August 2004 - 1.1A Boost Provides Soft-Start Capability in Tiny ThinSOT Package

DESIGN FEATURES
1.1A Boost Provides Soft-Start
Capability in Tiny ThinSOT Package
Introduction
by Brian Adolf
L1
2.7µH
C1
4.7µF
OFF ON
C3
0.047µF
4
D1
6
1
VIN
SW
SHDN
LT3467
5
SS
R1
402k
C4
3.3pF
VOUT
5V
765mA AT VIN = 4.2V,
540mA AT VIN = 3.3V,
360mA AT VIN = 2.6V
3
FB
R2
133k
GND
2
C2
15µF
C1, C2: X5R OR X7R, 6.3V
D1: ON SEMICONDUCTOR MBRM120
L1: SUMIDA CR43-2R7
95
90
VIN = 4.2V
85
EFFICIENCY (%)
Linear Technology’s new LT3467
step-up DC/DC converter provides
1.1A switch current limit in a SOT23—ample current in a small package.
It also features a soft-start feature to
limit the inrush current drawn from
the supply during startup. Figure 1
shows that the LT3467 can deliver up
to 540mA at 5V from an input of 3.3V
with 84% efficiency. The LT3467 is up
to 90% efficient for a single Li-Ion cell
to 5V boost converter.
The high switching frequency of 1.3
MHz means low cost and low height
inductors and capacitors can be used.
Moreover, the fixed frequency current
mode PWM architecture yields low
noise that is predictable and easy to
filter.
Application of the LT3467 is not
limited to boost configurations, but can
be used in a variety of other topologies
such as SEPIC and flyback converters. The LT3467 has an internal 1.1A
switch, and is capable of delivering up
to 40V output. It has a dedicated softstart pin: simply place an appropriately
valued external capacitor on that pin
to limit the inrush current by ramping
up the voltage slowly (typical startup
times are a few milliseconds).
The LT3467 is a pin-for-pin replacement of the LT1930 and LT1613, so
applications using these parts can be
easily updated to take advantage of the
soft-start feature, higher efficiency and
increased current capability.
VIN
2.6V TO
4.2V
80
VIN = 2.6V
VIN = 3.3V
75
70
65
60
55
50
100 200 300 400 500 600 700 800 900
IOUT (mA)
Figure 1. Single Li-Ion cell to 5V boost converter and its efficiency
If, during startup, a step-up DC/DC
converter’s switch is forced to turn on
for a shorter duration than normal,
the current in the inductor (and thus
from the supply) grows moderately.
During this time, the output voltage
is brought up to its final regulated
value much more slowly. Figures 2 and
3 show an example. Once the target
output voltage is reached, the switch
is no longer forced to short on-times,
and is allowed to act on its own so the
converter can regulate the output volt-
age under varying load conditions. This
function is known as soft-start.
To implement the soft-start feature
of the LT3467, simply place an external
capacitor on pin 5. Upon startup, this
capacitor is pulled high by an internal
250kΩ resistor. While the capacitor
is being pulled high, the duration the
switch is allowed to turn on is proportional to the voltage on the soft-start
capacitor. Thus the output voltage and
the supply current follow a segment
of the familiar RC exponential, before
Soft-Start
Normally, when a step-up DC/DC
converter is first turned on, the
output is low. This results in large
current spikes as the output is quickly
brought into regulation. Many applications with DC/DC converters use
a battery for VIN, or possibly another
voltage supply that can only provide
a limited amount of current, so these
large current spikes can collapse the
supply voltage.
Linear Technology Magazine • August 2004
VOUT
1V/DIV
VOUT
1V/DIV
ISUPPLY
0.5A/DIV
ISUPPLY
0.5A/DIV
0.1ms/DIV
Figure 2. Supply current of Figure 1 during
startup without soft-start capacitor
0.5ms/DIV
Figure 3. Supply current of Figure 1 during
startup with 47nF soft-start capacitor
5
DESIGN FEATURES
90
VIN
5V
C1
2.2µF
SHDN
4
D1
6
1
VIN
SW
SHDN
LT3467
5
SS
C3
0.047µF
VOUT
12V
270mA
R1
115k
FB
C4*
22pF
85
80
EFFICIENCY (%)
L1
4.7µH
C2
10µF
3
R2
13.3k
GND
2
75
70
65
60
55
C1: X5R OR X7R, 6.3V
C2: X5R OR X7R, 16V
D1: PHILIPS PMEG 2010
L1: SUMIDA CR43-4R7
*OPTIONAL
50
50
100
150
200
250
IOUT (mA)
300
350
Figure 4. 5V to 12V, 270mA step-up converter and its efficiency
the capacitor charges high enough that
the soft-start action goes away. Refer
to the data sheet for a more detailed
description of soft-start and a block
diagram explaining the operation of
the LT3467.
of bright light is occasionally useful,
such as in a camera flash for a cell
phone. An extra supply, VCC = 2.0V
±2% is required to reduce the power
dissipated in the sense resistor string.
Instead of placing the sense resistor
directly on the feedback pin, which will
servo to 1.255V and dissipate 1.255V
• IOUT(MAX), we use an extra 100µA to
level shift the sense resistor voltage.
By selecting R1 = 8.5kΩ, we reduce
the voltage by 0.85V, thus reducing
the power wasted in the sense resistor
chain to 0.405V • IOUT(MAX), less than a
third of the original power dissipated
in this resistor.
The current through the LED varies
according to:
Applications and Features
The LT3467 can be used in many
different applications where voltage
conversion is necessary. For example,
in a boost topology, any voltage up to
40V can be produced from a lower
voltage assuming the duty cycle of the
application is within the ability of the
part (see the data sheet for more information). Figure 4 shows a 5V to 12V
application capable of up to 270mA.
The efficiency peaks at 87%.
Figure 5 shows an application powering a Lumiled from a Li-Ion supply.
In this application, the output is the
current through the LED, which can
vary from 20mA for a “flashlight mode”
when the Flash signal is low, up to
280mA when the Flash signal is high
(at 2.5V). This is useful, for example,
when a small amount of light is normally required, but where a brief burst
VIN
2.6V TO
4.2V
IOUT =
6
By varying RS from 20Ω to 1.44Ω,
the current through the LED varies
from 20mA to 280mA. When switching
to this heavier load, the part reaches
VCC
2.0V ±2%
FLASH
OFF ON
C4, 0.01µF
R3
10k
R4
10k
4
5
Q1
C3
0.047µF
C1: X5R OR X7R 6.3V
C2: X5R OR X7R 10V
L1: SUMIDA CR43-2R7
1
VIN
SHDN
LT3467
SS
GND
2
SW
3
FB
R2
7.5k
Conclusion
D2
IOUT
20mA TO
280mA
C2
4.7µF
RS2
1.5Ω
R1
8.5k
RS1
20Ω
M1
D1: ON SEMICONDUCTOR MBRM120
D2: LUXEON LUMILED LXHL-LW6C
M1: INTERNATIONAL RECTIFIER IRLML2502
Figure 5. Li-Ion powered LED flash driver
6
R1
R1
+ 1.255 •
R2
R2
RS
where RS is the equivalent resistance of
RS1, RS2 and M1.
L1
2.7µH
C1, 4.7µF
1.255 – VCC •
current limit unless soft-start is used.
One solution is to briefly ground the
soft-start pin—which discharges the
soft-start cap—each time the Flash
signal goes high (shown in Figure 5).
Another interesting feature of the
LT3467 concerns its current limit. At
lower duty cycles, the switch is on for
less time and consumes less power
as a result. This fact can be taken
advantage of in a very useful way: by
making the current limit higher at
lower duty cycles, the part can deliver
more power than if it were simply fixed
at 1.1A. For example, at the lowest
duty cycle, around 10%, the minimum
current limit is guaranteed to be 1.4A
(typically it is around 1.8A.) In fact,
the current limit is guaranteed at all
operable duty cycles.
In addition to the features mentioned above, the LT3467 also comes
in a higher frequency version: the
LT3467A. This can be useful when a
switching frequency above 1.6MHz is
required, or when even smaller components must be used. The LT3467A
switches at 2.1MHz, and has a slightly
lower maximum duty cycle, but is
otherwise identical to the LT3467. See
the data sheet for more details.
FLASH
2.5V
0V
The LT3467 is an efficient boost
converter, both electrically, and in its
space requirements. Its operation at
a high fixed frequency allows external
components to be quite small, and with
the added soft-start feature, input supplies won’t be brought down when the
part is turned on. The LT3467 is ideal
in applications where high currents are
required in small spaces.
Linear Technology Magazine • August 2004
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