1.5A Rail-to-Rail Output Synchronous Step-Down Regulator Adjusts with a Single Resistor

1.5A Rail-to-Rail Output Synchronous Step-Down Regulator
Adjusts with a Single Resistor
Jeff Zhang
The LTC3600 features a
wide output range with
tight regulation over the
entire range. In most
regulators, the lowest
output voltage is limited
to the reference voltage.
The LTC3600, however,
uses a novel regulation
scheme with a precision
50µA current source and
a voltage follower, creating
an adjustable output from
“0V” to close to VIN. It
also features constant
loop gain, independent of
the output voltage, giving
excellent regulation at any
output and allowing multiple
regulators to be paralleled
for high output currents.
LTC3600
VIN
VIN
12V
RUN
50µA
10µF
BOOST
+
–
Figure 1. High efficiency,
12V to 3.3V 1MHz stepdown regulator with
programmable reference
2.2µH
SW
PWM CONTROL
AND SWITCH
DRIVER
ERROR
AMP
0.1µF
VOUT
3.3V
22µF
GND
VOUT
ISET
0.1µF
MODE/
SYNC INTVCC RT
RSET
66.5k
OPERATION
The LTC3600 is a current mode monolithic
step-down buck regulator with excellent
line and load transient responses. The
200kHz to 4MHz operating frequency can
be set by a resistor or synchronized to an
external clock. The LTC3600 internally
generates an accurate 50μA current source,
allowing the use of a single external
resistor to program the reference voltage
PGFB
ITH PGOOD
1µF
from 0V to 0.5V below VIN . As shown in
Figure 1, the output feeds directly back
to the error amplifier with unity gain.
The output equals the reference voltage
at the ISET pin. A capacitor can be paralleled with RSET for soft-start or to improve
noise while an external voltage applied
to the ISET pin is tracked by the output.
Figure 3. Efficiency of 12V input to 3.3V output
regulator in CCM and DCM mode
Figure 2. 0A to 1.5A load step response of the Figure 1 schematic
100
90
80
VOUT
100mV/DIV
IOUT
1A/DIV
EFFICIENCY (%)
VOUT
100mV/DIV
IOUT
1A/DIV
70
60
50
40
30
20
2VIN to 3.3VOUT (DCM)
2VIN to 3.3VOUT (CCM)
10
50µs/DIV
12VIN TO 3.3VOUT
INTERNAL COMPENSATION
ITH TIED TO INTVCC
30 | July 2012 : LT Journal of Analog Innovation
50µs/DIV
12VIN TO 3.3VOUT
ITH TIED TO EXTERNAL
RC COMPENSATION (R=56kΩ, C=68pF)
0
–0.1 0.1
0.3
0.5 0.7 0.9
ILOAD (A)
1.1
1.3
1.5
design ideas
VIN
BOOST
LTC3600
12V
RUN
50µA
+
10µF
0.1µF
PWM
CONTROL
AND
SWITCH
DRIVER
ERROR
AMP
–
2.2µH
SW
0.1Ω
GND
IOUT =
0A TO 1.5A
22µF
VOUT
Internal loop compensation stabilizes
the output voltage in most applications, though the design can be customized with external RC components.
The device also features a power good
output, adjustable soft-start or voltage tracking and selectable continuous/
discontinuous mode operation. These
features, combined with less than 1μA supply current in shutdown, VIN overvoltage protection and output overcurrent
protection, make this regulator suitable
for a wide range of power applications.
ISET
RT
PGFB
ITH
PGOOD
1µF
RSET
0k TO 3k
Figure 4. The LTC3600 as a programmable 0A to 1.5A current source
Figure 5. 15V to 2.5V, 3A dual phase regulator
VIN
4V
TO
15V
LTC3600
VIN
RUN
100k
50µA
10µF
BOOST
0.1µF
+
PWM CONTROL
AND SWITCH
DRIVER
ERROR
AMP
–
APPLICATIONS
Figure 1 shows the complete LTC3600
schematic in a typical application that
generates a 3.3V output voltage from
12V input. Figure 2 shows the load step
transient response using internal compensation and with external compensation. Figure 3 shows the efficiency in
CCM and DCM modes. Furthermore, the
LTC3600 can be easily configured to be
a current source, as shown in Figure 4.
By changing the RSET resistance from
0Ω to 3kΩ, the output current can
be programmed from 0A to 1.5A.
MODE/
SYNC INTVCC
22µF
VOUT
ISET
MODE/
SYNC INTVCC
RT
GND PGFB
VOUT = 2.5V
3A
ITH PGOOD
10pF
1µF
27k
VIN
4V
TO
15V
9
LTC3600
VIN
100k
RUN
50µA
5
10µF
150pF
BOOST
8
0.1µF
+
PWM CONTROL
AND SWITCH
DRIVER
ERROR
AMP
–
2.2µH
SW
7
VOUT
CONCLUSION
The LTC3600 uses an accurate internal
current source to generate a programmable reference, expanding the range of
output voltages. This unique feature gives
the LTC3600 great flexibility, making it
possible to dynamically change the output
voltage, generate current sources, and
parallel regulators for applications that
would be difficult to implement using a
standard DC/DC regulator configuration. n
2.2µH
SW
ISET
MODE/
SYNC INTVCC
6
1
10
24.9k
0.1µF
100k
RT
3
GND PGFB
13
1µF
V+
INTVCC
11
GND
SET
LTC6908-1*
4
ITH PGOOD
2
22µF
12
10pF
OUT1
OUT2
MOD
*EXTERNAL CLOCK FOR FREQUENCY SYNCHRONIZATION IS RECOMMENDED
July 2012 : LT Journal of Analog Innovation | 31
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