DN506 - 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
Design Note 506
Jeff Zhang
Introduction
A new regulator architecture the LTC3600 (first introduced with the LT3080 linear regulator) has wider output
range and better regulation than traditional regulators.
Using a precision 50µA current source and a voltage
follower, the output is adjustable from “0V” to close
to VIN. Normally, the lowest output voltage is limited
to the reference voltage. However, this new regulator
has a constant loop gain independent of the output
voltage giving excellent regulation at any output and
allowing multiple regulators to be paralleled for higher
output currents.
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
LTC3600
VIN
VIN
12V
resistor to program the reference voltage 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.
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.
L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
9
BOOST
0.1µF
8
RUN
50µA
5
10µF
+
–
7
ISET
0.1µF
MODE/
SYNC INTVCC RT
6
RSET
66.5k
VOUT
3.3V
22µF
GND
13
VOUT
1
2.2µH
SW
PWM CONTROL
AND SWITCH
DRIVER
ERROR
AMP
10
3
PGFB
4
11
ITH PGOOD
2
12
DN4JZ F01
1µF
Figure 1. High Efficiency, 12V to 3.3V 1MHz Step-Down Regulator with Programmable Reference
09/12/506
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.
VOUT
100mV/DIV
100
90
80
EFFICIENCY (%)
70
IOUT
1A/DIV
60
50
40
30
20
50µs/DIV
12VIN TO 3.3VOUT, INTERNAL COMPENSATION, ITH TIED TO INTVCC
12VIN to 3.3VOUT (CCM)
12VIN to 3.3VOUT (DCM)
10
0
–0.1
0.1
0.3
0.5
0.7
0.9
1.1
1.3
1.5
LOAD CURRENT (A)
Figure 3. Efficiency of 12V Input to 3.3V
Output Regulator in CCM and DCM Mode
VOUT
100mV/DIV
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.
IOUT
1A/DIV
50µs/DIV
12VIN TO 3.3VOUT, ITH TIED TO EXTERNAL
RC COMPENSATION (R=56kΩ, C=68pF)
Figure 2. 0A to 1.5A Load Step Response
of the Figure 1 Schematic
VIN
12V
9
LTC3600
BOOST
8
RUN
50µA
5
+
10µF
0.1µF
PWM
CONTROL
AND
SWITCH
DRIVER
ERROR
AMP
–
2.2µH
SW
7
GND
13
VOUT
MODE/
SYNC INTVCC
ISET
1
RSET
0k TO 3k
6
10
0.1Ω
IOUT =
0A TO 1.5A
22µF
11
RT
PGFB
ITH
PGOOD
3
4
2
12
1µF
DN4JZ F04
Figure 4. The LTC3600 as a Programmable 0A to 1.5A Current Source
Data Sheet Download
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear.com
For applications help,
call (408) 432-1900, Ext. 3845
dn506 AP/LT 0912 196k • PRINTED IN THE USA
 LINEAR TECHNOLOGY CORPORATION 2012
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