DN441 - Compact Triple Step-Down Regulator Offers LDO Driver and Output Tracking and Sequencing

Compact Triple Step-Down Regulator Offers LDO Driver and
Output Tracking and Sequencing – Design Note 441
Tiger Zhou
Introduction
Typical industrial and automotive applications require
multiple high current, low voltage power supplies to
drive everything from disk drives to microprocessors.
The LT®3507 triple step-down converter fits easily into
these applications. It is simple and compact compared
to multi-chip solutions.
The LT3507 is a single IC current mode triple stepdown regulator with internal power switches and a low
dropout linear regulator driver. The switching converters are capable of generating one 2.4A output and two
1.5A outputs. All three converters are synchronized to a
single oscillator, with the 2.4A output running antiphase
to the other two converters, thereby reducing input ripple
current. Each converter has independent shutdown and
soft-start circuits and generates a power good signal
when its output is in regulation, simplifying both supply
sequencing and the interface with microcontrollers and
DSPs. Separate input pins for each regulator offer additional flexibility; regulators can be cascaded to reduce
circuit size, or each regulator can draw power from a
different input source.
The switching frequency is set with a single resistor
between 250kHz to 2.5MHz. High switching frequency
allows the use of small inductors and capacitors resulting in a very compact triple output supply. The constant
switching frequency, combined with low impedance
ceramic capacitors, results in low output ripple. With
its wide input voltage range of 4V to 36V, the LT3507
regulates a broad array of power sources including 5V
logic rails, unregulated wall transformers, lead acid batteries and distributed power supplies.
6V to 36V Input to Four Outputs—1.8V, 3.3V, 5V
and 2.5V—One IC
The triple converter accommodates a 6V to 36V input
voltage range and is capable of supplying up to 2.4A,
1.5A and 1.5A, respectively. The 20mA LDO driver output can drive an NPN transistor to provide a fourth low
05/08/441
VIN
6V TO 36V
22μF
CMDSH-3
VOUT1
1.8V
2.4A
4.7μH
VIN1 VIN2 VIN3
PGOOD1
PGOOD2
PGOOD3
BOOST2
BOOST1
100k
CMDSH-3
0.22μF 8.2μH
0.22μF
B340A
SW1
SW2
FB1
FB2
B240A
12.4k
47μF
10k
31.6k
680pF
20k
680pF
LT3507
VC1
20k
VOUT2
3.3V
1.3A
22μF
10k
VC2
CMDSH-3
BOOST3
VOUT3
5V
1.5A
10μH
BIAS
0.22μF
B240A
SW3
DRIVE
21.5k
53.6k
FB3
47μF
10k
20k
FB4
680pF
10k
2.2μF
VOUT4
2.5V
0.2A
VC3
SHDN
107k
TRK/SS1
RUN1
TRK/SS2
RUN2
TRK/SS3
RUN3
RT/SYNC TRK/SS4
GND
0.22μF
DN441 F01
Figure 1. A 4-Output Supply Including a Low Noise LDO
noise rail. Figure 1 shows a typical application for four
outputs—1.8V at 2.4A, 3.3V at 1.3A, 5.0V at 1.5A and
2.5V at 0.2A—from a 6V-36V input supply.
Low Ripple High Frequency Operation Even at High
VIN/VOUT Ratios
High frequency operation minimizes solution size, but
one obstacle to high voltage (36V), high frequency
(MHz) operation of monolithic step-down regulators is
the minimum on-time constraint. Due to an internal logic
propagation delay, a step-down regulator must remain
on for a minimum time interval for proper operation.
Otherwise, the converter operates in pulse-skipping mode
at high input-to-output ratios, which has the undesirable
side effect of increasing output ripple. For example, the
application in Figure 1 best operates at 450kHz when the
input is 36V and the output is 1.8V.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
VIN
8V TO 18V
36V TRANSIENT
CMDSH-3
VOUT1
5V
1A
2.7μH
VIN1 VIN2 VIN3
PGOOD1
PGOOD2
PGOOD3
BOOST2
BOOST1
100k
11k
0.22μF 1μH
B340A
SW1
SW2
FB1
FB2
B240A
12.7k
680pF
20k
680pF
LT3507
VC1
0.33μF
TRK/SS1
0.1μF
DN441 F03
0.22μF
10k
6.81k
GND
CMDSH-3
53.6k
10μF
100k
LT3507
TRK/SS2
TRK/SS3
49.9k
TRK/SS4
PGOOD1
OVLO
UVLO
VINSW
22μF
20k
VOUT2
1.8V
1.3A
Figure 3. External Resistors Program the Input Voltage
Lockout; PGOOD1 Determines Sequencing and Tracking
10μF
10k
5VOUT
1V/DIV
VC2
CMDSH-3
BOOST3
VOUT3
1.2V
1.5A
1μH
BIAS
B240A
SW3
DRIVE
8.66k
4.99k
FB3
22μF
1.8VOUT
1.5VOUT 0.5V/DIV
1.2VOUT
VOUT1
0.22μF
10k
FB4
680pF
20k
10k
2.2μF
VOUT4
1.5V
0.2A
20ms/DIV
VC3
SHDN
15.8k
RUN1
TRK/SS1
RUN2
TRK/SS2
RUN3
TRK/SS3
RT/SYNC TRK/SS4
GND
0.22μF
DN441 F04
Figure 4. Channel 1 Starts First,
the Others Follow and Track
DN441 F02
Figure 2. Cascading Supplies Maintain High Frequency
Operation Even with High VIN/VOUT Ratios
An additional feature of LT3507 is the low noise LDO
output. Figure 5 shows the LDO output ripple is reduced
from the preregulated Channel 2 output.
However, the LT3507 has a built-in solution to this problem.
By cascading the first converter and the other two, as
shown in Figure 2, all three converters can be operated
at 2MHz without pulse-skipping mode.
Input Voltage Lockout and Sequencing
The LT3507’s under- and overvoltage lockouts can be
programmed with external resistors. When the schematic in Figure 2 is modified as in Figure 3, the LT3507
will accept VIN up to 36V but operate only when VIN is
between 8V to 18V. This prevents the IC from operating during unintended or fault conditions, allowing
the circuit designer to reduce the size of the external
components. Figure 3 also shows a simple sequencing
scheme: channel 1’s power good indicator is tied to the
tracking pins of the other three channels. Figure 4 shows
the resulting start-up sequence, with channel 1 starting
first and the remaining channels tracking during their
start-up. Other sequencing and tracking examples can
be found in the data sheet.
VOUT2
20mV/DIV
LDO
OUTPUT
20mV/DIV
2μs/DIV
DN441 F05
Figure 5. Low Noise LDO Output: Top Trace is Channel 2
Output, Bottom Trace is Channel 4 LDO Output
Conclusion
The LT3507 integrates three buck regulators and an LDO
driver in a QFN (5mm × 7mm) package, offering a compact
solution for multiple-rail systems. Separate inputs for each
converter offer wide design freedom, while separate PG
indicators and TRK/SS pins further extend tracking and
sequencing flexibility.
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 3223
www.linear.com
Linear Technology Corporation
dn441f LT/TP 0508 392K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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