March 2005 - 2A, 40V, SOT-23 Boost Converter Provides High Power in Small Spaces

DESIGN IDEAS
2A, 40V, SOT-23 Boost Converter
Provides High Power in Small Spaces
by Jeff Witt
Introduction
L1
1.8µH
VIN
2.3V TO 4.8V
C1
4.7µF
ON OFF
VIN
SOT-23 Boost with 2A Switch
Figure 1 shows the LT1935 generating
5V. Maximum load with VIN = 3.3V is
1A; from 2.5V the maximum load is
600mA. Note that the circuit efficiency
D1
SW
R1
29.4k
LT1935
SHDN
The small size eases system design
in many applications. Large digital
systems with dense layouts often
need point-of-load converters to generate secondary logic supplies. With
a minimum input voltage of 2.3V, the
LT1935 can convert power from 2.5V,
3.3V or 5V logic rails to a higher output
voltage. Even handheld electronics
such as cell phones, digital cameras
and music players require peak power
levels of several watts to drive LEDs,
audio amplifiers or large displays.
And space is always at a premium in
these products.
C3
150pF
C2
20µF
FB
R2
10k
GND
remains high even at low input voltage
and high load current. The LT1935’s
bipolar NPN power switch maintains
its low forward drop when the input
voltage is at its minimum of 2.1V (2.3V
max), unlike some MOS devices that
suffer increased RDS,ON with low gate
drive. The circuit in Figure 1 occupies
80mm2. Figure 2 shows a 12V circuit
that generates 600mA from 5V or
320mA from 3.3V. This higher power
circuit requires 100mm2 of PCB.
Soft-Start Reduces
Peak Input Current
During start-up, the input current of
an LT1935 circuit can reach 3A. This
can cause problems if the input source
is current-limited or if other circuits
are sensitive to disturbances at VIN.
The SHDN pin can be used to soft start
90
VOUT
5V
1A, VIN = 3.3V
0.6A, VIN = 2.5V
85
VIN = 3.3V
80
EFFICIENCY (%)
The LT1935 is a current mode boost
regulator in a tiny 5-lead ThinSOT
package. With its small package, high
switching frequency (1.2MHz) and
internal 2A, 40V power switch, the
LT1935 can deliver high power while
occupying very little circuit board
space. For instance, from a 5V input,
the LT1935 delivers 500mA average
and 600mA peak current at 12V (7.2W)
using only100mm2 of PCB.
The LT1935’s power switch drops
just 180mV at 2A, minimizing power
loss and temperature rise on the circuit board. Current mode control and
internal compensation allow the use
of small ceramic capacitors, resulting
in very low input and output ripple.
The input voltage range is 2.3V to
16V. Supply current is less than 1µA
in shutdown.
VIN = 2.5V
75
70
65
60
C1, C2: X5R OR X7R 6.3V
D1: ON SEMI MBRM120
L1: SUMIDA CR43-1R8
55
50
0
200
400
600
800
1000
1200
LOAD CURRENT (mA)
Figure 1. The LT1935 can deliver 1A at 5V from a 3.3V input in a circuit that occupies only 80mm2.
VIN
3V–11V
C1
4.7µF
ON OFF
VIN
90
D1
VOUT
12V
320mA, VIN = 3.3V
600mA, VIN = 5V
SW
47pF
LT1935
SHDN
R1
84.5k
FB
GND
D1: ON SEMI MBRM120
L1: SUMIDA CDRH5D28-4R2
R2
10k
C2
22µF
VIN = 5V
85
80
EFFICIENCY (%)
L1
4.2µH
VIN = 3.3V
75
70
65
60
55
50
0
100
200
300
400
500
600
700
LOAD CURRENT (mA)
Figure 2. The LT1935 delivers 600mA at 12V from a 5V input. High power density is achieved
using the internal 2A, 40V, 90mΩ switch and the high 1.2MHz operating frequency.
Linear Technology Magazine • March 2005
31
DESIGN IDEAS
the LT1935, reducing the maximum
input current during start-up.
The SHDN pin is driven through an
external RC filter to create a voltage
ramp at this pin. Figure 3 shows the
start-up waveforms with and without
the soft-start circuit. Without softstart, the input current peaks at ~3A.
With soft start, the peak current is
reduced to 1A. By choosing a large
RC time constant, the peak start-up
current can be reduced to the current
that is required to regulate the output,
with no overshoot. (The value of the
resistor should be chosen so that it
can supply 100µA when the SHDN
pin reaches 1.8V.)
ON OFF
VOUT
2V/DIV
IIN
1A/DIV
RUN
SHDN
GND
from cell phones to televisions. Power
requirements grow as well, but the
basic need for three supply voltages
C5
0.1µF
C3
1µF
D2B
D2A
16V
10mA
8V
450mA
SW
LT1935
SHDN
D1
R1
100k
C2
10µF
FB
GND
C1: X5R OR X7R 6.3V
C2, C4, C5, C6: X5R OR X7R 10V
C3: X5R OR X7R 25V
D1: MBRM120 OR EQUIVALENT
D2, D3: BAT-54S OR EQUIVALENT
L1: SUMIDA CDRH4D28-2R2
R2
18.7k
C6
0.1µF D3A
D3B
C4
1µF
–8V
10mA
Figure 4. This TFT-LCD supply produces three outputs using a single inductor.
LTC3709, continued from page 30
PLL monitors the switching of the two
channels and forces the switching
frequency of the second channel to
follow that of the first channel. The
interleaved operation of two channels
minimizes the input RMS current
and power loss along the input supply path.
A second PLL is provided for external
clock synchronization. The LTC3709
is synchronized by adjusting its ontime, indirectly adjusting its switching
frequency. When synchronized, the
LTC3709 combines the advantages
of constant frequency and constant
on-time architectures. The switch32
200µs/DIV
RUN
10k
0.22µF
SHDN
GND
Figure 3. The SHDN pin can be used to soft start the LT1935
reducing the peak input current during start up.
L1
2.2µH
VIN
VOUT
2V/DIV
20µs/DIV
TFT LCD display panels continue to
grow in size in every type of product
C1
4.7µF
RUN
5V/DIV
IIN
1A/DIV
More Power for
Larger LCD Panels
VIN
3.3V
RUN
5V/DIV
ing frequency stays constant despite
the changes of input voltage, output
voltage (if programmable) and load current. The LTC3709 can still respond to
load transients without clock latency
because of the indirect adjustment of
switching frequency during synchronization. The time constant of the PLL
is much longer than the load transient
duration, so the switching frequency of
the LTC3709 is temporarily altered to
take advantage of a constant on-time
architecture.
Other Features
The LTC3709 has a differential amplifier for remote sensing of both the high
remains. In Figure 4 the LT1935 produces three outputs using a single
inductor. From a 3.3V input, the boost
circuit produces the main output of
8V at 450mA. Two discrete charge
pumps produce the secondary outputs
of 16V and –8V.
Conclusion
By integrating a high frequency, current mode control with 2A, 90mΩ
switch in a SOT-23, the LT1935
delivers outsized power in a small
space. The 40V switch rating and the
wide input range (2.3V to 16V) allow
a wide variety input sources, output
voltages and circuit topologies, unlike
many regulators with restrictive 5V
ratings.
and low sides of the output voltage.
An output tracking function makes the
LTC3709 easy to use in multiple power
supplies applications. The LTC3709
also has a short-circuit shutdown
timer which is easily defeated.
Conclusion
The LTC3709 employs a constant ontime with PLLs and a valley current
control architecture. It has fast transient response, very short minimum
on-time and high efficiency from light
to full load. The LTC3709 is well suited
to high output current, high step-down
ratio applications.
Linear Technology Magazine • March 2005