20V, 2.5A Synchronous Monolithic Buck with Current and Temperature Monitoring

20V, 2.5A Synchronous Monolithic Buck with
Current and Temperature Monitoring
K. Bassett
The LTC3626 integrates a number of easyto-use, but powerful, features that would
normally require additional ICs and design
time to implement. Specifically, with the
addition of just a couple of passive components, the LTC3626 can be configured to
provide accurate measures of its output
current, input current, and on-die temperature. It can be just as easily programmed
to limit each measured parameter.
These built-in features expand the
designer’s insight into the performance
of the system and increase the level of
control with remarkably little extra
design investment. Additionally, optional
internal loop compensation is available to minimize the design effort.
32 | January 2013 : LT Journal of Analog Innovation
The LTC3626 also includes userselectable Burst Mode operation or
forced-continuous mode, resistorprogrammable switching frequencies
from 500kHz to 3MHz, power good
status output, output tracking capability, and external clock synchronization.
CURRENT MONITOR AND LIMIT
One way to measure the overall performance of a system is to is to monitor the
current at the output of the power supply.
Supply current monitoring also informs
designers if downsteam ICs are operating as expected—useful in design and
debug, and during normal operation.
The LTC3626 makes it easy to monitor
the supply current by producing a fraction of its average output current at its
IMONOUT pin, specifically, the current at
the IMONOUT pin is equal to the average output current divided by 16,000.
Figure 1 shows the typical performance
of the output current measurement
for an ambient temperature range of
–40°C to 85°C. Figure 2 shows the error
between the actual average output
CALCULATED OUTPUT CURRENT,
IMONOUT • 16000 (A)
156
VIN = 12V
VOUT = 1.8V
fO = 1MHz
2.25
2.00
125
1.75
1.50
94
1.25
1.00
TA = 85°C
TA = 25°C
TA = –40°C
0.75
63
0.50
31
0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5
OUTPUT CURRENT (A)
Figure 1. Output current monitor vs output current
current and the average output current as measured by the LTC3626.
The current at the IMONOUT pin can be
measured directly or converted to a
voltage by placing a resistor from the
IMONOUT pin to ground. Converting the
output of the IMONOUT pin to a voltage makes it easy to scale the output
for digitization via a microcontroller
or standalone ADC. Figure 3 shows
the LTC3626 configured to run with
Figure 2. Output current monitor error
vs output current
MEASURED OUTPUT CURRENT ERROR (%)
The LTC3626 is capable of supplying
2.5A of output current over an input
voltage range of 3.6V to 20V from a
tiny, 3mm × 4mm, 20-pin QFN package. Its patented controlled on-time
architecture yields outstanding transient
response and enables high step-down
ratios at high switching frequencies, minimizing board footprint.
2.50
5
VIN = 12V
VOUT = 1.8V
fO = 1MHz
4
3
2
1
0
–1
–2
–3
TA = 85°C
TA = 25°C
TA = –40°C
–4
–5
1
1.25
1.75
2
2.25
1.5
OUTPUT CURRENT (A)
2.5
IMONOUT CURRENT (µA)
Increases in digital IC integration, coupled with advances
in printed circuit board layout and assembly techniques,
continue to push system performance and power density
higher. Many of these systems, powered from a 12V rail or
battery stack, utilize point-of-load regulators to maximize
power chain efficiency while maintaining a small form
factor. The LTC3626 synchronous, monolithic step-down
regulator is ideally suited for these operating environments,
given its ability to provide a flexible, highly efficient DC/DC
conversion while occupying a very small footprint.
design ideas
VIN
12V
C1
47µF
0.1µF
C4
2.2µF
0.1µF
RPGD
200k
0.1µF
REFOUT COMP VCC
IN
CIOUT
1µF
LTC2460
GND
RIOUT
5.1k
BOOST
CBST
0.1µF
L1
1.5µH
LTC3626
INTVCC
SW
ITH
TRACK/SS
VON
TSET
TMON
FB
IMONIN
RT
PGOOD
IMONOUT MODE/SYNC
SGND
R1
40.2k
RT
324k
PGND
CF
22pF
VOUT
1.8V
COUT 2.5A
47µF
R2
20k
REF–
Figure 3. 12V input to 1.8V output, 2.5A regulator with digital output current monitoring
the output current monitor activated
while the LTC2460, 16-bit ADC, digitizes the result for digital processing.
is useful for applications that must limit
the average current drawn from the
input supply. Figure 4 shows the LTC3626
configured to limit the average input current to 475m A while producing an output
voltage of 2.5V from a 5V input voltage.
The LTC3626 also features an easily programmed average output current limit.
Specifically, the LTC3626 contains an onchip current limit amplifier with a reference of approximately 1.2V. To program
an average output current, simply size the
resistor from IMONOUT to ground such that
the resultant voltage is 1.2V for the current
at which the limit should be activated.
TEMPERATURE MONITOR AND LIMIT
The LTC3626 produces an estimate of
the on-die temperature at the TMON pin.
This feature can be used to determine the
quality of the ground connection to the
QFN exposed pad made during assembly.
The exposed pad for the QFN is intended
to provide a low impedance electrical
connection to the board as well as good
thermal contact. Visual inspection of
this critical connection can be difficult,
and a poor exposed pad connection may
not be apparent by simple observation
of the regulated output voltage even
though the on-die temperature may be far
Similar to the average output current,
the LTC3626 produces an estimate of the
average input current at the IMONIN pin.
That is, the current at the IMONIN pin is
an estimate of the average input current
divided by 16,000. Just like the average output current, the LTC3626 offers a
simple mechanism to program a limit for
the average input current. This feature
too high for reliable, long-term part
operation. Measurement of the TMON pin
however gives the user insight into the
exposed pad connection and hence the
internal part operating environment.
As an example, Figure 5 shows data
taken on two parts, one with a good
exposed pad connection to the PCB, the
other with a poor exposed pad connection. Though both parts regulate to the
expected output voltage, it is clear from
the internal temperature measurement that
the internal operating environment is very
different between the two parts. If placed
in a system with an ambient operating
temperature of say 70°C, the device with
the poor exposed pad connection will
clearly exceed the maximum allowed junction temperature of 125°C and will thus
have compromised long-term reliability.
CONCLUSION
The continuous push for higher performance and power density faced by today’s
system designers require small, flexible,
and efficient point-of-load converters to
maximize overall power chain efficiency.
The LTC3626’s combination of wide input
voltage range, output current capability,
flexible feature set, and very small form
factor make it ideal for many of today’s
point-of-load regulator applications. n
Figure 5. It is easy to determine the quality of the
exposed pad connection by examining temperature
measurements made by the LTC3626.
Figure 4. 5V Input to 2.5V output at 1MHz synchronized frequency
with input current monitor and 475mA input current limit
80
VIN
5V
C4
2.2µF
RCOMP
13k
CCOMP
220pF
RIIN
40.2k
C1
47µF
RPGD
100k
CIIN
1µF
PVIN
SVIN
RUN
BOOST
LTC3626
TRACK/SS
INTVCC
RT
SW
TSET
IMONOUT
VON
TMON
PGOOD
FB
ITH
IMONIN MODE/SYNC
SGND
PGND
CBST
0.1µF
L1
2.2µH
R1
127k
R2
40.2k
EXTERNAL
CLOCK
CF
22pF
COUT
47µF
VOUT
2.5V
2.5A
MEASURED ON-DIE TEMPERATURE (°C)
SCK
SDO
CS
PVIN
SVIN
RUN
C2
1µF
POOR EXPOSED CONNECTION
60
40
20
0
NORMAL EXPOSED CONNECTION
0
5
10
15
ILOAD (A)
20
25
January 2013 : LT Journal of Analog Innovation | 33
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