August 2004 - 2-Phase Dual Synchronous DC/DC Controller with Tracking Provides High Efficiency in a Compact Footprint

DESIGN FEATURES
2-Phase Dual Synchronous DC/DC
Controller with Tracking Provides High
Efficiency in a Compact Footprint
by Jason Leonard
Introduction
The LTC3736 is a 2-phase dual
synchronous step-down DC/DC
controller that requires few external
components. Its No RSENSE, constant
frequency, current mode architecture
eliminates the need for current sense
resistors and improves efficiency, with-
out requiring a Schottky diode. The two
controllers are operated 180 degrees
out of phase, reducing the required
input capacitance and power loss and
noise due to its ESR. A tracking input
allows the second output to track the
first output (or another supply) dur-
59k
187k
VIN
2.7V TO 8V*
CIN
10µF
×2
22
23
24
1
2
3
4
+ 21
SENSE1
SW1
PGND
IPRG1
BG1
VFB1
SYNC/FCB
ITH1
TG1
IPRG2
PGND
PLLLPF
TG2
SGND
LTC3736
5
RUN/SS
VIN
15k
10Ω
1µF 220pF VIN
100k
15k
MP1
L1
1.5µH
SW1
20
19
18
17
16
15
MN1
Si7540DP
VOUT1
2.5V
5A**
+
COUT1
150µF
14
13
BG2
9
12
PGND
PGOOD
7
11
SENSE2+
VFB2
8
ITH2
10
6
TRACK
SW2
PGND
MN2
Si7540DP
MP2
SW2
+
100pF
220pF
ing startup, allowing the LTC3736 to
satisfy the power-up requirements of
many microprocessors, FPGAs, DSPs
and other digital logic circuits. The
LTC3736 is available in a tiny 4mm ×
4mm leadless QFN package and 24lead narrow SSOP package.
L2
1.5µH
COUT2
150µF V
OUT2
1.8V
5A**
25
10nF 100pF
118k
59k
59k
118k
L1, L2: IHLP-2525CZ-01-1.5
MP1/MN1, MP2/MN2: Si7540P COMPLEMENTARY P/N
COUT1, COUT2: SANYO 4TPB150MC
* THE LTC3736 IS ABLE TO OPERATE WITH INPUT VOLTAGES UP TO 9.8V.
IN THIS CIRCUIT, VIN IS LIMITED TO 8V BY THE MAXIMUM VGS RATING OF THE POWER MOSFETS.
** MAXIMUM LOAD CURRENT IS DEPENDENT UPON INPUT VOLTAGE.
THIS CIRCUIT CAN PROVIDE 5A WITH A 5V INPUT, 4A WITH A 3.3V INPUT.
Figure 1. 5V input, 2.5V and 1.8V dual output step-down converter
100
100
95
95
95
90
90
85
85
VIN = 3.3V
85
VIN = 4.2V
80
EFFICIENCY (%)
EFFICIENCY (%)
90
VIN = 5V
75
70
65
60
75
70
VIN = 5V
SYNC/FCB = VIN
VOUT = 3.3V
VOUT = 2.5V
VOUT = 1.8V
VOUT = 1.2V
65
60
55
50
80
VOUT = 2.5V
1
10
100
1000
LOAD CURRENT (mA)
10000
55
50
1
10
100
1000
LOAD CURRENT (mA)
10000
EFFICIENCY (%)
100
Burst Mode
OPERATION
(SYNC/FCB = VIN)
80
75
PULSE SKIPPING MODE
(SYNC/FCB = 550kHz)
70
65
FORCED
CONTINUOUS
(SYNC/FCB = 0V)
VIN = 5V
VOUT = 2.5V
60
55
50
1
10
100
1000
LOAD CURRENT (mA)
10000
Figure 2. Measured efficiencies for Figure 1’s circuit for various input voltages, output voltages, and modes of operation
Linear Technology Magazine • August 2004
17
DESIGN FEATURES
LTC3736 Features
❑ 2-Phase, dual output synchronous controller
❑ No RSENSE current mode architecture
❑ No Schottky diodes required
❑ Internal/external soft-start or tracking input ramps VOUT
❑ Wide VIN range: 2.75V to 9.8V
❑ 0.6V ±1.5% over temperature reference
❑ Selectable frequency, current limit, and light load operation
❑ Power Good (PGOOD) indicator
❑ Available in 4mm × 4mm leadless QFN package or 24-lead narrow
SSOP package
Circuit Description
Figure 1 shows a typical application
for the LTC3736. This circuit provides
two regulated outputs of 2.5V and
1.8V from a typical input voltage of
5V, but it can also be powered from
any input voltage between 2.75V and
9.8V (depending on the voltage rating of
the power MOSFETs). This wide input
range makes the LTC3736 suitable for
a variety of input supplies, including
1- and 2-cell Li-Ion and 9V batteries,
as well as 3.3V and 5V supply rails.
The LTC3736 uses the drain to
source voltage (VDS) of the power Pchannel MOSFET to sense the inductor
current. The maximum load current
that the converter can provide is determined by the RDS(ON) of the PFET,
which is a function of the input supply
voltage (which provides the gate drive).
The maximum load current can also
be changed independently for each
channel using the three-state current
limit programming pins IPRG1 and
IPRG2. In this circuit, each output
can provide up to 5A from a 5V input
supply. Efficiency for this circuit is
as high as 95%, as shown in Figure
2. In drop-out, the LTC3736 can operate at 100% duty cycle, providing
maximum operating life in battery
powered systems.
At light loads, the LTC3736 offers several modes depending on the
needs of the application: Burst Mode®
operation, forced continuous operation, or pulse skipping mode (when
synchronized to an external clock).
The mode is selected at the SYNC/FCB
pin as seen in Figure 2c. Burst Mode
operation provides the highest efficiency, but at the expense of increased
18
output voltage ripple at light loads.
In forced continuous operation, the
power MOSFETs continue to switch
every cycle (constant frequency) and
inductor current is allowed to reverse,
providing small output ripple at the
expense of light load efficiency. In pulse
skipping mode, inductor current is
not allowed to reverse and cycles are
skipped only as needed to maintain
regulation, providing smaller output
ripple but lower efficiency than Burst
Mode operation. The inductor current
waveforms for these three modes are
shown in Figure 3.
Switching frequency may be selected from 300kHz, 550kHz, or 750kHz
using the PLLLPF pin, or the LTC3736
can be synchronized to an external
clock signal between 250kHz and
850kHz using the LTC3736’s phaselocked loop (PLL). High frequency
operation permits the use of smaller
inductors and capacitors, further
Burst Mode
OPERATION
SYNC/FCB = VIN
FORCED
CONTINUOUS
MODE
SYNC/FCB = 0V
IL
1A/DIV
PULSE
SKIPPING MODE
SYNC/FCB = 550kHz
VIN = 3.3V
VOUT = 1.8V
ILOAD = 200mA
4µs/DIV
Figure 3. Inductor current at light load
SW1
2V/DIV
SW1
2V/DIV
SW2
2V/DIV
SW2
2V/DIV
fSW = 550kHz
VIN = 5V
1µs/DIV
fSW = 550kHz
VIN = 3.3 V
1µs/DIV
Figure 4. SW node waveforms depicting out-of-phase (2-phase) operation
VIN = 5V
200µs/DIV
RLOAD1 = RLOAD2 = 1Ω
VOUT1
2.5V
VOUT2
1.8V
VOUT1
2.5V
VOUT2
1.8V
500mV/
DIV
500mV/
DIV
VIN = 5V
40ms/DIV
RLOAD1 = RLOAD2 = 1Ω
Figure 5. Startup waveforms showing soft-start and tracking (internal
1ms soft-start on the left and external 150ms soft-start on the right)
Linear Technology Magazine • August 2004
DESIGN FEATURES
VOUT
AC-COUPLED
100mV/DIV
VOUT
AC-COUPLED
100mV/DIV
VOUT
AC-COUPLED
100mV/DIV
IL
2A/DIV
IL
2A/DIV
IL
2A/DIV
VIN = 3.3V
100µs/DIV
VOUT = 1.8V
ILOAD = 300mA TO 3A
SYNC/FCB = VIN
VIN = 3.3V
100µs/DIV
VOUT = 1.8V
ILOAD = 300mA TO 3A
SYNC/FCB = 0V
VIN = 3.3V
100µs/DIV
VOUT = 1.8V
ILOAD = 300mA TO 3A
SYNC/FCB = 550kHz EXTERNAL CLOCK
Figure 6. Transient response to a 300mA to 3A load step (left to right, Burst Mode operation, forced continuous, and pulse skipping mode)
reducing the total solution size. The
2-phase switching behavior of the
LTC3736 is depicted by the SW node
waveforms in Figure 4.
Tracking
The LTC3736 features an internal
soft-start that ramps VOUT1 smoothly
from 0V to its final value in 1ms. This
soft-start time can be increased externally by connecting a capacitor on the
RUN/SS pin to ground. The startup of
VOUT2 can be programmed externally
(with two resistors) to track VOUT1 (or
RFB1A
59k
any other supply or reference) using
the LTC3736’s TRACK pin input. Use
of the TRACK pin permits ratiometric or tracking startup of VOUT2. The
open-drain PGOOD output indicates
when both outputs are within ±10%
of their regulated values. Figure 5
shows the startup waveforms for the
outputs of the Figure 1 circuit using
the internal soft-start and an optional
external soft-start capacitor, with
VOUT2 programmed to track VOUT1 in
a 1:1 ratio.
RFB1B
187k
VFB
CITH1A
100pF
VIN
3.3V*
CIN
10µF
×2
RITH1
CITH1 15k
220pF
RVIN 10Ω
CVIN 1µF
1M
CITH2
100pF
MITH
22
23
24
1
2
3
4
SW1
SENSE1+
IPRG1
PGND
VFB1
BG1
SYNC/FCB
ITH1
IPRG2
TG1
PLLLPF
PGND
SGND
TG2
LTC3736
5
VIN
RUN/SS
4700pF
BG2
9
PGND
PGOOD
7
SENSE2+
V
8 FB2
ITH2
6
TRACK
SW2
PGND
21
20
19
18
17
16
15
L1
1.5µH
MP1
MN1
COUT1
150µF
VOUT
2.5V
8A**
+
14
13
12
11
10
MN2
MP2
L2
1.5µH
25
L1, L2: IHLP-2525CZ-01-1.5
MITH: VN2222LL
MP1/MN1, MP2/MN2: Si7540P COMPLEMENTARY P/N
COUT1, COUT2: SANYO 4TPB150MC
* THE LTC3736 IS ABLE TO OPERATE WITH INPUT VOLTAGES UP TO 9.8V.
IN THIS CIRCUIT, VIN IS LIMITED TO 8V BY THE MAXIMUM VGS RATING OF THE POWER MOSFETS.
** MAXIMUM LOAD CURRENT IS DEPENDENT UPON INPUT VOLTAGE.
THIS CIRCUIT CAN PROVIDE 10A WITH A 5V INPUT, 8A WITH A 3.3V INPUT.
Figure 7. 3.3V to 1.8V at 8A 2-phase step-down converter
Stable with All Types
of Output Capacitors
The compensation components on the
ITH pins can be easily adjusted to make
LTC3736-based power supplies stable
for a wide variety of output capacitors, including tantalum, aluminum
electrolytic, and ceramic capacitors.
Figure 6 shows the transient response to a load step for the circuit
in Figure 1.
3.3V to 2.5V at 8A 2-Phase,
Single Output Regulator
Figure 7 shows the LTC3736 configured in a 2-phase, single output
converter. This regulator can provide
8A of load current to a 2.5V output
from a 3.3V input supply. The two
output stages of the LTC3736 continue
to operate out of phase, but supply
power to a single output. This 2-phase,
single output operation reduces not
only the required input capacitance
by up to 50%, but also the required
output capacitance.
Conclusion
LTC3736-based power supplies can
deliver high efficiency for input voltages up to 9.8V and output load
currents as high as 5A. The tracking input allows the two outputs to
smoothly track during startup. Its
2-phase, high frequency, No RSENSE,
synchronous current mode architecture results in a small solution size
with no Schottky diodes and no current sense resistors.
For more information on parts featured in this issue, see
http://www.linear.com/go/ltmag
Linear Technology Magazine • August 2004
19
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