Aug 1998 Fixed Frequency, 500kHz, 4.5A Step-Down Converter in an SO-8 Operates from a 5V Input

DESIGN FEATURES
Fixed Frequency, 500kHz, 4.5A
Step-Down Converter in an SO-8
Operates from a 5V Input
by Karl Edwards
D2
1N914
Introduction
5V to 3.3V Buck Converter
The circuit in Figure 1 is a step-down
converter suitable for use as a local
regulator to supply 3.3V logic from a
5V power bus. The high efficiency,
shown in Figure 2, removes the need
for bulky heat sinks or separate power
devices, allowing the circuit to be
placed in confined locations. Since
the boost circuit only needs 3V to
operate, the boost diode can still be
connected to the output, improving
efficiency. Figure 1’s circuit shows
the shutdown pin option. If this pin is
16
C2
0.68µF
INPUT
5V
C3
10µF TO
50µF
CERAMIC
BOOST
VIN
+
OPEN
OR
HIGH
= ON
L1
5µH
OUTPUT
3.3V
4A
VSW
LT1506-3.3
SHDN
GND
SENSE
VC
CC
1.5nF
+
D1
MBRS330T3
C1
100µF, 10V
SOLID
TANTALUM
1506 TA01
Figure 1. 5V to 3.3V step-down converter
pulled to a logic low, the output is
disabled and the part goes into shutdown mode, reducing supply current
to 20µ A. An internal pull-up ensures
correct operation when the pin is left
open. The SYNC pin, an option for the
DD package, can be used to synchronize the internal oscillator to a system
clock. A logic-level clock signal applied
to the SYNC pin can synchronize the
switching frequency in the range of
580kHz to 1MHz.
to the energy that needs to be stored
in the core. Three 4A inductors store
less energy (1/2Li2 ) than a single 12A
coil, so they are much smaller. In
addition, synchronizing three converters 120° out of phase with each
other reduces input and output ripple
currents. This reduces the ripple rating, size and cost of the filter
capacitors.
Current Sharing
Multiphase Supply
Current sharing is accomplished by
connecting the VC pins to a common
compensation capacitor. The output
of the error amplifier is a gm stage, so
any number of devices can be connected together. The effective gm of
the composite error amplifier is the
product of the individual devices. In
Figure 3, the compensation capacitor, C4, has been increased by 3×.
Tolerances in the reference voltages
cause small offset currents to flow
between the VC pins. The overall effect
is that the loop regulates the output
at a voltage somewhere between the
minimum and maximum references
of the devices used. Switch-current
matching between devices will be
typically better than 300mA over the
full current range. The negative
temperature coefficient of the VC-toswitch-current transconductance
prevents current hogging.
The circuit in Figure 3 uses multiple
LT1506s to produce a 5V, 12A power
supply. There are several advantages
to using a multiple switcher approach
compared to a single larger switcher.
The inductor size is considerably
reduced. Inductor size is proportional
90
85
EFFICIENCY (%)
The LT1506 is a 500kHz monolithic
buck mode switching regulator, functionally identical to the LT1374 but
optimized for lower input voltage applications. Its high 4.5A switch rating
makes this device suitable for use as
the primary regulator in small to
medium power systems. The small
SO-8 footprint and input operating
range of 4V to 15V is ideal for local
onboard regulators operating from
5V or 12V system supplies. The 4.5A
switch is included on the die, along
with the necessary oscillator, control
and logic circuitry to simplify design.
The part’s high switching frequency
allows a considerable reduction in
the size of external components, providing a compact overall solution.
The LT1506 is available in standard 7-pin DD and fused-lead SO-8
packages. It maintains high efficiency
over a wide output current range by
keeping quiescent supply current to
4mA and by using a supply-boost
capacitor to saturate the power switch.
The topology is current mode for fast
transient response and good loop stability. Full cycle-by-cycle short-circuit
protection and thermal shutdown are
provided. Both fixed 3.3V and adjustable output voltage parts are available.
80
75
70
0
1
2
3
LOAD CURRENT (A)
4
Figure 2. Efficiency vs load current for
Figure 1’s circuit
Current Sharing/
Split Input Supplies
Linear Technology Magazine • August 1998
DESIGN FEATURES
C1, C3: MARCON THCS50E1E106Z
D1: ROHM RB051L-40
D2: 1N914
L1: DO3316P-682
3-BIT RING
COUNTER
1.8MHz
INPUT
6V TO 15V
LT1506-SYNC
LT1506-SYNC
LT1506-SYNC
VC SYNC SW GND VIN BOOST FB
VC SYNC SW GND VIN BOOST FB
VC SYNC SW GND VIN BOOST FB
R1
5.36k
1% +
+
+
C3A
10µF
25V
D1A
C4
68nF
25V
+
L1B
6.8µH
C2B
330nF
10V
+
D2A
C1
10µF
25V
D1C
+
C2A
330nF
10V
R2
4.99k
1%
C3C
10µF
25V
D1B
+
L1A
6.8µH
+
C3B
10µF
25V
5V
12A
D2B
C2C
330nF
10V
L1C
6.8µH
D2C
1506 F15
Figure 3. Current-sharing 5V/12A supply
CURRENT
PHASE 2
TIME
CURRENT
PHASE 3
TIME
CURRENT
TOTAL
TIME
Figure 4. Input current
Linear Technology Magazine • August 1998
PHASE 1
CURRENT
A ring counter generates three synchronization signals at 600kHz, 33%
duty cycle, phased 120° apart. The
sync input will operate over a wide
range of duty cycles, so no further
pulse conditioning is needed. At full
load, each device’s input ripple current is a 4A trapezoidal wave at
600kHz, as shown in Figure 4. Summing these waveforms gives the
effective input ripple for the complete
system. The resultant waveform,
shown at the bottom of Figure 4,
remains at 4A but its frequency has
increased to 1.8MHz. The higher frequency eases the requirements on
the value of input filter without the 3×
increase in ripple current rating that
would normally occur. Although only
a single input capacitor is required,
practical layout restrictions usually
dictate an individual capacitor at each
device. Figure 5 shows the output
ripple current waveforms. The resultant 1.8MHz triangular waveform has
a maximum amplitude of 350mA at
an input voltage of 10V. This is
significantly lower than would be
expected for a 12A output. Interestingly, at inputs of 7.6V and 15V, the
TIME
PHASE 2
CURRENT
TIME
Synchronized
Ripple Currents
theoretical summed output ripple
current cancels completely. To reduce
board space and ripple voltage, C1
and C3 are ceramic capacitors. Loop
compensation capacitor C4 must be
adjusted when using ceramic output
capacitors, due to the lack of effective
series resistance (ESR). The typical
TIME
PHASE 3
CURRENT
CURRENT
PHASE 1
the backplane, copper losses, connectors and so on. The common VC
signal ensures that loading is still
shared between the devices.
TIME
TOTAL
CURRENT
A common VC voltage forces each
LT1506 to operate at the same switch
current, not at the same duty cycle.
Each device operates at the duty cycle
defined by its input voltage. This is a
useful feature in a distributed power
system. The input voltage to each
device could vary due to drops across
TIME
Figure 5. Output current
17
DESIGN FEATURES
tantalum compensation value of 1.5nF
is increased to 22nF (×3) for the
ceramic output capacitor. If synchronization is not used and the internal
oscillators free run, the circuit will
operate correctly, but ripple cancellation will not occur. Input and output
capacitors must be ripple rated for
the individual output currents.
Redundant Operation
The circuit shown in Figure 3 is fault
tolerant when operating at less than
8A of output current. If one power
stage fails open circuit, the output
will remain in regulation. The feedback loop will compensate by raising
the voltage on the VC pin, increasing
the switch current of the two remaining devices.
5V to 3.3V at 2.5A
on 0.25in2 of board space,
0.125in High
higher ripple current can be tolerated, allowing the use of small, low
value, high current inductors. A
ceramic output capacitor also reduces
board area and improves voltage
ripple. Using Figure 1’s circuit with
the SO-8 LT1506 and the component
changes in Table 1, a very small, low
profile, step-down converter can be
implemented.
In many space-sensitive applications,
the component that dominates both
board area and overall height is the
inductor. One of the factors affecting
inductor value choice is maximum
ripple current. Using the high current switch rating of the LT1506,
Conclusion
Table 1. Component changes for a low profile
version of Figure 1’s circuit
Part
Value
C1, C3
22µF, 10V
CC
22nF
L1
2.2µH
The LT1506 is a compact, easy to
use, monolithic switcher. The internal 4.5A switch covers a wide range of
medium power applications. Its input
operating range of 4V to 15V and
availability in SO-8 or DD packages
make it ideal for very space-efficient,
local onboard DC/DC converters.
Vendor/
Part#
Tokin
1210ZG226Z
Sumida
CD43 2R2
Authors can be contacted
at (408) 432-1900
RF 4.7Ω
1
VIN = 20V
VOUT = 2.5V
INTVCC
EFFICIENCY (%)
95
CSS
LTC1625
0.1µF
2
3
4
90
CC1
1nF
LTC1435
RC1
5
10k
6
CC2 220pF
85
7
8
LTC1625
16
EXTVCC
VIN
SYNC
TK
RUN/SS
SW
FCB
TG
ITH
BOOST
SGND
INTVCC
VOSENSE
VPROG
BG
PGND
15
13
12
1
2
3
LOAD CURRENT (A)
4
**DB
11
10
CB
O.1µF
VIN
12V–28V
CIN
22µF
35V
×2
L1* 39µH
D1
MBRS140T3
CVCC
4.7µF
R2
35.7k
1% +
R1
3.92k
1%
9
80
0
M1
Si4412DY
14
+
100
+
CF
0.1µF
VOUT
12V/2.2A
COUT
100µF
16V
0.030Ω ESR
M2
Si4412DY
5
DI_1068_02a. EPS
* L1 = SUMIDA CDRH127-390MC
** DB = CMDSH-3
Figure 4. Efficiency vs load current
Figure 5. 12V/2.2A adjustable-output supply
LTC1625, continued from page 4
Conclusion
A circuit demonstrating the wide
output range of the LTC1625 is shown
in Figure 5. This application uses
Si4412DY MOSFETs to deliver a 12V
output at up to 2.2A. Note that the
SYNC pin is tied high for 225kHz
operation in order to reduce the
inductor size and ripple current.
The LTC1625 step-down DC/DC controller offers true current mode control
without the expense and difficulty of
using a sense resistor. Popular features from Linear Technology’s other
controllers, such as fixed frequency
operation, N-channel MOSFET drive,
Burst Mode operation, soft-start and
output voltage programming make
18
this controller useful in a variety of
applications. By eliminating the power
loss in the sense resistor, even higher
efficiencies can be achieved than were
previously possible, making the
LTC1625 an excellent choice for
DC/DC converter designs requiring
the highest performance.
Linear Technology Magazine • August 1998
Similar pages