DN457 - Monolithic Synchronous Step-Down Regulator Delivers up to 12A from a Wide Input Voltage Range

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Monolithic Synchronous Step-Down Regulator Delivers up to 12A
from a Wide Input Voltage Range – Design Note 457
Charlie Zhao and Henry Zhang
Typical Application Example
Figure 1 shows a typical application schematic of the
LTC3608. This 7mm × 8mm regulator supplies 2.5V at 8A
maximum load current from a 4.5V to 20V input source.
This application switches at a nominal 650kHz, which
allows the use of low profile inductor and capacitors while
maintaining high efficiency. The switching frequency can
be easily adjusted by a resistor (RON in Figure 1, connected
to the ION pin). Figure 2 shows the operating efficiency.
Introduction
The LTC ®3610 is a high power monolithic synchronous step-down DC/DC regulator that can deliver up
to 12A of continuous output current from a 4V to 24V
(28V maximum) input supply. It is a member of a high
current monolithic regulator family (see Table 1) that
features integrated low RDS(ON) N-channel top and bottom MOSFETs. This results in a high efficiency and high
power density solution with few external components.
This regulator family uses a constant on-time valley
current mode architecture that is capable of operating
at very low duty cycles at high frequency and with very
fast transient response. All are available in low profile
(0.9mm max) QFN packages.
The FCB pin is connected to ground to force continuous
mode operation at light load for both low noise and small
output ripple. The FCB pin can also be tied to INTVCC to
enable discontinuous mode for higher efficiency at light
load. Soft-start is programmable with a capacitor from
the RUN/SS pin to ground. Forcing the RUN/SS pin below
0.8V shuts down the device.
Table 1. Comparison of LTC36XX Family Members
LTC3608 LTC3609 LTC3610 LTC3611
INPUT VOLTAGE 4 to 20
4.5 to 32
4 to 24
4.5 to 32
RANGE (V)
MAX LOAD
8
6
12
10
CURRENT (A)
QFN PACKAGE 7 × 8 × 0.9 7 × 8 × 0.9 9 × 9 × 0.9 9 × 9 × 0.9
SIZE (mm)
VIN
4.5V TO 20V
10μF
25V
10μF
25V
10μF
25V
GND
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
1μH
IHLP2525CZ-01
SW
PVIN
0.22μF
CMDHS-3 25V
INTVCC
The LTC36XX family are valley current mode regulators
so they inherently limit the cycle-by-cycle inductor current. The inductor current is sensed using the RDS(ON)
100μF
6.3V
LTC3608
GND
SW
4.7μF
INTVCC 10V
BOOST
INTVCC
RUN/SS
0.1μF
VOUT
EXTVCC
2200pF
60.4k
1%
PGND
ION
SGND
0.47μF
25V
VOUT
VFB
ITH
FCB
SVIN
SVIN
VRNG
100pF
VIN
1Ω
VON
PGOOD
19.1k
1%
12k
fSW = 650kHz
RON
162k
1%
DN457 F01
SVIN
Figure 1. 4.5V to 20V Input to 2.5V/8A High Density Step-Down Converter
01/09/457
100μF
6.3V
VOUT
2.5V
8A
limiting feature to further limit current in the event of
a short circuit. If the output drops more than 25%, the
maximum sense voltage is lowered to about one sixth
of its original value.
100
VIN = 5V
95
EFFICIENCY (%)
90
85
VIN = 20V
Paralleling Regulators for >12A
These parts can be easily paralleled for high output
current applications. Figure 3 shows a 1.2V/24A
application using two parallel LTC3610s. Because of the
valley current mode control architecture, the paralleled
regulators can operate at very low duty cycles with fast
transient response and excellent load balance.
80
75
70
65
60
VOUT = 2.5V
FREQ = 650kHz
55
50
0
1
2
7
6
4
3
5
LOAD CURRENT (A)
8
AN457 F02
Figure 2. Efficiency vs Load Current of Figure 1
of the bottom MOSFET —no additional current sense
resistor is required. The current limit is also adjustable
with the voltage at the VRNG pin. When the VRNG pin is
tied to ground, in the Figure 1 example, the current limit
is set to about 16A.
An open-drain logic power good output voltage monitor
(PGOOD) is pulled low when the output voltage is outside
±10% of the regulation point. In the case of overvoltage,
the internal top MOSFET is turned off and the bottom
MOSFET is turned on until the overvoltage condition
clears. The LTC36XX also includes a foldback current
The current sharing is simple. Connect the ITH pins
together, since the ITH pin voltage determines the cycleby-cycle valley inductor current. The feedback pins of
paralleled LTC3610s share a single voltage divider. The
RUN/SS pins are connected so that the LTC3610s start
up with same slew rate. The paralleled LTC3610s have
excellent thermal balance due to good current sharing.
Conclusion
With broad input and output ranges, high current capability
and high efficiency, these monolithic regulators provide
small size, low external component count power solutions
for many applications from communications infrastructure to industrial distributed power systems.
10μF
PVIN
RUN/SS
3.83k
150μF
0.33μH
SW
LTC3610
ITH
10μF
VIN
11V TO 13V
GND
+
ITH
PGND
22pF
VFB
SGND
VFB
60.4k
100pF
1800pF
10μF
PVIN
RUN/SS
0.1μF
100pF
ITH
PGND
47μF
s8
+
150μF
GND
0.33μH
SW
22pF
LTC3610
ITH
10μF
VOUT
1.2V
24A
VFB
SGND
VFB
DN457 F03
fSW = 750kHz
Figure 3. Two LTC3610s in Parallel Can Provide 24A Output Current
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 3258
www.linear.com
Linear Technology Corporation
dn457 LT/TP 0109 116K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2009
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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