April 2010 - 2MHz Dual Buck Regulator Operates Outside of AM Radio Band When Delivering 3.3V and 1.8V from 16V Input

2MHz Dual Buck Regulator Operates Outside of AM Radio
Band When Delivering 3.3V and 1.8V from 16V Input
Pit-Leong Wong
The number of microprocessor-based control units continues
to grow in both automobiles and industrial systems. Because
the amount of required processing power is also increasing,
even modest processors require a low voltage core supply
in addition to 3.3V or 5V memory, I/O and analog supplies.
In automotive systems, power comes
from the battery, with its voltage typically between 9V and 16V. Including cold
crank and double battery jump-starts, the
minimum input voltage may be as low
as 4V and the maximum up to 36V, with
even higher transient voltages. Likewise,
a 24V industrial supply may be as high
as 32V. With these high input voltages,
linear regulators cannot be used for supply currents greater than 200mA without
overheating the regulator. Instead, high
efficiency switching regulators must be
used to minimize thermal dissipation.
There are challenges in applying switching regulators in these systems. A small
circuit is desired, and certain operating
frequencies may be unacceptable. At
high step-down ratios, switching regulators typically operate at frequencies in
the AM radio band. One solution is to
dynamically move the power converter
switching frequency (and harmonics)
away from the tuned AM frequency, but
moving the switching frequency can
lead to unexpected EMI problems.
A cleaner solution is to simply set the
switching frequency higher than the top
of the AM radio band, which is at 1.8MHz.
This is easier said than done, since most
existing buck converters cannot meet the
low (<100ns over temperature) minimum
20 | April 2010 : LT Journal of Analog Innovation
timer to supervise microprocessors.
The LT3640 is offered in 4mm × 5mm
QFN and 28-lead FE packages.
DUAL BUCK REGULATOR
The LT3640 is a dual channel, constant
frequency, current mode monolithic
buck switching regulator with power-on
reset and watchdog timer. Both channels
are synchronized to a single oscillator
with frequency set by RT. The adjustable frequency ranges from 350kHz
to 2.5MHz. The internal oscillator of
the LT3640 can be synchronized to an
external clock signal on the SYNC pin.
on-time required to produce the step-down
ratio from a 16V input to a 3.3V output.
The LT3640 solves this problem with a
fast non-synchronous high voltage buck
converter and a high efficiency synchronous low voltage buck converter. With a
typical minimum on time of about 60ns
over temperature, the high voltage channel in the LT3640 can deliver 3.3V from
16VIN at 2MHz with comfortable margin.
The synchronous low voltage channel
in the LT3640 can be cascaded from the
3.3V channel to generate the other lower
voltage buses such as 2.5V, 1.8V or 1.2V.
The high voltage channel is a non-synchronous buck with an internal 1.7A NPN top
switch that operates from an input of
4V to 35V. Above 35V, an internal overvoltage lockout circuit suspends switching,
protecting the LT3640 and downstream
circuits from input faults as high as 55V.
The low voltage channel is a synchronous
buck with internal CMOS power switches
The LT3640 also includes a programmable power-on reset timer and watchdog
0.22µF
Figure 1. 2MHz 3.3V/0.8A
and 1.8V/0.8A step-down
regulators
VIN
5V TO 35V CIN
4.7µF
D2
L1,3.3µH
EN/UVLO VIN
SYNC
WDE
PGOOD
SW
BST SW1
D1 80.6k
DA
FB1
49.9k
VOUT1
100k
100k
LT3640
RST1
RST2
WDO
WDI
µP
CIN: TAIYO YUDEN UMK316BJ475KL
COUT1, COUT2: TAIYO YUDEN JMK212BJ226MD
L1: VISHAY IHLP2020BZER3R3
L2: VISHAY IHLP1616BZER1R0
D1, D2: DIODES INC. B240A
D2: CENTRAL SEMICONDUCTOR CMDSH-4E
VOUT1
3.3V/0.8A
COUT1
22µF
VIN2
EN2
L2,1µH
SW2
100k
CWDT
CPOR
1.5nF
RT GNDSS2
1.5nF
32.4k
FB2
SS1
1nF
1nF
49.9k
VOUT2
1.8V/0.8A
COUT2
22µF
design features
The high voltage buck regulator in the LT3640 has
a very fast minimum on time of about 60ns. This
enables the LT3640 to operate at a high step-down
ratio while maintaining high switching frequency.
90
90
VIN2 = 3.3V
85
VIN = 16V
80
VIN = 24V
75
80
75
fSW = 2MHz
3.3V CHANNEL
70
0
0.2
0.4
0.6
0.8
1.0
VOUT1 CURRENT (A)
FAST, HIGH VOLTAGE
BUCK REGULATOR
The high voltage buck regulator in the
LT3640 has a very fast minimum on time
of about 60ns. This enables the LT3640 to
operate at a high step down ratio while
maintaining high switching frequency.
Figure 3 shows the waveform of the
LT3640 operating from input voltage of
35V to regulate a 3.3V output at 2MHz. The
on time of the top power switch is about
60ns, which is also flat over temperature.
85
VIN = 12V
EFFICIENCY (%)
EFFICIENCY (%)
providing high efficiency without the
need of an external Schottky diode and
accepts an input of 2.5V to 5.5V. Typically,
the low voltage channel can operate from
the output of the high voltage channel to
form a cascaded structure as shown in
Figure 1, but it can also operate from a
separate supply source. The low voltage
channel only switches when the high voltage channel output is within regulation.
1.2
fSW = 2MHz
1.8V CHANNEL
70
0
0.2
0.4
0.6
VOUT2 CURRENT (A)
0.8
1
Figure 2. Efficiency of the circuit in Figure 1
Besides the fast minimum on time, the
high voltage buck regulator has fast
switching edges to minimize switching
losses and improve conversion efficiency
at high frequency. Figure 4 shows the efficiency of the high voltage buck regulator
at 2MHz operation for different input voltages. For 5V output voltage, the high voltage buck maintains an efficiency of higher
than 86% for input voltage up to 24V.
OUTPUT SHORT-CIRCUIT
ROBUSTNESS
The LT3640 monitors the catch diode current to guarantee the output short-circuit
robustness for the high voltage buck
converter. The LT3640 waits for the catch
diode current to fall below its limit before
starting a new cycle. The top NPN does
not turn on until the catch diode current
is below its limit. This control scheme
90
85
EFFICIENCY (%)
VSW1
10V/DIV
SW1
10V/DIV
80
75
IL1
0.5A/DIV
fSW = 2MHz
VOUT1 = 5V
100ns/DIV
Figure 3. Fast high voltage buck regulator
70
0
0.2
VIN = 12V
VIN = 16V
VIN = 20V
VIN = 24V
0.4
0.6
0.8
1.0
VOUT1 CURRENT (A)
IL1
0.5A/DIV
1.2
Figure 4. Efficiency of the high voltage buck regulator
1µs/DIV
VIN = 30V
VOUT1 = SHORT
RT SET = 2MHz
Figure 5. Output shorted robustness, high voltage
channel
April 2010 : LT Journal of Analog Innovation | 21
ensures cycle-by-cycle current limit, providing protection against shorted output.
The switching waveform for VIN = 30V and
VOUT = 0V is shown in Figure 5.
LOW VOLTAGE SYNCHRONOUS
BUCK REGULATORS
The low voltage channel is a synchronous
buck with internal CMOS power switches
providing high efficiency without the
need of external Schottky diode. This
channel only switches when the high
voltage channel output is within regulation. The output can be programmed
as low as 0.6V, covering any core voltage in modern microprocessors.
The low voltage buck has a similar scheme
as the high voltage buck of monitoring the current in the bottom NMOS to
guarantee shorted output robustness.
However, when the bottom NMOS current
exceeds its limit, the oscillator frequency
is not affected. The low voltage buck
simply skips one cycle to avoid interference with the high voltage buck.
At light load the low voltage buck also
operates in low ripple Burst Mode operation to minimize output ripple and power
loss. Although the two channels in the
LT3640 share a common oscillator, they
may require different light load operation
frequencies to optimize efficiencies at their
respective loads. In this case, the oscillator always runs at the higher frequency,
with the channel requiring the lower
frequency skipping cycles. Figures 6 and 7
show the light load switching waveforms
of two channels running at same reduced
frequency and at different frequencies,
respectively. Output ripple for both channels remains below 10mVP–P. No-load
quiescent current from the input is only
300µA with both outputs in regulation.
BENEFITS OF CASCADING
As described above, there are clear
advantages of cascading two switchers to
generate I/O and core supplies. The higher
operating frequency reduces circuit size
22 | April 2010 : LT Journal of Analog Innovation
SW1
10V/DIV
SW1
10V/DIV
IL1
0.5A/DIV
IL1
0.5A/DIV
SW2
5V/DIV
SW2
5V/DIV
IL2
0.5A/DIV
IL2
0.5A/DIV
500ns/DIV
VIN = 12V
VOUT1 = 3.3V/25mA
VIN2 = VOUT1
VOUT2 = 1.8V/30mA
2µs/DIV
VIN = 12V
VOUT1 = 3.3V/0mA
VIN2 = VOUT1
VOUT2 = 1.8V/30mA
Figure 6. Two channels running in discontinuous
mode at light load remain synchronized.
Figure 7. At still lighter loads, the two channels
switch at different frequencies to maintain high
efficiency and low output ripple.
and provides faster transient response
for better regulation. The low voltage
technology used for the core supply
switcher further reduces solution size.
Figure 1 against two non-synchronous
bucks operating from VIN, the overall
efficiency is nearly identical. If the core
voltage is reduced from 1.8V to 1.2V, the
LT3640 circuit is actually more efficient.
Although the core supply is generated
via two conversions, with two efficiency
hits, keep in mind that the core supply is often low power, even if it is high
current, so total power loss is minimal.
Also, a buck converter generating the
core voltage directly from the input
does not typically operate in an efficient
region anyway, and it would be larger
and slower. Comparing the circuit in
Figure 8. Power-on reset
and watchdog timing
POWER-ON RESET AND
WATCHDOG TIMER
In high reliability systems, a supervisor monitors the activity of the microprocessor. If the processor appears to
stop, due to either hardware or software
faults, the supervisor resets the microprocessor in an attempt to restore the
system to a functional state. While some
FB
tRST
tUV
RST
a. Power-on reset timing
WDI
WDO
tDLY
t < tWDL tRST
b. Watchdog timing
t < tWDU
tWDL < t < tWDU
tWDU
tRST
design features
modern processors include internal
supervisor functions, it is better practice
to separate the two. Typical supervisor functions are voltage monitors with
power-on resets to qualify supply voltages and watchdog timers to monitor
software and hardware functions.
function for higher reliability. If the falling
edges on the WDI pin are grouped too close
together or too far apart, the WDO pin
is pulled down for a period the same as
the power-on reset timeout period before
the watchdog timer is started again. The
timing diagrams of the power-on reset and
watchdog timer are shown in Figure 8.
The LT3640 includes one power-on reset
timer for each buck regulator and one
common watchdog timer. Power-on
reset and watchdog timers are both
adjustable using external capacitors.
LOW NOISE DATA ACQUISITION
SUPPLY
Figure 9 shows a 4-output supply that
generates low noise 5V and 3.3V rails for
analog circuits, along with 3.3V I/O and
1.5V core supplies for digital circuits.
The high voltage channel of the LT3640
converts the input to a 5.7V intermediate
bus, the low voltage channel bucks to the
1.5V core, and a few LDOs regulate the
5V and 3.3V outputs. The 5.7V bus voltage
gives the 5V regulator suitable headroom
for good PSSR and transient performance.
Once the high voltage buck output voltage reaches 90% of its regulation target,
the high voltage channel reset timer is
started and the RST1 pin is released after
the reset timeout period. The low voltage channel reset timer is started once the
low voltage buck output voltage reaches
92% of its regulation voltage, and releases
RST2 after the reset timeout period.
Diode D2 performs two functions. First,
it lowers the intermediate voltage to a
value below the 5.5V maximum operating
voltage of the synchronous buck regulator. Second, it isolates high frequency
ripple current flowing into the VIN2 and
The watchdog circuit monitors a microprocessor’s activity. As soon as both RST1
and RST2 are released and an additional
delay has expired, the watchdog starts
monitoring the signal at the WDI pin. The
LT3640 implements windowed watchdog
Figure 9. This circuit generates two low
noise analog rails plus I/O and core
supplies for a microprocessor.
BST pins of the LT3640 from the inputs
of the LDOs regulating the analog supplies, resulting in quiet analog rails.
Total current draw from the 5.7V rail
is 820mA, so about 30% more power
is available from this output. RST1 and
RST2 indicate power is good when the
1.5V and 5.7V rails are in regulation.
The watchdog function is not used here,
and is disabled by tying WDE to VIN2.
The associated pins, not shown on this
schematic, should be left floating.
CONCLUSION
The LT3640 is a dual channel buck regulator. The high voltage channel buck is
capable of converting 16V input voltage
to 3.3V output at 2MHz with comfortable
margin. The high voltage buck maintains efficiency above 86% for delivering
5V from up to 24V input at 2MHz switching frequency. The low voltage channel
buck input ranges from 2.5V to 5.5V. The
LT3640 also includes power-on reset and
watchdog timer to monitors a microprocessor’s activity. The high frequency
high efficient buck converters and the
programmable timers make the LT3640
ideal for automotive applications. n
LOW NOISE, LDO
MICROPOWER REGULATORS
IN
OUT
LT1962-5
SHDN BYP
GND
D3
VIN
7V TO 35V
CIN1
10µF
VIN
BST
EN/UVLO
SW
SW1
226k
49.9k
DA
FB1
POR
1nF
CIN1: TAIYO YUDEN UMK325BJ106MM
COUTSW1: TAIYO YUDEN JMK212BJ226MD
CIN2, COUT1, COUT2, COUT3 : TAIYO YUDEN JMK212B7106KG
COUT4 : TAIYO YUDEN JMK212BJ476MG
L1: VISHAY IHLP2525CZER8R2
L2: VISHAY IHLP1616BZER2R2
D1: UPS140
D2: CMMR1-02
D3: PMEG4005
D1 178k
1nF
1nF
82.5k
VIN2
RST1
RST2
WDE
CPOR
EN2
SHDN BYP
GND
RT
SYNC
GND
FB2
VOUT2
3.3V/100mA
COUT2
10µF
10nF
VOUT3
3.3V/300mA
COUT3
10µF
VOUT4
1.5V/800mA
SW2
SS2
SHDN BYP
GND
IN
OUT
LT1763-3.3
CIN2
10µF
75k
10nF
D2
L2
2.2µH
SS1
IN
OUT
LT1761-3.3
COUTSW1
22µF
LT3640
100k
VOUT1
5V/100mA
COUT1
10µF
1µF
L1,8.2µH
49.9k
VOUT3
10nF
COUT4
47µF
49.9k
April 2010 : LT Journal of Analog Innovation | 23
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