LT3470
0.22μF
6.3V
33μH
22pF
VOUT
3.3V
22μF 200mA
6.3V
CERAMIC
X5R
324k
1%
FB
200k
1%
GND
L: TOKO A003AS-270M=P3
VIN
4V TO 23V
OFF ON
2.2μF
25V
CERAMIC
X5R
VIN BOOST
SHDN VSW
BIAS
LT3470
0.22μF
25V
15μH
22pF
VOUT
1.8V
22μF 200mA
6.3V
CERAMIC
X5R
147k
1%
FB
332k
1%
GND
DN373 F01
L: MURATA LQH32CN150K53
Figure 1. Typical 3.3V and 1.8V Output Applications
90
1000
70
100
VIN = 12V
80 VOUT = 5V
60
10
50
40
POWER LOSS (mW)
Current Mode Control
The LT3470 uses a hysteretic current control scheme
in conjunction with Burst Mode operation to provide
low output ripple and low quiescent current while using a tiny inductor and ceramic capacitors. The switch
turns on until the current ramps up to the level of the
top current comparator, then turns off and the inductor current ramps down through the catch diode until
the bottom current comparator trips and the minimum
off-time has been met.
OFF ON
EFFICIENCY (%)
The LT3470 accepts an input voltage from 4V to 40V
and delivers up to 200mA to load. Micropower bias current and Burst Mode ® operation enable it to consume
merely 26μA with no load and a 12V input. Hysteretic
current mode control and single-cycle bursts result in
very low output ripple and stable operation with small
ceramic capacitors. The combination of small circuit
size, low quiescent current and 40V input makes the
LT3470 ideal for automotive and industrial applications.
VIN
4V TO 40V
5.5V
START-UP
1
30
20
10
0.1
0.1
1
10
LOAD CURRENT (mA)
100
DN373 F02
Figure 2. Efficiency and Power Loss vs Load Current
Design Flexibility with Integrated Boost Diode
A high side NPN power switch in a buck regulator design
needs a driver voltage that is at least a few volts higher
than the switch or input voltage. When there are no other
high voltage lines available, a bootstrapping method of
providing several volts of boost to the IC is required.
When there is at least 2.5V on the output, the boost
voltage can be most efficiently derived from the output.
If the output voltage is too low, 1.8V for example, the
boost voltage must be derived from the input.
via the BIAS pin. In most cases, this is a simple connection to either the input, when the output voltage is
below 2.5V, or the output, for output voltages above
2.5V. Additional Zener diode voltage drop in the boost
diode path or a transistor bias supply as shown in
Figure 4 protects the IC from BOOST pin overvoltage
when there is a wide input voltage range.
Conclusion
The LT3470 is a wide input voltage range, hysteretic
mode, fully integrated monolithic 300mA step-down
DC/DC converter. The onboard high side NPN power
switch, Schottky boost diode, and Schottky catch diode
combined with the small ThinSOT package and high
40V input voltage make this a simple and versatile IC
to use for many step-down applications with less than
200mA load current.
Integration of the high side bootstrapping boost diode
into the IC does not limit boost diode flexibility. Boost
diode flexibility such as the ability to connect to various
sources and/or the inclusion of a Zener blocking diode
is needed for both high and low output voltages with
and without wide input voltage ranges. The anode of
the boost diode can be connected to different sources
NO LOAD
200mA LOAD
VOUT
20mV/DIV
VOUT
20mV/DIV
IL
100mA/DIV
IL
100mA/DIV
1μs/DIV
1ms/DIV
10mA LOAD
150mA LOAD
VOUT
20mV/DIV
VOUT
20mV/DIV
IL
100mA/DIV
IL
100mA/DIV
DN373 F03a
5μs/DIV
1μs/DIV
(3a)
DN373 F03b
(3b)
Figure 3. (3a) Burst Mode Operation—Single Pulse Burst Mode Operation Has Only 20mVP-P Ripple.
(3b) Continuous Operation—Extremely Low Output Voltage Ripple
VIN
OFF ON
VOUT7
VIN BOOST
SHDN VSW
BIAS
LT3470
VOUT
VIN
BIAS DIRECT
VBOOST(MAX) = VINt
VIN
BIAS ZENER
VBOOST(MAX) = VINto7Z
VIN
FB
BIAS
GND
TRANSISTOR BIAS
VBOOST(MAX) = VIN + VZ – VBE
VIN
OFF ON
VIN BOOST
SHDN VSW
BIAS
LT3470
VOUT
BIAS DIRECT
VBOOST(MAX) = VIN + VOUT
VOUT
BIAS ZENER
VBOOST(MAX) = VIN + VOUT – VZ
VOUT
VOUTö7
FB
GND
DN373 F04
Figure 4. BIAS and BOOST Pin Connection Variations Provide Input and Output Voltage Range Flexibility
Data Sheet Download
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call (408) 432-1900
dn373f_conv LT/TP 1005 409K • PRINTED IN THE USA
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