Jan 2008 - I 2 C Quad Buck Regulator Packs Performance, Functionality, Versatility and Adaptability in a 3mm × 3mm QFN

L DESIGN IDEAS
I2C Quad Buck Regulator Packs
Performance, Functionality, Versatility
and Adaptability in a 3mm × 3mm QFN
by Joe Panganiban
I2C Programmable
The LTC3562 is an I2C quadruple erating modes to satisfy the various Output Voltages
Introduction
step-down regulator composed of four
extremely versatile monolithic buck
converters. Two 600mA and two 400mA
highly adjustable step-down regulators
provide a total of 2A of available output
current, all packed inside a 3mm × 3mm
QFN package. All four regulators are
2.25MHz, constant-frequency, current
mode switching buck converters whose
output voltages and operating modes
can be independently adjusted through
I2C control. The 2.7V to 5.5V input
voltage range makes it ideally suited for
single Li-Ion battery-powered applications requiring multiple independent
voltage supply rails.
I2C Programmable
Operating Modes
All four LTC3562 step-down regulators have the unique ability to be
programmed into four distinct op-
noise/power demands of a variety of
applications. These four modes are
pulse skipping mode, Burst Mode operation, forced Burst Mode operation,
and LDO mode.
Pulse skipping mode allows the
regulator to skip pulses at light load
currents, providing very low output
voltage ripple while maintaining high
efficiency. Burst Mode operation and
forced Burst Mode operation deliver
bursts of current to the buck output and regulate the output voltage
through hysteretic control, giving the
highest efficiency at low load currents.
In LDO mode, the bucks are converted
to DC linear regulators and deliver
continuous power from the switch
pins through the inductor, providing
the lowest possible output noise as
well as the lowest no-load quiescent
current.
Another unique feature of the LTC3562
is its ability to adjust the output voltage
of each regulator through I2C control.
The chip contains two different flavors
of output adjustable regulators. The
Type A regulators (R600A, R400A)
have programmable feedback servo
voltages, while the Type B regulators (R600B, R400B) have directly
programmable output voltages that
do not need external programming
resistors.
The Type A regulators use external
feedback resistors to set the output
voltage based on a programmable
feedback servo voltage. The feedback
voltage values can be programmed from
800mV (full scale) down to 425mV in
25mV steps. This results in 16 possible
feedback servo voltages, and thus 16
different output voltage settings for the
same external programming resistors.
Table 1. Feature comparison of the LTC3562’s four integrated regulators (two 600mA and two 400mA)
R600A
R400A
R600B
R400B
Type
A
A
B
B
Output Current
600mA
400mA
600mA
400mA
I2C Programmable
Operating Modes
Pulse Skip
Burst
Forced Burst
LDO
Pulse Skip
Burst
Forced Burst
LDO
Pulse Skip
Burst
Forced Burst
LDO
Pulse Skip
Burst
Forced Burst
LDO
Feedback Servo
Voltage
I2C Programmable
425mV–800mV
25mV steps
(16 settings)
I2C Programmable
425mV–800mV
25mV steps
(16 settings)
600mV (Fixed)
600mV (Fixed)
I2C Programmable
600mV–3.775V
25mV steps
(128 settings)
No
34
Output Voltage
Adjustable using
External Resistors
Adjustable using External
Resistors
I2C Programmable
600mV–3.775V
25mV steps
(128 settings)
RUN Pins
Yes
Yes
No
Linear Technology Magazine • January 2008
DESIGN IDEAS L
100k
C5
10µF
Li-Ion BATTERY
3.4V TO 4.2V
SDA
VIN
VOUT 600B
3.3V
600mA
VOUT 400B
1.2V
400mA
L3
3.3µH
C3
10µF
C4
10µF
DVCC
R5
100k
LTC3562
SW600B
L1
3.3µH
POR600A
SW600A
OUT600B
L4
4.7µH
SCL
R1
634k
FB600A
RUN600A
SW400B
OUT400B
VOUT 600A
1.8V
600mA
C6
10pF
C1
10µF
POR SCL SDA
VCC CORE
VCC I/O
MICROPROCESSOR
R2
499k
VOUT 400A
2.5V
400mA
L2
4.7µH
RUN400A
SW400A
R3
1070k
FB400A
PGND AGND
C7
10pF
C2
10µF
R4
499k
L1, L3: TOKO 1098AS-4R7M
L2, L4: TOKO 1098AS-3R3M52
Figure 1. The LTC3562 configured in a quad step-down converter with pushbutton control and power sequencing.
RUN pins and
Default Settings
I2C applications generally have a
microprocessor in charge of the I2C
communications between the various system blocks. A multi-channel
buck converter such as the LTC3562
provides an excellent solution for
efficiently stepping down the microprocessor’s core and I/O supply
voltages from a higher input supply
or battery. At the surface, using an
I2C controllable voltage converter to
generate the microprocessor’s power
supplies seems to pose a bootstrap
problem at system start-up. If the
microprocessor initially has no power
and thus there is no I2C control, what
programs the LTC3562’s output to the
proper voltage for the patiently waiting
microprocessor?
Linear Technology Magazine • January 2008
100
90
80
EFFICIENCY (%)
The Type B regulators (R600B,
R400B) do not require external programming resistors at all because they
are integrated inside the chip. These
internal feedback resistors not only
save valuable board space, they are
also I2C programmable. The values
of the internal feedback resistors can
be adjusted through I2C control to
directly program the regulator output
voltages from 0.6V to 3.775V in 25mV
increments. That is 128 possible output voltage settings for each Type B
regulator.
FORCED
Burst Mode
OPERATION
600mA
BUCKS
70
60
50
40
PULSE SKIP
Burst Mode
OPERATION
30
20
VIN = 3.8V
VOUT = 2.5V
10
0
0.01
0.1
1
10
IOUT (mA)
100
1000
Figure 2. Efficiency of the 2.5V regulator
The LTC3562 gets around this
start-up issue by providing individual
RUN pins for the two Type A regulators. These RUN pins bypass the I2C
controls and enable the regulators if
I2C is unavailable. When a RUN pin
is used, the corresponding Type A
regulator is enabled in a default setting, which is 800mV for the feedback
voltage and pulse skipping mode for
the operating mode. Once I2C becomes
available to the system, these default
settings can always be modified on the
fly through I2C.
Pushbutton Control and
Power Sequencing
Figure 1 shows an application circuit
that uses the LTC3562 to power the
core and I/O supplies of a system microprocessor. The RUN pin of R600A
connects to a pushbutton circuit with
a pull-up resistor used to power on the
system. When the button is pushed,
the RUN pin goes low which enables
R600A to ramp up the power supply
for the microprocessor’s core. The RUN
pin of R400A is tied to R600A’s poweron-reset output signal (POR600A).
Once R600A reaches regulation,
POR600A goes high after a 230ms
time delay, which would then enable
R400A to power the I/O supply of the
microprocessor.
After both the core and I/O supplies
are up, the microprocessor could then
communicate back to the LTC3562
through I2C to program the part such
that it keeps R600A enabled even after
the pushbutton stimulus is removed.
The microprocessor then can enable
regulators R600B and R400B in any
mode and program the output voltages
to desired levels.
Low Power Adaptability
The ability to change the operating
modes and output voltages at any
time allows the LTC3562 to adapt to
the constantly changing demands of
many high performance systems. An
example of this adaptability would be
during lower power standby operation
in handheld battery-powered systems.
When going into standby mode, the
regulators can be programmed into
Burst Mode operation or forced Burst
continued on page 37
35
DESIGN IDEAS L
synchronization, user adjustable
switching frequency and a SHARE pin
for paralleling modules. The LTM8022
also employs Burst Mode operation,
drawing only 50µA quiescent current
at no load while maintaining only
30mV of output voltage ripple. Like the
LTM8020, the quiescent current when
shut down is less than 1µA. The schematic is very simple, with examples of
3.3V and 8V output designs shown in
Figures 3 and 4, respectively.
VIN
5.5V TO 36V
VOUT
RUN/SS
BIAS
AUX
2.2µF
ADJ
RT
49.9k
operation and low quiescent current.
The LTM8022 and LTM8023 share
the same footprint and pin pattern,
so even if you start a design with
the LTM8022 but later find that you
Max Load
VOUT Range
Size
LTM8020EV
4V to 36V
200mA
1.25V to 5V
6.25 × 6.25 × 2.32mm
LTM8022EV
3.6V to 36V
1A
0.8V to 10V
11.25 × 9 × 2.82mm
LMT8023EV
3.6V to 36V
2A
0.8V to 10V
11.25 × 9 × 2.82mm
the output is drawing full load. Its
efficiency is shown in Figure 7.
Conclusion
The LTC3813 and LTC3814-5’s
synchronous architecture and high
voltage capability make them ideally
suited for high voltage high power
boost converters. They decrease comLinear Technology Magazine • January 2008
154k
need more current, you can simply
drop in the LTM8023. In most cases,
the design will use identical passive
components as the LTM8022, as seen
in the 3.3V example in Figure 5.
Conclusion
VIN Range
LTC3813 and LTC3814-5, continued from page 21
GND SYNC
Figure 5. The LTM8023 produces 3.3V at 2A with the same footprint
and components required for the LTM8022 producing 1A.
Part Number
Mode operation to maximize power
efficiency at light loads. Under noload conditions, the regulators can
also be programmed into LDO mode,
which provides the lowest quiescent
current (all four regulators in LDO
mode only draw a combined 80µA for
the entire chip).
To save even more power, the
LTC3562 can be programmed to reduce the regulators’ output voltages
in Burst Mode operation or forced
Burst Mode operation during light load
conditions. Since power dissipation
22µF
SHARE
Table 1. Summary of LTM8000 series µModule regulators
LTC3562, continued from page 35
VOUT
3.3V
2A
LTM8023
…Or, Even More Power…
The LTM8023 is the big brother of the
LTM8022, capable of producing up to
2A of output current. The LTM8023
has the same input, output voltage
range, and control features as the
LTM8022. It also features Burst Mode
VIN
is directly proportional to the supply
voltage multiplied by the load current,
dropping the supply voltage effectively
reduces the circuit’s total power dissipation. If the output load is resistive
in nature, reducing the supply voltages
has an even greater effect, since power
dissipation in the load is proportional
to the supply voltage squared.
Conclusion
The LTC3562 is a highly flexible I2C
quad step-down converter composed
of two 600mA and two 400mA buck
plexity by eliminating the requirement
for a large diode package and heat
sink to dissipate its high power loss.
Programmable frequency and current
limit, wide output voltage range, and
ability to drive logic-level or higher
threshold MOSFETs provide maximum flexibility in using them for a
variety of boost applications. Other
The LTM8020, LTM8022 and LTM8023
µModule regulators make power supply development fast and easy. Their
broad input and output voltage ranges,
load capabilities and small size (see
Table 1) make them readily fit into a
wide variety of applications. L
regulators in a 3mm × 3mm QFN
package. The output voltages of the
regulators can be switched on the fly
using servo control or I2C control. Each
regulator can also be switched on the
fly into four possible high efficiency or
low-noise operating modes. This is a
perfect device for high performance
applications that require constant
control of the power supply. It can also
be used to simplify design, build and
test cycles, since output voltages can
easily be changed without changing
components. L
features such as such as strong gate
drivers to minimize transition losses,
an accurate voltage reference, accurate
cycle-by-cycle current limit, and an
on-chip bias supply controller make
the LTC3813 and LTC3814-5 the obvious choice for high performance, high
power boost converters. L
37