4mm × 7mm IC Produces Seven Regulated Outputs and a Dual-String LED Driver

4mm × 7mm IC Produces Seven Regulated Outputs
and a Dual-String LED Driver
Aspiyan Gazder
The LTC3675 is a space-saving single-chip power solution for multirail applications
that run from a single Li-ion cell. Its 4mm × 7mm QFN contains two 500mA buck
regulators, two 1A buck regulators, a 1A boost regulator, a 1A buck-boost regulator,
a boost LED driver capable of driving dual LED strings up to 25mA, and an alwayson 25mA LDO for powering a housekeeping microprocessor. All regulators can be
controlled via I2C. Figure 1 shows an eight-rail solution run from a single Li-ion battery.
SWITCHING REGULATOR FEATURES
All of the voltage regulators in the LTC3675
are internally compensated monolithic
synchronous regulators. The buck regulators and the buck-boost regulator can be
enabled via enable pins or I2C, whereas
the boost regulator is enabled via I2C only.
The feedback regulation voltage of
the regulators can be programmed via
I2C from 425mV to 800mV in 25mV steps.
Each regulator offers two modes of light
load operation. The buck regulators offer
Burst Mode operation for the greatest efficiency and pulse skipping-mode for more
predictable EMI. The boost and buck-boost
regulators offer Burst Mode operation
and PWM mode. Each regulator’s operating mode can be programmed via I2C.
The regulators also feature
I2C-programmable slew rate control
on the switch edges, where fast switching produces higher efficiency and slow
switching improves EMI performance.
PARALLEL BUCK REGULATORS
FOR INCREASED LOAD CURRENT
CAPABILITY
Any two successively numbered buck
regulators of the LTC3675 can be combined
in parallel to produce a single regulator output with a combined load current
capability. For instance, buck regulators 1
26 | October 2010 : LT Journal of Analog Innovation
(capable of 1A) and 2 (capable of 1A) can
be paralleled to produce a single buck
regulator capable of delivering up to
2A of load current. Similarly, buck regulators 2 and 3 can be paralleled to make a
single buck regulator with load capability
up to 1.5A and buck regulators 3 and 4
can be paralleled to make a single buck
regulator with load capability up to 1A.
When two buck regulators are combined,
the lower numbered buck regulator serves
as the master and controls the power
stage of the higher numbered slave buck
regulator. The behavior of the combo
buck regulator is programmed via the
master (lower numbered) regulator. To
configure a buck regulator as a slave, its
feedback pin must be connected to VIN and
the switch nodes of the master and slave
buck regulators are shorted together to a
common inductor. The trace impedances
of both master and slave must be kept the
same from the switch pins to the inductor
to obtain better current flow distribution
in the two power stages. Unequal trace
impedance may compromise on the load
capability of the combo buck regulator.
Figure 2 shows an application in which
buck regulators 1 and 2 have been paralleled with buck regulator 1 as the master and buck regulator 2 as the slave.
LED DRIVER FEATURES
The LED driver is capable of driving two
LED strings with up to 10 LEDs each. The
LED driver may alternately be configured as a high voltage boost regulator.
When configured as a dual string
LED driver, the lower of the voltages at
the LED1 or LED2 pins is the regulation
point. In Figure 1, the 20k resistor at the
LED_FS pin programs the LED full-scale
current to 25mA. Better than 1% matching
between the two LED strings is achieved at
this current level. An automatic gradation
circuit allows the LED current to change
levels at a rate programmed by the user.
For applications that require LEDs to be
biased at currents higher than 25mA, the
programmed current can be doubled by
setting a bit in the program register via
I2C. For a LED_FS resistor of 20k, setting this bit programs a full-scale current of 50mA. When used in this mode
the output voltage is limited to 20V.
LED DRIVER CONFIGURED AS A
HIGH VOLTAGE BOOST REGULATOR
The LED driver can be configured to operate as a high voltage boost regulator using
an I2C command. The LED_OV pin acts as
the feedback pin. An output voltage up
to 40V can be programmed using external resistors. In Figure 2 the LED driver
design features
Li-Ion
CELL
2.7V
TO 4.2V
VIN
1µF
1.2V
25mA
VIN
LDO_OUT
10µF
SW1
L1
2.2µH
10µF
324k
FB1
649k
649k
VIN
1µF
Figure 1. Li-ion cell to eight power rails,
including an LED driver, using a single IC
VIN
L5
2.2µH
5V
1A
22µF
22µF
200k
L6
2.2µH
10µF
309k
LTC3675
VIN
322k
*
590k
FB3
105k
1µF
L3
2.2µH
SW3
FB6
I2C
CONTROL
10µF
SW4
DVCC
10µF
511k
FB4
is configured as a boost regulator that
provides a 12V output. To maintain stability, the average inductor current must
not exceed 750mA. For a 12V output, up
to 150mA of load current can be supplied
over the entire input voltage range.
PUSHBUTTON INTERFACE AND
SEQUENTIAL POWER UP
The LTC3675 can be powered up or powered down using the ONB pin. All timing
related to the ONB, RSTB and WAKE pins
are programmed by the CT capacitor.
In the discussion below, a CT capacitor of 0.01µF is assumed.
PUSHBUTTON
ONB
Regulators may be started up sequentially using the pushbutton interface and
precision enable thresholds. When all
regulators are off, the enable pin threshold is 650mV. Once a regulator has been
enabled either via I2C or its enable pin,
the thresholds of the remaining enable
pins is set to precisely 400mV. This allows
a well controlled sequential power up.
After initial power up and if no regulator
has yet been enabled, holding the ONB pin
low for 400ms causes the WAKE pin to go
high for five seconds. The WAKE pin may
be hard tied to an enable pin to power up
any individual regulator, whose output
10µF
L7
10µH
SW7
4.7µF
50V
D1
UP TO •
10 LEDS ••
0.01µF
D1: DIODES INC. PD3S140
L1, L2, L5, L6: COILCRAFT XFL4020-222
L3, L4: TOKO MDT2520-CR2R2
L7: VISHAY IHLP 2020BZER10R
*ALL PULL UP RESISTORS ARE 100k
1.6V
500mA
511k
*
IRQB
RSTB
WAKE
PBSTAT
EN1
EN2
EN3
EN4
EN6
CT
EXPOSED PAD
10µF
1.8V
500mA
475k
L4
2.2µH
SCL
SDA
MICROPROCESSOR
CONTROL
10µF
2.5V
1A
FB2
SWAB6
SWCD6
VOUT6
3.3V
1A
22µF
665k
1.05M
FB5
L2
2.2µH
SW2
SW5
VOUT5
22µF
22µF
324k
LDOFB
1.2V
1A
LED1
LED2
LED_OV
LED_FS
•
•
•
1.96M
20k
42.2k
3675 F06
may then be used to power up another
regulator. In this fashion, the LTC3675 can
be sequentially powered up as shown in
Figure 3. Figure 4 shows the sequential
power up of buck regulator 1 followed
by buck regulator 2 and then by buck
regulator 3. Before the WAKE pin goes
LOW, an I2C command must be written to
reinforce the enabled status of buck 1.
Otherwise, when WAKE is pulled low,
buck regulator 1 shuts off, causing buck
regulators 2 and 3 to power down as well.
If the LTC3675 has one or more regulators enabled, pressing the ONB pin for
five seconds generates a hard reset. A
October 2010 : LT Journal of Analog Innovation | 27
Li-Ion
CELL
2.7V
TO 4.2V
VIN
VIN
1µF
10µF
1.2V
25mA
L1
2.2µH
LDO_OUT
10µF
324k
SW2
649k
Figure 2. Paralleling buck regulators 1
and 2 ups the load current capability.
The 12V output is produced using the
boost typically used for LED strings.
22µF
L4
2.2µH
200k
LTC3675
10µF
22µF
332k
*
FB3
649k
DVCC
FB4
SCL
SDA
LED_FS
*
D1: DIODES INC. PD3S140
L1: TOKO FDV0530-2R2
L2: TOKO MDT2520-CR2R2
L3: COILCRAFT XFL4020-222
L4: VISHAY IHLP 2020BZER10R
*ALL PULL UP RESISTORS ARE 100k
hard reset causes all enabled regulators
to power down for one second. After one
second, the hard reset state is exited and
the I2C registers are all set to their default
state. A hard reset may also be generated
using the RESET_ALL bit via I2C command.
The PBSTAT pin reflects the status of
the ONB pin once the LTC3675 is in the
ON state. At initial power up, if the
ONB pin is pulled low and all regulators are off, PBSTAT remains in the
high impedance state. If a regulator is
28 | October 2010 : LT Journal of Analog Innovation
SW7
PUSHBUTTON
10µF
20V
D1
12V
150mA
1.87M
LED_OV
LED1
LED2
0.01µF
ONB
10µF
L4
10µH
IRQB
RSTB
WAKE
PBSTAT
EN1
EN2
EN3
EN4
ENBB
CT
MICROPROCESSOR
CONTROL
1.2V
1A
324k
SW4
105k
1µF
10µF
SW3
FB6
I2C
CONTROL
VIN
L2
2.2µH
SWAB6
SWCD6
VOUT6
3.3V
1A
10µF
FB2
1.05M
FB5
22µF
22µF
VIN
SW5
VOUT5
22µF
22µF
309k
L3
2.2µH
5V
1A
665k
FB1
VIN
1µF
2.5V
2A
SW1
LDOFB
133k
EXPOSED PAD
enabled, ONB going low for at least 50
ms will cause PBSTAT to also go low.
I 2C FEATURES
preset undervoltage warning thresholds and one of three preset die temperature warning thresholds.
The I2C interface provides both programmability and status reporting via 11
program registers and 2 status registers.
The contents of these registers can be read
at any time to ensure proper operation.
The I2C port is also used to reset the
IRQB pin and the latched status register
bits in the event that a fault has occurred.
Each switching regulator is associated
with a single program register while
the LED driver is controlled by two
program registers. The UVOT program
register is used to select one of eight
The LTC3675’s RSTB and IRQB pins are
pulled low when reporting an error
condition—otherwise they remain in a
high impedance state. Reported error
conditions include out-of-regulation
ERROR CONDITION REPORTING—
USING RSTB AS A POWER ON RESET
design features
Li-Ion CELL
2.7V TO 4.2V
VIN
1µF
1.2V
25mA
VIN
LDO_OUT
10µF
SW1
324k
649k
10µF
324k
LDOFB
FB1
VIN
VIN
L5
2.2µH
Figure 3. Single string
LED driver with regulator
start-up sequencing
SW5
22µF
5V
1A
SW2
200k
L6
2.2µH
VIN
10µF
L2
2.2µH
10µF
SW4
511k
10µF
1.6V
500mA
FB4
511k
SCL
SDA
L7
10µH
IRQB
RSTB
WAKE
PBSTAT
EN1
EN2
EN3
EN4
ENBB
CT
ONB
10µF
SW7
4.7µF
50V
D1
UP TO •
10 LEDS ••
LED1
LED2
0.01µF
Each voltage regulator has an internal
power good (PGOOD) signal that indicates
the status of its output voltage. The output
voltage of a regulator is defined as bad if it
is enabled and the output voltage is below
its programmed value by more than 7.5%.
The PGOOD bit is set to zero indicating
the output voltage is bad. The LED driver
PGOOD signal is used only when it is configured as a high voltage boost regulator.
1.8V
500mA
475k
*
output voltages, input undervoltage and overtemperature warnings.
10µF
L4
2.2µH
*
PUSHBUTTON
2.5V
1A
FB3
DVCC
D1: DIODES INC. PD3S140
L1, L2, L5, L6: COILCRAFT XFL4020-222
L3, L4: TOKO MDT2520-CR2R2
L7: VISHAY IHLP 2020BZER10R
*ALL PULL UP RESISTORS ARE 100k
511k
511k
L3
2.2µH
590k
332k
MICROPROCESSOR
CONTROL
22µF
SW3
105k
1µF
511k
309k
FB6
I2C
CONTROL
1.2V
1A
LTC3675
SWAB6
SWCD6
VOUT6
10µF
511k
FB2
1.05M
FB5
22µF
649k
665k
VOUT5
22µF
×2
3.3V
1A
10µF
L1
2.2µH
EXPOSED PAD
1.96M
LED_OV
LED_FS
A PGOOD bit going low will pull RSTB low
if unmasked. When the error condition is
cleared, the RSTB pin goes back to its high
impedance state. The user can selectively
mask out an error condition from pulling
RSTB low by programming the RSTB mask
register. As an example, if the boost regulator is enabled but the user does not need
to know the status of its output, the user
can program the RSTB mask register such
that a bad output at the boost regulator
will not cause RSTB to be pulled low.
20k
42.2k
The RSTB pin may be used to implement
a power on reset function. After a regulator has been enabled, the RSTB pin is
pulled low and stays low until the output
voltage has been above its PGOOD threshold for 200ms. After that, the RSTB pin
returns to its high impedance state.
The above example assumes that the
RSTB mask register contents are such that
the PGOOD signal of the enabled regulator is allowed to pull the RSTB pin low.
October 2010 : LT Journal of Analog Innovation | 29
The LTC3675 is ideally suited for applications that
require multiple power rails from a single Li-ion
battery source. Six regulators combined with a
dual string LED driver set the LTC3675 apart from
competing power management solutions.
Figure 4. Sequenced start-up of
the four buck regulators
OVERTEMPERATURE FAULT
WARNING AND SHUTDOWN
WAKE
5V/DIV
VOUT1
1V/DIV
VOUT2
2V/DIV
VOUT3
1V/DIV
100µs/DIV
The IRQB pin is also pulled low when an
error is generated and stays low even if the
error condition has been corrected. The
IRQB pin is cleared using an I2C command.
In addition to reporting a bad regulator
output voltage, the IRQB is also pulled low
if either the input undervoltage or overtemperature warning thresholds have been
exceeded. By programming the IRQB mask
register, it is possible to selectively mask
the error conditions that cause IRQB to
be pulled low. The input undervoltage
warning and overtemperature warning conditions cannot be masked.
The data in the real time status and
latched status registers reveal the exact
nature of the fault. The condition of
the error reporting bits in the real time
status register changes as the error
conditions change. The latched status
register information is latched when an
unmasked error condition occurs—the
contents of the register do not change
after the latching event. The contents
of the latched status register are cleared
during an IRQB clear command.
30 | October 2010 : LT Journal of Analog Innovation
INPUT UNDERVOLTAGE FAULT
WARNING AND SHUTDOWN
The LTC3675 is capable of operating at
input voltages down to 2.7V. Nevertheless,
other devices may need to shut down
or enter a low power state before the
Li-ion discharges all the way to 2.7V. The
LTC3675 includes an input undervoltage warning signal, with a threshold set
to one of eight levels via I2C. When the
input voltage drops to the programmed
threshold voltage, the IRQB pin is pulled
low, indicating a fault. The status register can be read to determine the fault
and take any corrective action needed.
The LTC3675 also includes an input
undervoltage shutdown, which turns off
all enabled regulators if the input supply
voltage drops below 2.45V. The contents
of the program registers are reset to their
default state. Operation resumes once
the input voltage increases above 2.55V.
The LTC3675 is capable of delivering
more than 15W of output power in a very
small amount of board space. Even with
its high efficiency regulators, the combined efficiency losses produce dissipated
heat, which raise the die temperature. To
protect the die and other components, the
LTC3675 includes four I2C-selectable die
temperature warning thresholds. When
the die temperature exceeds the selected
warning threshold, the IRQB pin pulls
low. In the event of a warning, the status
register can be read to determine the fault.
If the die temperature exceeds 150°C, all
enabled regulators are shut down and
the program registers are reset to their
default state. Operation resumes once
the die temperature drops below 135°C.
CONCLUSION
The LTC3675 is ideally suited for applications that require multiple power rails
from a single Li-ion battery source. Six
regulators combined with a dual string
LED driver set the LTC3675 apart from
competing power management solutions.
I2C programmability and fault reporting give system designers the ability to
maximize battery run time with efficient
battery power usage and active thermal
management. The LTC3675 is available in a
space saving 4mm × 7mm QFN package. n
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