August 2004 - Synchronous Switching Regulator Controller Allows Inputs up to 100V

DESIGN FEATURES
Synchronous Switching Regulator
Controller Allows Inputs up to 100V
by Greg Dittmer
Introduction
Industrial, automotive, and telecom
systems create harsh, unforgiving
environments that demand robust
electronic systems. In telecom systems
the input rails can vary from 36V to
72V, with transients as high as 100V.
In automotive systems the DC battery voltage may be 12V, 24V, or 42V
with load dump conditions causing
transients up to 60V.
Until now, no synchronous buck
(or boost) control IC has been capable
of operating at 100V, so solutions
have been limited to low-side drive
topologies that utilize expensive and
bulky transformers. The LTC3703 is a
100V synchronous switching regulator
controller that can directly step-down
high input voltages using a single
inductor, thus providing a compact
high performance power supply for
harsh environments.
Key Features for
High Voltage Applications
The LTC3703 drives external Nchannel MOSFETs using a constant
frequency, voltage mode architecture.
A high bandwidth error amplifier and
patented line feed forward compensation provide very fast line and load
transient response. Strong 1Ω gate
drivers allow the LTC3703 to drive
multiple MOSFETs for higher current
applications. A precise internal 0.8V
reference provides 1% DC accuracy.
The operating frequency is user programmable from 100kHz to 600kHz
and can also be synchronized to an
external clock for noise-sensitive applications. Selectable Pulse Skip Mode
operation improves light load efficiency. Current limit is user programmable
and utilizes the voltage drop across
the synchronous MOSFET to eliminate
the need for a current sense resistor.
A low minimum on-time allows high
input-to-output step-down ratios such
as 72V-to-3.3V at 200kHz. Shutdown
mode reduces supply current to 50µA.
An internal UVLO circuit guarantees
that the driver supply voltage is high
enough to sufficiently enhance the
MOSFETs before enabling the controller (UV+ = 8.7V, UV– = 6.2V). The
LTC3703 is available in a 16-pin narrow SSOP package or, if high voltage
100Ω
L1:
CIN1:
CIN2:
COUT:
SGND PGND
VISHAY IHLP5050EZ
SANYO 100MV68AX
TDK C4532X7R2A105M
OSCON 16SP270M
+
1
RC1
10k
RSET 30.1k 2
CC1
470pF
CC2
1000pF
RMAX 15k
R2
8.06k
1%
RC2
100Ω
CC3
2200pF
R1
113k
1%
22µF
25V
4
5
7
8
FB
IMAX
INV
RUN/SS
GND
SW
VCC
DRVCC
BG
BGRTN
VIN
36V TO 72V
12V
DB
BAS21
CIN2 1µF
100V X7R ×2
+
15
FSET
BOOST
LTC3703
3
14
COMP
TG
6
CSS
0.1µF
MODE/SYNC VIN
16
20k
Q1
FZT600
CB
0.1µF
13
10
9
RF
10Ω
L1
8µH
M2
Si7852DP
CDRVCC
10µF
CVCC
1µF
Figure 1. 36V–72V to 12V/5A synchronous step-down converter
Linear Technology Magazine • August 2004
CMDSH-3
M1
Si7852DP
12
11
CIN1
68µF
100V
COUT
270µF
16V
+
D1
MBR1100
VOUT
12V
5A
pin spacing is required, in a 28-pin
SSOP package.
Strong Gate Drivers
and Synchronous Drive
for High Efficiency
Because switching losses are proportional to the square of input voltage,
these losses can dominate in high
voltage applications with inadequate
gate drive. The LTC3703 has strong 1Ω
gate drivers that minimize transition
times and thus minimize switching
losses, even when multiple MOSFETs
are used for high current applications.
Dual N-channel synchronous drives
combined with the strong drivers results in power conversion efficiencies
as high as 96%.
The LTC3703 provides a separate
return pin for the bottom MOSFET
driver (see Figure 1), allowing the use
of a negative gate drive voltage in the
off state. In high voltage switching converters, the switch node dv/dt can be
many volts/ns, which pulls up on the
gate of the bottom MOSFET through
its Miller capacitance, especially in
applications with multiple MOSFETs.
If this Miller current, times the combined internal gate resistance of the
MOSFET plus the driver resistance,
exceeds the threshold of the MOSFET,
shoot-through will occur, degrading
efficiency. By using a negative supply
on this pin, the gate can be pulled
below ground when turning the bottom MOSFET off. This provides a few
extra volts of margin before the gate
reaches the turn-on threshold of the
MOSFET.
Fast Load Transient Response
The LTC3703 uses a fast 25MHz op
amp as an error amplifier. This allows the compensation network to
be optimized for better load transient
response. The high bandwidth of the
amplifier, along with high switching
frequencies and low value inductors,
13
DESIGN FEATURES
VOUT
5V/DIV
VOUT
50mV/DIV
VOUT
50mV/DIV
RUN/SS
5V/DIV
VIN
20V/DIV
IOUT
2A/DIV
50µs/DIV
VIN = 50V
VOUT = 12V
1A TO 5A LOAD STEP
IL
5A/DIV
IL
2A/DIV
1ms/DIV
VOUT = 12V
20µs/DIV
ILOAD = 1A
25V TO 60V VIN STEP
Figure 2. Load transient performance
Figure 3. Line transient performance
Figure 4. Short circuit performance
allow very high loop crossover frequencies. Figure 2 illustrates the transient
response of a 50V input, 12V output
power supply (1A to 5A load step).
equals the current limit. During the
transient period while the capacitor is
being discharged to the proper duty
ratio, a cycle-by-cycle comparator
guarantees that the peak inductor
current remains in control by keeping the top MOSFET off when the
VDS of the bottom MOSFET exceeds
the programmed limit by more than
50mV. The top MOSFET stays off until
the inductor current decays below the
limit (VDS < VIMAX). Figure 4 shows the
inductor current waveforms during a
short-circuit condition.
Figure 5 shows a peak efficiency
of almost 95% at 50V input and 93%
at 75V input. The loop is compensated for a 50kHz crossover frequency
which provides ~10µs response time
to load transients. The IC and driver
bias supply is derived from the 12V
output when the output is in regulation, improving the efficiency. During
startup or in a short circuit condition
when the 12V output is not available,
Q1 provides this IC bias voltage from
the input supply.
For input voltages >30V, the practical choices for input capacitors are
limited to ceramics and aluminum
electrolytics. Ceramics have very low
ESR but bulk capacitance is limited,
while aluminum electrolytics have
higher bulk capacitance but with
much higher ESR. To meet RMS ripple
and bulk capacitance requirements,
using a combination of the two types
is usually the best approach and
also prevents excessive LC ringing
at the input (by lowering the high Q
of the ceramics) when the supply is
connected.
Another consideration in high
voltage converters such as this one
is the boost diode. Low leakage and
fast reverse recovery is essential. In
order to limit power dissipation when
this diode is reverse biased at high
voltage, ultra-fast reverse recovery
silicon diodes such as the BAS21 are
recommended.
Overcurrent Protection
Current limiting is very important in
a high voltage supply. Because of the
high voltage across the inductor when
the output is shorted, the inductor
can saturate quickly causing excessive currents to flow. The LTC3703
has current limit protection that
uses VDS-sensing of the bottom-side
MOSFET to eliminate the need for a
current sense resistor. The current
limit is user programmable with an
external resistor on the IMAX pin to
set the maximum VDS at which the
current limit kicks in.
Current limit works by discharging
the RUN/SS capacitor when the VDS
exceeds the programmed maximum.
The voltage on RUN/SS controls the
LTC3703’s maximum duty cycle, so
discharging this capacitor reduces
the duty ratio until the output current
14
Application Examples
36V–72V to 12V/5A
Synchronous Step-Down Regulator
The circuit shown in Figure 1 provides direct step-down conversion of
a typical 36V-to-72V telecom input
rail to 12V at 5A. With the 100V
maximum rating of the LTC3703 and
the MOSFETs, the circuit can handle
input transients of up to 100V without requiring protection devices. The
frequency is set to 250kHz to optimize
efficiency and output ripple.
100
VIN = 25V
VIN = 50V
95
EFFICIENCY (%)
Outstanding Line
Transient Rejection
The LTC3703 achieves outstanding
line transient response using a patented feedforward correction scheme.
With this circuit the duty cycle is adjusted instantaneously to changes in
input voltage without having to slew
the COMP pin, thereby avoiding unacceptable overshoot or undershoot. It
has the added advantage of making
the DC loop gain independent of input
voltage. Figure 3 shows how large
transient steps at the input have little
effect on the output voltage.
VIN = 75V
90
85
80
0
1
3
2
LOAD (A)
4
5
Figure 5. Efficiency of the circuit in Figure 1
48V-to-12V 360W
Isolated Power Supply
The circuit shown in Figure 6 can be
used to generate a loosely regulated
12V, 30A isolated power supply for a
Linear Technology Magazine • August 2004
DESIGN FEATURES
VIN
24V MMBZ5252B
100Ω
FZT600
10k
.56µH
D01813P-561HC
0.33µF
X7R
100V
×2
VCC
4.7µF
X5R
16V
13V MMBZ5243B
VIN
36V–53V
0.33µF
X7R
100V
VIN
BAS19
MODE/SYNC
30.1k 1%
100k 1%
1000pF
20k
FSET
BOOST
LTC3703
COMP
TG
FB
100k
1%
0.22µF
16V
Si7852DP
PA0486
SW
10Ω
IMAX
0.1µF
16V
1Ω
INV
VCC
N=2
DRVCC
RUN/SS
GND
•
•
22µH
CDEP105-0R2NC-50
N=1
•
Si7852DP
N=1
BG
BGRTN
0.1µF
X5R
16V
20k
N=2
1µF
X5R
16V
0.22µF
X5R
16V
1Ω
•
1Ω
VBUS
12V
22µF 30A
X5R
16V
x2
B180
B180
Si7884DP
Si7884DP
1µF
X7R
100V
×2
10k
200k
0.1µF
X5R
25V
+
VCC
2Ω
LT1797
–
0.1µF
16V
1µF
X7R
100V
×2
6.8V
MMBZ5235B
13V
MMBZ5243B
•
1k
1µF
X5R
16V
B140
1µF
X5R
16V
13V
MMBZ5243B
N=1
•
N=1
210k
10.0k
1%
Figure 6. 48-to-12V 360W isolated power supply
Linear Technology Magazine • August 2004
step-down ratio thus generates an output voltage equal to 0.25 • VIN. Running
open loop in this fashion eliminates the
need for complex feedback circuitry
100
95
EFFICIENCY (%)
360W intermediate bus that can then
be stepped down with additional buck
regulators to generate multiple low
voltage high current outputs. Using
this LTC3703-based DC/DC pushpull converter allows one to replace a
conventional power module at a lower
cost, smaller size and with superior
efficiency. The push-pull topology has
the advantage over forward/flyback
topology of less voltage stress on
the MOSFETs, allowing the use of a
lower voltage, lower RDS(ON) device to
improve efficiency.
The LTC3703 runs open loop using
the LT1797 amplifier to force 50%
duty cycle by driving the FB input of
the LTC3703. The 2-to-1 transformer
90
85
80
0
5
10
15
20
25
LOAD CURRENT (A)
30
Figure 7. Efficiency of Figure 6
35
over the isolation barrier. The second
stage step-down regulators can then
convert this intermediate bus voltage
to more tightly regulated outputs.
Figure 7 shows that an efficiency of
almost 94% can be achieved at 30A.
High Efficiency 12V-to-24V 5A
Synchronous Step-Up Regulator
Synchronous boost converters have a
significant advantage over non-synchronous boost converters in higher
current applications due to the low
power dissipation of the synchronous
MOSFET compared to that of the diode
in a non-synchronous converter. The
high power dissipation in the diode
requires a much larger package,
15
DESIGN FEATURES
100
R1
113k 1%
RSET 30.1k
RC1
10k
CC2
0.1µF
R2
3.92k
1%
CC1
100pF
15
FSET
BOOST
LTC3703
3
14
COMP
TG
5
6
CSS
0.1µF
+
2
4
RMAX 15k
MODE/SYNC VIN
DB
CMDSH-3
16
7
8
FB
SW
IMAX
VCC
INV
RUN/SS
GND
DRVCC
BG
BGRTN
SGND PGND
CB
0.1µF
X7R
13
M1
B240A
RF
10Ω
10
CDRVCC
10µF
X7R
9
L1:
COUT1:
COUT2:
CIN:
M1, M2:
CVCC
1µF
X7R
M2
COUT2
10µF
50V
X5R
×2
L1
3.3µH
12
11
VOUT
24V
5A
COUT1
220µF
35V
×3
CIN
180µF
20V
×2
+
VIN
10V
TO 15V
VISHAY IHLP5050EZ
SANYO 35MV220AX
UNITED CHEMICON NTS60X5RIH106MT
OSCON 20SP180M
Si7892DP
Figure 8. 12-to-24V, 5A synchronous boost converter
e.g. D2PAK, than the small SO8-size
package required for the synchronous
MOSFET to carry the same current.
Figure 8 shows the LTC3703
implemented as a synchronous 12Vto-24V/5A step-up converter that
achieves a peak efficiency over 96%.
The LTC3703 is set to operate as a
synchronous boost converter by simply connecting the INV pin to greater
than 2V. In boost mode, the BG pin
becomes the main switch and TG, the
synchronous switch; and aside from
this phase inversion, its operation is
similar to the buck mode operation. In
boost mode, the LTC3703 can produce
output voltages as high as 80V.
LT1941, continued from page 12
operate. The LT1941 can supply all
of them. Figure 9 shows a typical
SLIC. The two step-down switching
regulators provide the 3.3V and 1.8V
logic supplies. The inverting switching
regulator generates both the –21.6V
and –65V outputs using a charge
pump configuration. The PGOOD3
pin indicates if the –21.6V output is
useful because a switching regulator
without soft-start can trip a current
limited input supply during startup.
SLIC Power Supply
with Soft-Start
SLICs (Subscriber Line Interface
Circuits) require many voltages to
Conclusion
The LTC3703 provides a set of features
that make it an ideal foundation for a
high input voltage, high performance,
high efficiency power supplies. Those
features include: 100V capability,
synchronous N-channel drive, strong
gate drivers, outstanding line and load
VOUT1 (1.8V)
2V/DIV
VOUT2 (3.3V)
5V/DIV
VOUT3 (–21.6V)
50V/DIV
VOUT4 (–65V)
100V/DIV
IVIN(AVG) (–65V)
2A/DIV
5ms/DIV
Figure 10. SLIC start-up waveforms with soft-start
16
95
EFFICIENCY (%)
1
90
85
80
0
1
2
3
LOAD CURRENT (A)
4
5
Figure 9. Efficiency of Figure 8’s circuit
regulation, overcurrent protection,
and 50µA shutdown current. It is
particularly well suited to the harsh
environments presented by automotive, telecom, avionics and industrial
applications.
Its ability to directly step-down
input voltages from up to 100V without requiring bulky transformers, or
external protection, makes for low cost
and compact solutions.
The LTC3703 is also versatile—easily applied to a wide variety of output
voltages and power levels—mainly
due its low minimum on-time (which
allows low duty ratios), programmable
frequency, programmable current
limit, step-up or step-down capability,
and package options.
in regulation. Figure 10 shows the
output voltages and input current
during startup. Soft-start helps limit
the peak input current.
Conclusion
The LT1941 is a monolithic triple output switching regulator that has the
features and size to fit in a wide variety
of applications. The high switching
frequency allows the use of small
external components, minimizing the
total solution size. An internal op amp
allows the part to directly regulate
negative voltages. The wide input range
of 3.5V to 25V and soft-start feature
allow the LT1941 to regulate a broad
array of power sources. Power good
indicators and 2-phase switching help
the LT1941 to work with almost any
system.
Linear Technology Magazine • August 2004
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