May 2001 LT1766: 1.5A Converter Runs Directly from 60V DC

DESIGN FEATURES
LT1766: 1.5A Converter Runs Directly
from 60V DC
by Mark W. Marosek
Introduction
With a maximum input voltage capability of 60V and peak switch current
of 1.5A, the LT1766 is targeted for
high voltage, industrial and automotive applications. The 60V maximum
operating input voltage makes the
LT1766 ideal for 12V, 24V and (future)
42V automotive systems, which must
survive load-dump input transients
as high as 60V. At high input-tooutput voltage differentials, power
dissipation is minimized by providing
very fast switch edge rates and by
using a BIAS pin connected to the
regulated output to supply internal
control circuitry. At low input-tooutput voltage dif ferentials, a
VIN
supply-boost capacitor is used to fully
saturate an internal 200mΩ power
switch to reduce DC switch losses.
The LT1766 runs at a fixed frequency
of 200kHz and can be externally synchronized. In addition, a pin is
provided for programming undervoltage lockout and placing the part into
micropower shutdown. The LT1766
maintains the 1.5A peak switch current over the full duty cycle range.
This is achieved by the use of patented circuitry that cancels the effect
of slope compensation on peak switch
current without affecting frequency
compensation.
The LT1766 is packaged in a 16-pin
SSOP (GN-16) package, which occupies the same space as an SO-8
package. The LT1766 provides a
wide range of functions, such as
undervoltage lockout and micropower
shutdown, along with external synchronization capability, without
sacrificing the power saving features
of a BIAS or BOOST pin. Also, the
GN-16 package, with its fused corner
pins, improves thermal performance,
reducing the θJA of the package from
120°C/W (typical SO-8) to 95°C/W.
4
RSENSE
2.9V BIAS
REGULATOR
INTERNAL
VCC
Σ
+
BIAS 10
–
RLIMIT
CURRENT
COMPARATOR
SLOPE COMP/
BOOST
ANTI-SLOPE COMP
6
200kHz
OSCILLATOR
SYNC 14
S
RS
R FLIP-FLOP
DRIVER
CIRCUITRY
Q1
POWER
SWITCH
SHUTDOWN
COMPARATOR
–
+
2 SW
FREQUENCY
FOLDBACK
5.5µA
0.4V
+
SHDN 15
LOCKOUT
COMPARATOR
–
×1
Q2
12 FB
+
2.38V
Q3
–
VC MAX
CLAMP
FOLDBACK
CURRENT
LIMIT
CLAMP
11 VC
ERROR AMPLIFIER
gm = 2000µ
1.22V
GND
Ω
1, 8, 9, 16
Figure 1. LT1766 block diagram
Linear Technology Magazine • May 2001
5
DESIGN FEATURES
AVX TPSD107M010R0100
AVX X7R 0805YC334KAT1A
MARCON TCCR70EA475M
AVX X7R 08055C332KAT
(207) 282-5111
(843) 946-0362
(847) 696-2000
(843) 946-0362
90
VOUT
5V/1.0A
R1
15.4k
D1
60V
1.5A
C4
1nF, 50V
C1:
C2:
C3:
C4:
C2
0.33µF, 16V
+
R2
4.99k
C1
100µF, 10V
SOLID
TANTALUM
80
70
60
50
0.00
D1: IR 10MQ060N
D2: ZETEX FMM914TA
L1: COILTRONICS UP2-680
0.25
0.50
0.75
1.00
LOAD CURRENT (A)
1.25
Figure 2b. Efficiency vs load current
for Figure 2a’s circuit
(310) 322-3331
(516) 543-7100
(561) 241-7876
LT1766 Features
Figure 2a. 42V to 5V step-down converter
Circuit Description
feature of the LT1766, referred to as
“anti-slope compensation” in the block
diagram, eliminates the effects of slope
compensation on the peak switch
current at high duty cycles without
affecting the frequency compensation. For high duty cycle applications,
this is a significant benefit over competing current mode converters with
similar peak switch current limits.
The LT1766 also includes an accurate 1.2V reference. This reference is
scaled to provide an accurate 2.38V
threshold on the SHDN pin, allowing
programming of undervoltage lockout; a second, lower threshold (0.4V)
allows shutdown with the input supply current reduced to 25µA. The part
can be externally synchronized to
frequencies up to 700kHz.
The LT1766 is optimized to provide
high efficiency for both high and low
input-to-output voltage differentials
in a buck-mode switching regulator
topology. The block diagram in Figure
1 shows all of the key functions of the
LT1766 step-down converter. A current mode architecture is used to
provide fast transient response and
good loop stability. Two feedback loops
exist to control the duty cycle of the
power switch using a transconductance error amplifier and a current
sense comparator that monitors
switch current on a cycle-by-cycle
basis. Nonlinear slope compensation
has been added to the current sense
signal to prevent the subharmonic
oscillation associated with current
mode control when the regulator duty
cycle is greater than 50%. An added
EFFICIENCY (%)
C3
4.7µF
100V
CERAMIC
100
L1 68µH
2
SW
BOOST
4
LT1766
VIN
10
15
BIAS
SHDN
12
14
FB
SYNC
11
1, 8, 9, 16
VC
GND
6
VIN = 42V
D2
D2
The LT1766 provides the following
features:
❏ Wide input range: 5.5V to 60V
❏ Constant 200kHz switching
frequency
❏ 1.5A peak switch current
❏ 0.2Ω saturating switch
❏ Peak switch current maintained
over full duty cycle range
❏ 25µA shutdown current
❏ 1.2V feedback reference
❏ Easily synchronizable
42V to 5V Buck Converter
The LT1766 was designed to address
the need for high efficiency over a
wide range of input voltages. A typical
high input voltage application, a 42V
to 5V converter, is shown in Figure
2a. To achieve high efficiency at high
input voltages, fast output-switch
edge rates are required; the LT1766
achieves edge rates of 1.2V/ns (rise)
and 1.7V/ns (fall). In addition, light
continued on page 17
C2
0.33µF, 16V
VIN = 12V
(TRANSIENTS
TO 60V)
C3
4.7µF
100V
CERAMIC
C4
1nF, 50V
100
L1 33µH
VOUT
5V/1.0A
90
R1
15.4k
D1
60V
1.5A
R2
4.99k
+
C1
100µF, 10V
SOLID
TANTALUM
EFFICIENCY (%)
2
SW
BOOST
4
LT1766
VIN
10
15
BIAS
SHDN
12
14
FB
SYNC
11
1, 8, 9, 16
VC
GND
6
80
70
60
C1:
C2:
C3:
C4:
AVX TPSD107M010R0100
AVX X7R 0805YC334KAT1A
MARCON TCCR70EA475M
AVX X7R 08055C332KAT
(207) 282-5111
(843) 946-0362
(847) 696-2000
(843) 946-0362
D1: IR 10MQ060N
D2: ZETEX FMM914TA
L1: SUMIDA CDHR6D38-330M
Figure 3a. 12V to 5V step-down converter
6
(310) 322-3331
(516) 543-7100
(847) 956-0667
50
0.00
0.25
0.50
0.75
1.00
LOAD CURRENT (A)
1.25
Figure 3b. Efficiency vs load current
for Figure 3a’s circuit
Linear Technology Magazine • May 2001
DESIGN FEATURES
TRACE A
VPROG = 4.5V
ILAMP = 9mARMS
TRACE A
VPROG = 4.5V
ILAMP = 9mARMS
TRACE B
VPROG = 1.125V
ILAMP = 1mARMS
TRACE B
VPROG = 1.125V
ILAMP = 1mARMS
1ms/DIV
100µs/DIV
Figure 3a. CCFL current for Figure 1’s circuit
Figure 3b. CCFL current for Figure 1’s circuit
(expanded time scale)
be set so that the LT1768 runs in
linear mode over the most widely used
operating range. In the example in
Figure 1, PWM mode runs from VPROG
= 1V to VPROG = 3.0V with CCFL current modulated between 0mA and
6mA. The PWM modulation frequency
is set to 220Hz by capacitor C3.
When combined, these five modes
of operation allow the creation of a
DC-controlled CCFL current profile
that can be tailored to each particular
display. With linear mode, CCFL current control over the most widely used
current range and with PWM mode at
the low end, the LT1768 enables wide
dimming ratios while maximizing
CCFL lifetimes.
LT1768 Fault Modes
Additional Features
The LT1768 also has fault detection
to ensure that lamp current and Royer
transformer ratings are not exceeded
under fault conditions. If the current
in either CCFL is less than 125µA for
a minimum of one PWM clock cycle,
the FAULT pin will be activated and
ICCFLMAX will be halved. This function
ensures that the maximum CCFL
current set by RMAX will not be
exceeded even under fault conditions.
If current in both CCFLs is less than
125µA, and the voltage on the VC pin
reaches its clamp value (indicating
an open-load condition) for a minimum of one PWM cycle, the gate drive
will be latched off. The latch can be
cleared by setting VPROG to 0V or
placing the LT1768 in shutdown
mode.
The LT1768 also contains a temperature-compensated 5V reference, an
undervoltage lockout feature for VIN <
8V, thermal shutdown and a logiccompatible shutdown pin that reduces
supply current to 50µA when activated. The LT1768 is available in a
GN16 package.
motive electronics. A typical 12V to
5V converter is shown in Figure 3a.
The key to achieving high efficiency
for low input-to-output voltage conversions is to use a saturating switch
design. A prebiased capacitor, connected between the BOOST and SW
pins, generates a boost voltage above
the input supply during switching.
Prebias is derived via a diode, D2,
from the regulated output. Driving
the switch from this boost voltage
allows the 200mΩ power switch to
fully saturate. The worst-case minimum BOOST voltage required to fully
saturate the internal power switch is
3V above input supply. This means
that an output voltage as low as 3.3V
is ideal for prebiasing of the boost
supply capacitor via a simple diode.
The efficiency for a 12V to 5V conversion, which peaks at 90%, is shown in
Figure 3b.
Authors can be contacted
at (408) 432-1900
LT1766, continued from page 6
loads at high input voltages require
minimal quiescent current to be drawn
from the input. A BIAS pin allows the
internal control circuitry to be supplied from the regulated output if
greater than 3V. At a duty cycle of
12%, for example, this technique
reduces input quiescent current from
4.5mA to approximately 1.8mA. The
efficiency for a 42V to 5V conversion
(>80%) is shown in Figure 2b.
12V to 5V Converter
The LT1766 is capable of excellent
efficiencies at lower input voltages;
combined with its ability to withstand
voltage transients up to 60V, this
makes it ideal for power conversion in
harsh environments such as autoLinear Technology Magazine • May 2001
Conclusion
The LT1768 high power CCFL controller, with its unique patented
dimming control scheme, accurate
minimum and maximum lamp currents and lamp fault protection
enables wide dimming ratios and
maximizes CCFL lifetime in single- or
multiple-lamp LCD displays.
References
1. Williams, Jim. November 1995. A Fourth Generation of LCD Backlight Technology. Linear
Technology Corporation, Application Note 65.
Conclusion
Having a peak switch current of 1.5A
and a maximum input voltage capability of 60V, the LT1766 provides an
ideal solution for 12V, 24V or (future)
42V automotive electronics in addition
to 48V nonisolated telecom applications, where input voltages as high as
60V must be accommodated. Packaged in a 16-pin SSOP (GN16), which
occupies the same space as an SO-8,
and running at a fixed frequency of
200kHz, the LT1766 provides a compact solution for high voltage
step-down power conversions.
17