DN1020 - Dual Output Monolithic Synchronous Buck Regulator Operates from 3.6V to 15V Inputs and Delivers 3A Per Channel

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Dual Output Monolithic Synchronous Buck Regulator Operates
from 3.6V to 15V Inputs and Delivers 3A Per Channel – Design Note 1020
Tom Gross
Introduction
The LTC3633 is a high efficiency, dual output monolithic
synchronous step-down switching regulator, capable of
delivering 3A of output current per channel from input
voltages between 3.6V and 15V. Its compact 4mm × 5mm
QFN package has very low thermal impedance eliminating
the need for an external heat sink. Its high input voltage
capability makes the LTC3633 ideal for 12V distributed
power systems.
1.8V and 3.3V 2MHz Buck Regulator
Figure 1 shows a LTC3633 12VIN to 1.8VOUT and 3.3VOUT,
3A, buck regulator. With the frequency setting pin, RT,
connected to the internal 3.3V regulator rail (INTVCC), this
circuit runs at 2MHz —the LTC3633 switching frequency
is programmable from 500kHz to as high as 4MHz, and
can also be synchronized to an external clock—reducing
the value and size of the output capacitors and inductor.
Even with this high switching frequency, the efficiency of
this circuit is good: 87% at full load for the 3.3V regulator and 82% at full load for the 1.8V regulator, at 12VIN
(see Figure 2).
VIN 3.6V TO 15V
CIN3
22µF
16V
1206
VISHAY
IHLP-2020BZER1R0M01
VOUT1
1.8V
3A
20
BOOST1
CBST1
0.1µF 23
SW1
R3
20k
1%
COUT3
22µF
6.3V
1206
COUT5
10µF
6.3V
0805
CIN1
47µF
16V
1210
L1
1µH
VOUT1
COUT1
22µF
6.3V
1206
CIN2
47µF
16V
1210
24
25
10
CFFW1
10pF
28
27
R5
10k
1%
94
92
VOUT = 3.3V
EFFICIENCY (%)
90
88
86
VOUT = 1.8V
84
82
80
VIN = 12V
FSW = 2MHz
Burst Mode® OPERATION
78
76
0
0.5
DN1020 F02
By connecting the phase mode pin (PHMODE) to
INTVCC, the two internal regulators operate 180 degrees
out-of-phase with respect to each other, reducing the
input current ripple, and thus, the number and value of
input capacitors. Due to a controlled on-time design,
the LTC3633 achieves very fast load transient response
L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered
trademarks of Linear Technology Corporation. All other trademarks are the property of
their respective owners.
6
RUN2
15
16
VIN2 VIN2
17
BOOST2
SW2
SW1
SW2
LTC3633EUFD
VON2
TRACKSS2
VFB2
VFB1
PGND
TRACKSS1
SGND
14
13
POWER
GROUND
RPG1
100k
RPG2
100k
VOUT2
VISHAY
IHLP-2020BZER1R0M01
12
COUT2
22µF
6.3V
1206
R4
61.9k
1%
CFFW2
10pF
9
COUT4
22µF
6.3V
1206
29
R6
13.7k
1%
7
COUT6
10µF
6.3V
0805
PGOOD2
Figure 1. LTC3633: 1.8VOUT and 3.3VOUT, 2MHz Buck Regulator
11/10/1020
SIGNAL
GROUND
L2
CBST2
1µH
1µF
CSS1
4700pF PGOOD1 ITH1 RT PHMODE INTVCC MODE/SYNC V2P5 ITH2 PGOOD2
1
26 5
2
19
4
18
11
8
PGOOD1
3.5
3
Figure 2. High Efficiency from Each Channel for the
Circuit in Figure 1
21
22
3
VIN1 VIN1 RUN1
VON1
1
1.5
2
2.5
LOAD CURRENT (A)
VOUT2
3.3V
3A
DN1020 F01
1.2VOUT, 6A, Out-of-Phase Supply
The LTC3633’s two internal regulators can be combined
form a single output circuit, operating out-of-phase to
deliver high total output current with minimal input and
output capacitance—useful for distributed power systems. This design also generates lower high-frequency
EMI noise than a single-phase design, due to the lower
input ripple current caused by the out-of-phase switching
currents with respect to each other (see Figure 4). The
12VIN to 1.2VOUT dual-phase LTC3633 regulator is shown
in Figure 4, with Figure 5 showing its load step response.
with minimal output capacitance, as shown in Figure
3. With only 50µF of output capacitance, it takes only
10µs to recover from a 3A load step that results in less
VOUT
100mV/DIV
AC-COUPLED
IOUT
1A/DIV
20µs/DIV
DN1020 F03a
VOUT1: 12VIN TO 1.8VOUT LOAD STEP
3A LOAD STEP, 2MHz SWITCHING FREQUENCY
FORCED CONTINUOUS MODE,
INTERNAL COMPENSATION
Conclusion
The dual 3A output LTC3633 provides a high performance
compact regulator solution for powering low voltage
loads in 12V intermediate bus systems. Configured as a
two-phase, single output regulator, it can deliver up to
6A with minimum input ripple and an effective switching
frequency up to 8MHz.
VOUT
100mV/DIV
AC-COUPLED
IOUT
1A/DIV
20µs/DIV
DN1020 F03b
VOUT2: 12VIN TO 3.3VOUT LOAD STEP
3A LOAD STEP, 2MHz SWITCHING FREQUENCY
FORCED CONTINUOUS MODE,
INTERNAL COMPENSATION
VOUT
100mV/DIV
AC-COUPLED
Figure 3. Load Step Responses for the Figure 1 Circuit
IOUT
2A/DIV
than 100mV of output voltage deviation. Note that compensation is internal, set up by tying the compensation
pin (ITH) to INTVCC. This connects an internal series RC
to the compensation point of the loop, eliminating the
need for external compensation components. To further
optimize the loop, an external RC circuit can be applied
to the ITH pin.
VIN 3.6V TO 15V
CIN3
22µF
16V
1206
PGOOD1
CIN2
47µF
16V
1210
CIN1
47µF
16V
1210
RPG1
100k
21 22
3
VIN1 VIN1 RUN1
1
26
RITH
6.49k
1%
27
CSS1
4700pF
CITH
470pF
19
CVCC
1µF
18
5
11
10
SIGNAL
GROUND
15
16
VIN2 VIN2
PGOOD1
PGOOD2
ITH1
BOOST1
TRACKSS1
SW1
INTVCC
V2P5
CSS2
4700pF
LTC3633EUFD
SW1
VON1
VFB1
PHMODE
VFB2
RT
BOOST2
8
20
23
24
SW2
TRACKSS2
SW2
7
PGND
29
VON2
RPG2
100k
PGOOD2
L1
CBST1
0.47µH
0.1µF
VOUT1
VISHAY
IHLP-2525CZERR47M01
CFFW1
10pF
25
R1
20k
1%
28
9
17
ITH2
SGND
DN1020 F05
Figure 5. Load Step Response
4 MODE/SYNC
2
POWER
GROUND
6
RUN2
20µs/DIV
DUAL VOUT: 12VIN TO 1.2VOUT LOAD STEP
6A LOAD STEP, 2MHz SWITCHING FREQUENCY
FORCED CONTINUOUS MODE, EXTERNAL COMPENSATION
14
13
CFFW2
10pF
CBST2 L2
0.1µF 0.47µH
R2
20k
1%
VOUT2
VISHAY
IHLP-2525CZERR47M01
COUT2
22µF
6.3V
1206
12
COUT4
22µF
6.3V
1206
COUT1
22µF
6.3V
1206
COUT3
22µF
6.3V
1206
COUT5
10µF
6.3V
0805
COUT6
10µF
6.3V
0805
VOUT
1.2V
6A
DN1020 F04
Figure 4. 6A Out-of-Phase Buck Regulator Using the LTC3633
Data Sheet Download
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