DN511 - 20V, 2.5A Monolithic Synchronous Buck SWITCHER+ with Input Current, Output Current and Temperature Sensing/ Limiting Capabilities

20V, 2.5A Monolithic Synchronous Buck SWITCHER+ with
Input Current, Output Current and Temperature Sensing/
Limiting Capabilities
Design Note 511
Tom Gross
Introduction
The LTC ®3626 synchronous buck regulator with
current and temperature monitoring is the first of
Linear’s SWITCHER+™ line of monolithic regulators. It is a high efficiency, monolithic synchronous
step-down switching regulator capable of delivering
a maximum output current of 2.5A from an input
voltage ranging from 3.6V to 20V (circuit shown in
Figure 1). The LTC3626 employs a unique controlled
on-time/constant-frequency, current-mode architecture, making it ideal for low duty cycle applications
and high frequency operation, while yielding fast
response to load transients (see Figure 2). It also
features mode setting, tracking and synchronization
capabilities. The LTC3626’s 3mm × 4mm package
has such low thermal impedance that it can operate
without an external heat sink even while delivering
maximum power to the load.
VIN
3.6V TO 20V
C2
47µF
25V
1210
PGOOD
PVIN
PVIN
SVIN
RUN
BOOST
ITH = INTVCC
IL
1A/DIV
F02
20µs/DIV
12VIN TO 1.8VOUT LOAD STEP RESPONSE,
2.5A LOAD STEP, 2MHz SWITCHING FREQUENCY,
FORCED CONTINUOUS MODE, INTERNAL COMPENSATION
Figure 2. Load Step Response for Figure 1 Circuit
Beyond its impressive regulator capabilities, the
LTC3626’s current and temperature monitoring functions stand out. They offer both monitoring and control
capabilities with minimal additional components.
L, LT, LTC, LTM, Linear Technology, and the Linear logo are registered trademarks
and SWITCHER+ is a trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
C3
0.1µF
SW
SW
LTC3626
PGOOD
VOUT
50mV/DIV
AC-COUPLED
L1
1µF
VISHAY
IHLP-2020BZ-ER-1ROMO1
VON
FB
C4
22pF
R2
100k
1%
R3
100k
INTVCC
C5
2.2µF
TRACK/SS
ITH
RT
INTVCC
C1
47µF
6.3V
1206
R1
200k
1%
IOUT
IMONOUT
IIN
IMONIN
TMON
C8
1µF
TSET
MODE/
SYNC SGND PGND
R4
432k
C7
1µF
R7
54.9k
C6
1µF
VOUT
1.8V
2.5A
VIOUTMON
R6
7.32k
VIINMON
TMON
R5
665k
TSET
F01
Figure 1. 20V Maximum Input, 2.5A, 2MHz Buck Regulator with Current and Temperature Monitoring
02/13/511
Output/Input Current Sensing
The LTC3626 senses the output current through the
synchronous switch during the switch’s on-time and
generates a proportional current (scaled to 1/16000)
at the IMONOUT pin. Figure 3 shows the accuracy of
the IMONOUT output by comparing the measured
output of the IMONOUT pin with calculated values.
Error remains less than 1% over most of the output
current range.
Likewise, this same sense current signal is combined
with the buck regulator’s duty cycle to produce a
current proportional to the input current— again
by 1/16000—at the IMONIN pin. A precision of better
than 5% is achieved over a wide current range (see
Figure 4).
Both current signals are connected to internal voltage
amplifiers, referenced to 1.2V, that can shut down the
part when tripped. So the input and output current
limits are set by simply connecting a resistor to the
IMONIN or IMONOUT pins, respectively, as shown in
Figure 1. The relationship between the current limit
and the resistor is:
1.2V • 16000
RLIM
For example, a 10k resistor sets a current limit of
approximately 2A.
This simple scheme allows both monitoring and active
control of the input and output current limits—the latter
200
OUTPUT SENSE CURRENT (µA)
16
140
14
120
12
100
10
80
8
60
6
40
4
20
2
0
0.5
1
2
2.5
1.5
OUTPUT CURRENT (A)
3
Choosing a maximum temperature limit of 125°C
equates to an approximate 2V setting on the TSET
pin—the IC will shut down once the die temperature
TJ reaches this limit.
Conclusion
The LTC3626 combines current and temperature
monitoring capabilities with a high performance buck
regulator in a compact package. A microprocessor or
other external control logic can supervise conditions via
easy-to-use input and output current and temperature
monitor pins, and it can shut itself down by setting
a threshold voltage on the temperature set limit pin.
30
0
F03
Figure 3. Output Current vs Output Current Monitor
Data Sheet Download
www.linear.com/3626
Linear Technology Corporation
TJ + 273
200°K/V
25
CALCULATED
MEASURED
ERROR
30
25
20
20
15
15
10
10
5
5
0
0
100 150 200 250 300 350 400 450
INPUT CURRENT (mA)
50
OUTPUT CURRENT MONITOR ERROR (%)
18
OUTPUT CURRENT MONITOR ERROR (%)
160
VTSET =
20
CALCULATED
MEASURED
ERROR
180
Temperature Sensing
The LTC3626 generates a voltage proportional to
its own die temperature, which can be used to set a
maximum temperature limit. The voltage at the temperature monitor pin (TMON) is typically 1.5V at room
temperature. To calculate the die temperature, TJ,
multiply the TMON voltage by the temperature monitor
voltage-to-temperature conversion factor of 200°K/V,
and subtract the 273°C offset. The LTC3626 also has
a temperature limit comparator fed by the temperature
limit set pin, TSET, and the TMON pin. Hence, by applying
a voltage to the TSET pin, a maximum temperature limit
can be set according to the following:
OUTPUT SENSE CURRENT (µA)
ILIM 

can be implemented via external control circuitry, such
as a DAC with a few passive components.
F04
Figure 4. Input Current vs Input Current Monitor
For applications help,
call (408) 432-1900, Ext. 3229
dn511 LT/AP 0213 196K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2013
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