DN478 - Dual Output Step-Down Controller Produces Accurate, Efficient and Reliable High Current Rails

Dual Output Step-Down Controller Produces 10% Accurate,
Efficient and Reliable High Current Rails – Design Note 478
Mike Shriver and Theo Phillips
Introduction
The LTC ®3855 makes it possible to generate high current rails with the accuracy and efficiency to satisfy the
most demanding requirements of today’s leading edge
network, telecommunications and server applications.
This 2-phase, dual output synchronous buck controller
includes strong gate drivers that support operation with
per-phase currents above 20A. The accurate 0.6V ±0.75%
reference and its integrated differential amplifier (diff
amp) allow remote sensing of the output of critical rails.
This controller has an output voltage range from 0.6V to
12.5V when used without the diff amp and from 0.6V to
3.3V with the diff amp.
The LTC3855 uses the reliable peak current mode architecture to achieve a fast and accurate current limit and real
time current sharing. Its current sense comparators are
designed to sense the inductor current with either a sense
resistor or with inductor DCR sensing. DCR sensing offers
17.4k
RNTC2
100k
NEXT
TO L2
the advantage of reduced conducted power losses, since
the current is measured using the voltage drop across the
already-present inductor DC resistance—eliminating the
losses incurred by adding a sense resistor. The trade-off is
that DCR sensing is less accurate than a dedicated sense
resistor because the DCR varies from part to part and over
temperature. The LTC3855 uses an innovative scheme to
improve the accuracy of DCR sensing by compensating
for the DCR’s variation with temperature.
1.5V/20A and 1.2V/20A Buck Converter with
Remote Sensing and NTC Compensated DCR
Sensing
Figure 1 shows a 1.5V/20A and 1.2V/20A dual phase
converter with DCR sensing, operating at 325kHz. High
efficiency is achieved with the strong gate drivers,
L, LT, LTC, LTM, Burst Mode, PolyPhase, Linear Technology and the Linear logo are
registered trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
17.4k
49.9k
RNTC1
100k
NEXT
TO L1
0.1μF
49.9k
82.5k
325kHz
20k
ITH1
BOOST1
VFB1
PGND1
SGND
TK/SS2
EXTVCC
SENSE2–
PGND2
34.0k
+
4.42k
L1
0.47μH
DCR = 1.0mΩ TYP, 1.2mΩ MAX
CMDSH-3
2.2Ω
4.7μF
0.1μF
COUT1
100μF
6.3V
s2
M2
RJK0330DPB
s2
+
COUT2
330μF
2.5V
s2
VOUT1
1.5V
20A
L1, L2: VISHAY IHLP5050FD-01, 0.47μH
COUT1, COUT3: MURATA GRM31CR60J107ME39l
COUT2, COUT4: SANYO 2R5TPE330M9
RNTC1, RNTC2: MURATA NCP18WF104J03RB
0.1μF
CMDSH-3
DCR = 1.0mΩ TYP, 1.2mΩ MAX
M3
RJK0305DPB
100k
PGOOD1
PGOOD2
100k
M4
RJK0330DPB
s2
L2
0.47μH
4.42k
34.0k
COUT3
100μF
6.3V
s2
DN478 F01
Figure 1. Dual 1.5V/20A and 1.2V/20A Converter Operating at fSW = 325kHz.
The Entire Circuit Fits within 1.7in2 with Both Sides of the Board Populated
05/10/478
VIN
4.5V TO
14V
180μF
16V
BOOST2
SW2
20k
NC
DIFFP
0.1μF
PGOOD2
BG2
PGOOD1
SENSE2+
ILIM2
0.1μF
INTVCC
ILIM1
1.2nF
0.1μF
VIN
LTC3855
ITH2
DIFFN
5.23k
VFB2
RUN2
20k
M1
RJK0305DPB
BG1
DIFFOUT
150pF
SW1
CLKOUT
TG1
TG2
1nF
150pF
PHSASMD
FREQ
TK/SS1
6.04k
MODE/PLLIN
ITEMP2
RUN1
0.1μF
ITEMP1
SENSE1–
30.1k
SENSE1+
22μF
s3
+
COUT4
330μF
2.5V
s2
VOUT2
1.2V
20A
95
30
5A
25
IOUT(MAX)
20
DCR SENSING IMPLEMENTED
WITH TEMPERATURE
COMPENSATION
WITHOUT TEMPERATURE
COMPENSATION
15
6
EFFICIENCY
10
90
5
1.5V
1.2V
85
4
1.5V
1.2V
80
3
POWER LOSS
75
2
VIN = 12V
fSW = 325kHz
MODE = CCM
70
POWER LOSS (W)
EFFICIENCY (%)
35
CURRENT LIMIT (A)
optimized dead-time and DCR sensing. The typical full
load efficiency for the 1.5V and 1.2V rails is 89.5% and
87.8%, respectively (see Figure 2). The 1.2V output is
remotely sensed with the diff amp. As a result, the 1.2V
rail’s output accuracy is unaffected by the voltage drops
across the VOUT and GND planes. The load step response
for the 1.2V rail is shown in Figure 3.
1
65
0
0
5
10
15
LOAD CURRENT (A)
20
25
DN478 F02
Figure 2. Efficiency and Power Loss of the 1.5V/20A
and 1.2V/20A Converter
1.2VO(AC)
100mV/DIV
20A
LOAD STEP
10A/DIV
10A
50μs/DIV
DN478 F03
Figure 3. 50% to 100% Load Step Response
for the 1.2V Rail at VIN = 12V
The LTC3855 features precise current limit thresholds of
30mV, 50mV and 75mV, selected via the ILIM pins. The
current limit threshold can be raised by biasing the ITEMP
pins below 500mV. Since the ITEMP pins source 10μA of
current, the peak current sense voltage can be increased
by inserting a resistance of less than 50k from the ITEMP
pin to ground. By placing an inexpensive NTC thermistor
next to the inductor and connecting this thermistor to
a linearization network from the ITEMP pin to ground,
the current limit temperature coefficient can be greatly
reduced. As Figure 4 illustrates, the compensated current
limit is 20% higher than the uncompensated current limit
at 110°C. Another use for the ITEMP pins is to increase
the current limit for conventional DCR sense and RSENSE
applications.
PolyPhase® Operation
The LTC3855 provides inherently fast cycle-by-cycle current sharing due to its peak current mode architecture,
Data Sheet Download
www.linear.com
0
100
40
60
80
20
INDUCTOR TEMPERATURE (°C)
120
DN478 F04
Figure 4. Measured Current Limit of the 1.2V Rail
Over Temperature with and without Temperature
Compensation
plus very tight DC current sharing for single output
PolyPhase applications. Up to 12-phase operation can
be achieved by daisy chaining the CLKOUT and MODE/
PLLIN pins and by programming the phase separation
with the PHASMD pins. A major advantage of PolyPhase
operation is the reduction of the required input and output capacitance due to ripple current cancellation. Also,
single output PolyPhase applications have a faster load
step response due to a smaller clock delay.
Other Important Features
The switching frequency of the LTC3855 can be programmed between 250kHz and 770kHz with a resistor
placed from the FREQ pin to ground or synchronized
to an external clock in this frequency range using its
internally compensated phase lock loop. High efficiency
at light load is achieved by selecting either Burst Mode®
operation or discontinuous mode operation, as opposed to
continuous conduction mode. The LTC3855 can be used
for inputs up to 38V, and its 100ns typical minimum ontime allows for high step-down ratios. The LTC3855 has
a TK/SS pin for programmable soft-start or rail tracking,
and dedicated RUN and PGOOD pins for each channel.
The LTC3855 comes in either a 6mm × 6mm QFN or a
thermally enhanced 38-lead TSSOP package.
Conclusion
The LTC3855 is a high performance dual output buck
converter intended for low output voltage, high output
current supplies. It provides the user with the benefits
of a precise 0.6V 0.75% reference, an accurate current
limit and high efficiency.
For applications help,
call (408) 432-1900, Ext. 3720
Linear Technology Corporation
dn478f LT/TP 0510 116K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2010
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
Similar pages