DN1024 - Step-Down Converter Delivers 25A at 12VOUT from Inputs Up to 60V

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Step-Down Converter Delivers 25A at 12VOUT from Inputs Up to 60V
Design Note 1024
Victor Khasiev
Introduction
The LTC ® 3890 (dual outputs) and LTC3891 (single
output) step-down DC/DC controllers directly accept
inputs from 4V to 60V. This wide input range covers
input voltages for single or double battery automotive
environments, thus eliminating the need for snubbers
and voltage suppression circuitry typically required to
protect ICs during load dumps. This range also encompasses 48V telecom applications. If no galvanic isolation
is required between the input and output voltages, the
LTC3890 and LTC3891 can replace expensive and bulky
transformer-based converters. When compared to a
transformer-based solution, an LTC3890 or LTC3891
step-down converter increases efficiency, reduces
power loss in the supply lines, simplifies layout and
significantly reduces the bill of materials.
5
30.1k
2
7
0.1µF
8
29
47pF
4.7nF
9.76k
13
30
12
47pF
35.7k
499k
10pF
21
27
1µF
PLLIN
LTC3890
FREQ
VIN
14
28
20
22
SENSE1–
1
SENSE1+
32
RUN2
SS1
TG1
SS2
SW1
ITH1
ITH2
BOOST1
VFB1
VFB2
PGND
PGOOD1
INTVCC
TG2
SW2
PGOOD2
ILIM
2.2Ω
2.2pF
RUN1
BG1
31
11
VOUT
Although the ITH pins are connected together, each is
terminated to a separate 47pF capacitor to compensate
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
VIN, 16V TO 60V
1M
57.6k
High Efficiency 2-Phase Converter Produces 12V
at 25A
Figure 1 shows the LTC3890 in a 2-phase single output
step-down converter configuration that delivers 25A
at 12V, which can be scaled up to 75A by adding more
LTC3890 ICs to increase the number of power phases.
For lower output current, the single-phase LTC3891
can be used. Implementing a 2-phase converter simply
requires tying together the independent channel pins
of the LTC3890, namely, FB1 and FB2, TRACK/SS1 and
TRACK/SS2, RUN1 and RUN2, ITH1 and ITH2.
BOOST2
EXTVCC
BG2
SENSE2+
SENSE2–
2.2µF/100V
×4
1µF/100V
VIN
100Ω
26
RJK0651DPB
25
1
0.1µF DFLS1100
INT
23
19
+
150µF
VOUT
12V AT 25A
VIN
RJK0651DPB
L2 10µH
0.1µF DFLS1100
3m
10µF
×2
INT
18
9
10µF
×2
4.7µF
16
10
3m
RJK0653DPB
INT
15
17
L1
10µH
RJK0653DPB
100Ω
L1, L2: WÜRTH 7443631000
2.2pF
DN1024 F01
Figure 1. High Efficiency Converter Produces 25A at 12VOUT from Inputs Up to 60V
0513/1024
+
150µF
for possible noise from interconnecting traces. A
relatively low switching frequency, around 150kHz,
and a relatively high phase inductance of 10µH are
used to reduce switching losses at high input voltages.
The output voltage is fed to the EXTVCC pin to reduce
losses associated with biasing the chip and internal
gate drivers at high input voltages.
Circuit Performance
Efficiency is shown in Figure 2, measured without
cooling air flow. Efficiency peaks close to 98% in the
middle of the load range and declines to 96% at the
25A maximum load. Figure 3 shows the average input
current vs input voltage at no load in Burst Mode ® operation. The value of this current is below 0.5mA. Figure
4 shows a thermal map of the board with no air flow
present at VIN = 20V and VOUT = 12V at 25A (300W).
99.0
20V
36V
50V
98.5
EFFICIENCY (%)
98.0
97.5
97.0
96.5
96.0
95.5
95.0
1
6
11
16
21
LOAD (A)
26
DN1024 F02
Figure 2. Efficiency at VIN = 20V, 36V and 50V
0.22
INPUT CURRENT (mA)
0.20
Figure 4. Temperature Hot Spots with No Air Flow
Component Selection
Two values define selection of the inductor: RMS current (IRMS) and saturation current (IPK ):
∆I2
12
(V – V )•D
∆I= IN OUT
L•f
V
D= OUT
VIN
I
IPH = k • OUT
2
∆I
IPK =IPH +
2
IRMS = (IPH)2 +
where f is the switching frequency and k is a coefficient
defined by the current imbalance between the phases.
For converters based on the LTC3890, k = 1.08, assuming current sense resistors with a 1% tolerance.
Selection of power MOSFETs and input/output capacitors is described in detail in the LTC3890 data sheet.
It is important to note that the typical internal VCC
voltage and, consequently, the MOSFET gate voltage
is 5.1V. This means that logic level MOSFETs must be
used in the design.
0.18
0.16
0.14
0.12
0.10
20
25
30
35
VIN
40
45
50
DN1024 F03
Figure 3. Average Input Current vs Input Voltage at
No Load. VOUT is 12V.
Data Sheet Download
www.linear.com/LTC3890
Conclusion
The LTC3890 dual output, synchronous step-down
converter can be easily configured as a single output,
dual phase converter for high input voltage, high output
current automotive and telecom applications.
For applications help,
call (408) 432-1900, Ext. 3161
Linear Technology Corporation
dn1024 LT 0513 REV A • PRINTED IN THE USA
(408) 432-1900
 LINEAR TECHNOLOGY CORPORATION 2012
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
FAX: (408) 434-0507 ● www.linear.com
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