Dec 2002 Tiny 1.25MHz Monolithic Boost Regulator Has 1.5A Switch and Wide Input Voltage Range

DESIGN FEATURES
Tiny 1.25MHz Monolithic Boost
Regulator Has 1.5A Switch and
Wide Input Voltage Range by Keith Szolusha
Introduction
The LT1961 is a monolithic, currentmode, boost converter with a very
high switching frequency and an onboard monolithic, high-current power
switch. Because the power switch is
included in the tiny MSOP 8-pin exposed leadframe package, layout and
board space shrink dramatically for
most designs. The high 1.25MHz
switching frequency further reduces
the size requirement for the surrounding inductors and capacitors, by
allowing the use of chip inductors
and low profile capacitors and inductors. It operates within a 3V to 25V
input voltage range and can be synchronized up to 2MHz. The built in
0.2Ω, 35V switch allows up to 1.5A
switch current at high efficiency. In
battery-powered applications the extremely low 6µA shutdown current
maintains high efficiency and long
battery life. The shutdown pin also
provides an undervoltage lockout
option to limit battery source current
when low on charge.
The current-mode topology of the
IC allows for fast transient response
and simple loop compensation techniques that can take advantage of a
variety of ceramic output capacitors
to cover a wide range of output voltages. Ceramic capacitors have the
L1
6.8µH
D1
UPS120
VIN
5V
C3
2.2µF
CERAMIC
VIN
OPEN
OR
HIGH
= ON
VSW
LT1961
SHDN
SYNC
GND
C3: TAIYO YUDEN EMK316BJ225ML
C1: TDK C3225X5R1C106M
L1: TOKO A915AY-6R8M
advantage of smaller size and they
can handle high RMS ripple current,
the overriding requirement in sizing
the output capacitor for boost and
flyback topologies.
A High Efficiency 12V Boost
Converter with all Ceramic
Capacitors
Figure 1 shows a typical application
for the LT1961; a 12V boost converter
using only ceramic capacitors. This
circuit provides a regulated 12V output from a typical input voltage of 5V,
but can also be powered from any
input voltage between 3V and 12V.
EFFICIENCY (%)
70
20mV/
DIV
40
30
20
10
250ns/DIV
0
100
400
200
300
LOAD CURRENT (mA)
500
600
Figure 2. Efficiency of the circuit
shown in Figure 1 is as high 87%.
Linear Technology Magazine • December 2002
C1
10µF
CERAMIC
(408) 573-4150
(408) 392-1400
(847) 297-0070
Figure 1. 5V input, 12V output boost converter with ceramic input and output capacitors
80
0
C4
100pF
R2
10k
1%
*MAXIMUM OUTPUT CURRENT IS SUBJECT TO THERMAL DERATING.
90
50
FB
C2
6800pF
R3
6.8k
100
60
VC
R1
90.9k
1%
VOUT
12V
0.5A*
Figure 3. At 500mA load current, the
output ripple voltage of Figure 1’s
circuit is an extremely low 60mV
peak-to-peak.
The maximum load current changes
as a function of input voltage. Efficiency for the circuit is as high as 87%
for a 5V input, as shown in Figure 2.
Figure 3 shows the extremely low
60mVP–P output voltage ripple at
500mA load. Low-ESR ceramic capacitors and high switching frequency
help reduce the peak-to-peak output
voltage ripple, even in the normally
noisy boost configuration.
Low Profile 3.0mm SEPIC
Has Wide Input Voltage
Range
Although the LT1961 is configured as
a boost, or step-up, converter, its
internal low-side, asynchronous,
1.5A, 35V switch is versatile enough
to be used in other applications such
as a SEPIC or flyback. SEPIC solutions
typically use the basic single-output
flyback topology and a transformer
with its two windings capacitively
coupled together to generate a fixed
output voltage from an input voltage
that can be either above, equal to, or
below the output voltage. However,
much of the advantage of the LT1961’s
high frequency and correspondingly
small external components is lost by
25
DESIGN FEATURES
CCOUP
1µF
25V
X5R
CERAMIC
VIN
3V TO 20V
VIN
C3
2.2µF
25V
X5R
CERAMIC
900
800
D1
UPS140
700
VOUT
5V
VSW
L2
10µH
LT1961EMS8E
SHDN
SYNC
GND
VC
R1
31.6k
FB
C3
15nF
R3
1.0k
R2
10.0k
C4
100pF
C2
10µF
6.3V
X5R
CERAMIC
LOAD CURRENT (mA)
L1
10µH
600
500
400
300
200
100
0
0
5
10
15
VIN (V)
L1, L2: SUMIDA CDRH4D28-100 (847) 956-0667
90
VIN = 3V
VIN = 5V
80
EFFICIENCY (%)
low profile solution with a wide input
voltage range. The maximum load
current depends on the input voltage
(see Figure 5). The two 10µH inductors limit the ripple. However, each
inductor can be sized independently
to increase the output current capability. Figure 6 shows that typical
efficiency is over 70% and improves
as the input voltage approaches the
output voltage (5V). In this configuration, the two inductor currents are
summed in the switch during the
switch on-time and then through the
catch diode and the output during
the switch off-time. This effectively
doubles the switch and catch diode
losses compared to the typical boost
application. At high input voltages,
the duty cycle is low and the catch
diode conducts current for a greater
proportion of the overall time. At low
VIN = 12V
70
60
VIN = 20V
50
40
input voltages, the duty cycle is high
and the switch conducts current for a
greater proportion of the overall time.
The low switch VceSAT relative to the
forward voltage of the catch diode is
the reason for the increase in converter efficiency at lower input
voltages.
Dual Polarity Output SEPIC
Figure 7 is a 5V to 9V input to ±12V
dual polarity output converter. As
discussed above, the low-side boost
converter switch is ideal for flyback
converters that usually use a transformer to couple energy from the
primary (input) side to the secondary
(output) side. For dual polarity output
flyback converters, this transformer
has at least three windings coupled
together on the same core, one for the
primary side, and one for each outCCOUP1
1µF
16V
X5R
CERAMIC
L1
10µH
VIN
5V TO 9V
100
C1
2.2µF
16V/25V
X5R
CERAMIC
25
Figure 5. Maximum load current of the low
profile SEPIC shown in Figure 4 increases
with input voltage.
Figure 4. 3V–20V input, 5V output SEPIC saves space by using
two low profile inductors and all ceramic capacitors.
using a transformer, which is typically tall and occupies a large amount
of board space. Instead, Figure 4
shows a way to use two separate
inductors to create a low profile SEPIC
solution with less than 3.0mm
height—desirable for many of today’s
handheld and portable computer applications. The coupling capacitor
replaces the transformer core as the
low-impedance path for energy to
move from the primary to the secondary side. The coupling capacitor
charges up to a steady state voltage—
equal to the input voltage—and has
enough capacitance to maintain its
charge within 5% during switch on
and off-times while high ripple current passes back and forth between
the primary and secondary sides.
The circuit shown in Figure 4 is a
3V–20V input to 5V output low profile
SEPIC featuring the LT1961 with less
than 3.0mm height and all ceramic
capacitors. This is a tiny, low cost and
20
VIN
D1
B0530
VOUT1
12V
VSW
R1
90.9k
LT1961EMS8E
SHDN
SYNC
GND
VC
FB
•
C3
15µF
R3
1.0k
L2A
CTX15-1A
C4
100pF
R2
10.0k
C2
10µF
6.3V
X5R
CERAMIC
30
CCOUP2
1µF
25V
X5R
CERAMIC
20
10
0
0
100 200 300 400 500 600 700 800 900
LOAD CURRENT (mA)
Figure 6. Efficiency of the low profile SEPIC
shown in Figure 4 is as high as 76% and
increases as the input voltage approaches the
output voltage.
26
L1: SUMIDA CDRH4D28-100 (847) 956-0667
L2A, L2B: COILTRONICS CTX15-1A (561) 752-5000
D2
B0530
•
L2B
CTX15-1A
C5
10µF
16V
X5R
CERAMIC
VOUT2
–12V
Figure 7. Dual polarity output SEPIC for 5V–9V input to ±12V output. This circuit uses one low
profile inductor and one 1:1 off-the-shelf transformer for the two outputs.
Linear Technology Magazine • December 2002
DESIGN FEATURES
13.0
13.0
12.8
12.8
100
90
VIN = 7V
VOUT2 LOAD = 185mA
80
12.4
VIN = 9V
VOUT1 LOAD = 215mA
12.2
12.0
VIN = 7V
VOUT2 LOAD = 185mA
12.2
VIN = 5V
VOUT2 LOAD = 150mA
12.0
VIN = 7V
VOUT1 LOAD = 185mA
VIN = 5V
VOUT1 LOAD = 150mA
11.8
12.4
50
100
150
200
VOUT2 LOAD CURRENT (mA)
250
LTC4257, continued from page 10
Two additional features add flexibility to LTC4257 designs. An
open-drain PWRGD output indicates
that the voltage drop across the internal power MOSFET has dropped below
1.5V, indicating that any input capacitance has charged, the output
has reached its final value, and it is
safe to turn on the system. This helps
systems that draw the maximum input power stay below the inrush limits
at turn on. A SIG_DISA input allows
Linear Technology Magazine • December 2002
40
VIN = 9V
VOUT2 LOAD = 215mA
10
50
100
150
200
VOUT1 LOAD CURRENT (mA)
250
Figure 9. Cross-regulation of Figure 7’s
circuit with fixed VOUT2 load current and
varying VOUT1 load current.
MAXIMUM MATCHED LOAD CURRENT (mA)
put. Such a transformer, at the power
level required (1.5A total parallel current and 3.3µH to 22µH per winding),
negates most of the space savings
provided by the high frequency
LT1961. The solution is to capacitively couple energy from the input to
the output transformer like the single
output of the low-profile SEPIC using
two separate inductors. This not only
gets the job done, but reduces the
height of the inductive components
and provides layout flexibility. 1:1
transformers with only two windings
are more readily available and much
smaller than transformers with at
least three windings.
Cross-regulation is excellent in this
converter as shown in Figures 8 and
9. With only a single feedback pin, the
positive output voltage always maintains regulation, but the negative
output voltage (VOUT2) regulation
changes as a function of the differ-
VIN = 5V
VOUT2 LOAD = 150mA
50
0
0
Figure 8. Cross-regulation of Figure 7’s
circuit with fixed VOUT1 load current and
varying VOUT2 load current.
60
20
11.6
0
70
30
VIN = 9V
VOUT2 LOAD = 215mA
11.8
11.6
EFFICIENCY (%)
12.6
|VOUT2 (V)|
|VOUT2 (V)|
12.6
250
200
150
LOAD CURRENT = IVOUT1 = IVOUT2
100
50
0
5
6
7
VIN (V)
8
9
Figure 11. Maximum individual output load
current (with equal loads on VOUT1 and VOUT2)
for the circuit of Figure 7, at various input
voltages.
ence in the load currents of the two
outputs. As one output becomes
heavily loaded and other lightly
loaded, cross-regulation can become
slightly compromised due to differences in losses in the catch diodes
and inductors. Figure 8 shows that
extremely light loads on VOUT2 can
the PD to disable the 25k signature
resistance if desired, allowing it to opt
not to receive power from the PSE if it
is getting it from another source, such
as a wall transformer.
Conclusion
The LTC4257 contains virtually all of
the circuitry needed to connect a powered device to an IEEE 802.3af Power
Over Ethernet network. Signature,
classification, power switching, inrush, and fault protection are all
0
50
100
150
200
VOUT1 LOAD CURRENT (mA)
250
Figure 10. Efficiency of the circuit
shown in Figure 7 is typically 75%.
result in a loss of regulation, so a
preload may be required. However,
Figure 9 shows that VOUT1 can go to
zero load current without a loss in
regulation on VOUT2. The overall converter efficiency remains high for a
flyback or SEPIC-type design as shown
in Figure 10. The maximum load current on each output varies as a
function of the load on the other
output. Figure 11 shows the maximum matched load current (the same
load current on both outputs). If one
load current is decreased, the other
can be increased without exceeding
current limit.
Conclusion
The LT1961 is a tiny, monolithic,
1.5A boost converter with a wide input voltage range that can be used in
many applications. Its high switch
frequency and onboard switch help
minimize circuit size and cost.
included, thus simplifying the required circuitry between the input
transformers and the PD voltage regulator. The LTC4257 accomplishes all
of this in a space-saving 8-pin SO or
DFN package with only one external
component, a resistor to program the
class current (not needed for class 0).
Part 3 of this series will cover the
details of detection and classification
from the PSE end of the power network.
27
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