SC2434

SC2434
POWER MANAGEMENT
Applications Information (Cont.)
PCB Layout Consideration
Good layout is necessary for successful implementation of the SC2434 based 3 tri-phase topology. There are few
general rules:
·
Reserve enough PCB space for the power supply (1.2~1.5 square inch for every 10A of load current);
·
Place enough high frequency ceramic capacitors inside and around the CPU socket (please follow CPU manufacture’s
decoupling guideline);
·
Place bulk output capacitors around the CPU socket as uniformly as possible. The connection copper between
these capacitors and the CPU socket must be short and wide to minimize inductance and resistance;
·
Always place the high power parts first;
·
Always use a ground plane or ground planes;
·
Always try to minimize the stray inductance of the high pulsating current loop which is formed by input capacitors
and the MOSFET half-bridges.
The following layout guideline gives details on how to achieve a good layout:
·
Input filter should contain mixed electrolytic capacitors and MLC capacitors. For every 20A of load current, use
about 10uF of MLC caps. Put MLC caps close to current sensing resistor;
·
Use surface mount current sensing resistor (typically 3~5 mOhm in surface mount package with low temperature
coefficient and low package inductance, typically less than 0.3nH);
·
Try to minimize the stray inductance from the current sensing resistor to the drains of the top FETs by using wide
trace (>0.5” wide and no more than 3” long). This trace can run on inner1 layer, for example, if the inner2 layer is
the ground plane, assuming the FETs are on the top layer. This arrangement forms so called strip line structure for
the pulsating power current, which yields least amount of stray inductance. The concept is depicted in Fig. 7;
·
Keep the layout as electrically symmetrical as possible, as shown in Fig. 8, to avoid very uneven stray inductance
from the sensing resistor to the drains of the top FETs;
·
Use a pair of closely paralleled traces to pick up the sensing voltage across the sensing resistor. The sensing traces
server as differential input to the OC+ and OC- pins of the SC2434 controller. These traces should run on a routing
layer (e.g., bottom layer for 4 layer PCB case) to avoid picking up strong AC magnetic field due to power current flow.
In this case, the differential sensing traces are shielded by the ground layer. The filter cap across the OC+ and OCpins should be placed as close as possible to the controller. Pay close attention that never allow power current
flowing on or running close by the sensing traces. Please see Fig. 8;
·
Separate power ground from analog ground to prevent power current from running over the analog ground plane.
The SC2434 controller should be placed on the quite analog ground area. The analog ground should be singlepoint connected to the PGND near the output capacitor or the CPU socket to provide best possible ground sense.
Refer to the application schematics for those components should be connected directly to the AGND (Vcc decoupling
caps, cap on BGOUT pin, resistors on OSCREF pin, DACREF pin, FB pin, and PGIN pin).
Rsense
VIA
MLC
D
TOP FET
S
D
BOT FET
S
VIA
Ground Plane
Fig. 7 - Use MLC capacitors and strip line structure to minimize the stray inductance for the switching current loop.
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SC2434
POWER MANAGEMENT
Applications Information (Cont.)
PRELIMINARY
Fig. 8 - Layout concept for input current sensing: (a) use MLC input capacitors; (b) minimize inductance; (c)
keep electrical symmetry; and (d) use differential sensing traces.
A Reference Design Example For Intel Pentium IV
Processor
Brief specifications of this design are listed below:
•
Vin=12V
•
Vout=1.725V +/- 25mV at 0A load
•
Vout droop slope is 1.5 mOhm
•
Vout tolerance is +/-25mV for all load conditions
•
Iout = 60A max
•
VID [4:0] = 00100
The schematic is shown on the cover page of this data
sheet.
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SC2434
POWER MANAGEMENT
Bill of Materials - Reference Design
Item
Qty. Reference
Value
Description/Part No.
P ackag e
Vendor
47pF
10V, X7R,MLCC,VJ603Y470KXXAT
0603
VISHAY
1
1
C_COMP
2
1
C1
0.33uF
25V, X7R,MLCC,VJ805Y334KXXAT
0805
VISHAY
3
3
C2,C3,C4
2200uF
15V Al. Elec. cap
CPCYL/D.400/
LS.200/.034
Sanyo
4
8
C5,C9,C13,
C15,C18,
C26,C30,C38
1uF
16V, Y5V,MLCC,PCC1849CT-ND
0805
Panasonic
5
3
C6,C7,C16
4.7uF
16V, Y5V,MLCC,PCC1900CT-ND
1206
Panasonic
6
12
C8,C10,C14,
C19,C20,C22,
C23,C27,C28,
C31,C33,C35
1500uF
6.3V Al. Cap Rybucon MBZ
CPCYL/D.325/
LS.125/.034
Rubicon
7
3
C11,C24,C36
1nF
16V, X7R,MLCC,VJ603Y102KXXAT
0603
VISHAY
8
3
C12,C25,C37
2.2nF
50V, X7R,MLCC,VJ805Y222KXXAT
0805
VISHAY
9
2
C17,C29
0.33uF
25V, X7R,MLCC,VJ803Y334KXXAT
0805
VISHAY
10
1
C 21
10nF
10V, X7R,MLCC,VJ603Y103KXXAT
0603
VISHAY
11
1
C 32
100nF
16V, X7R,MLCC,VJ603Y104KXXAT
0603
VISHAY
12
1
C 34
470pF
10V, X7R,MLCC,VJ603Y471KXXAT
0603
VISHAY
13
6
D1,D2,D3,D4,
D5,D6
3A
30V SM Schottly DL4148MSCT-ND
DO213AC
DIGI-KEY
14
4
L1,L2,L3,L4
638nH
638nH, 20A , Inductor TTIF1305-638
IN/L500/W400/.10
Falco
15
3
M1,M3,M5
F D B 6036B L
MOSFET
TO-263AB
Fairchild
16
2
M2,M4
F D B 7045L
MOSFET
TO-263AB
Fairchild
17
1
M6
F D P 7045L
MOSFET
TO-263AB
Fairchild
18
1
R_COMP
29.4K
SM 1% CRCW06032942F
0603
VISHAY
19
1
R_DAC
37.4K
SM 1% CRCW06033742F
0603
VISHAY
20
1
R_DRP
187K
SM 1% CRCW06031873F
0603
VISHAY
21
1
R_FB
10.0K
SM 1% CRCW06031002F
0603
VISHAY
22
1
R_OS
46.4K
SM 1% CRCW06034642F
0603
VISHAY
23
1
R_OSC
31.6K
SM 1% CRCW06033162F
0603
VISHAY
24
1
R1
3m
SM Sensing R 1% RL7520W
2512
CYNTEC
25
4
R2,R5,R8,R13
2R2
SM 1% CRCW06032R2F
0603
VISHAY
26
1
R3
20
SM 1% CRCW060320R0F
0603
VISHAY
27
3
R4,R10,R15
1R0
SM 1% CRCW060331R0F
0603
VISHAY
28
1
R6
100
SM 1% CRCW06031000F
0603
VISHAY
29
3
R7,R11,R16
1R0
SM 1% CRCW12061R0F
1206
VISHAY
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SC2434
POWER MANAGEMENT
Bill of Materials - Reference Design (Cont.)
Item
Qty. Reference
PRELIMINARY
Value
Description/Part No.
Package
Vendor
30
1
R9
NO POP
SM 1% CRCW06031R0F
0603
VISHAY
31
1
R12
5.1K
SM 1% CRCW06035111F
0603
VISHAY
32
2
R14,R18
1.0K
SM 1% CRCW06031001F
0603
VISHAY
33
1
R17
750
SM 1% CRCW06037500F
0603
VISHAY
34
3
U1, U3, U4
SC1205
Dual FET Driver
SOIC-8
SEMTECH
35
1
U2
SC2434
Tri-Phase Current Mode Controller
w/ Power Good
SOIC-20 or
TSSOP-20
SEMTECH
Note 1: Magnetic Cool Mu 77041, 5 turns AWG #16 (800nH@0A, 600nH@25A)
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SC2434
POWER MANAGEMENT
Applications Information (Cont.)
Typical Performance Of The Reference Design
The reference design implemented 1.5mOhm output droop impedance as shown in Fig. 9.
Load Line (Vin=12V, VID=00100)
1.76
1.74
Vo(V)
1.72
Vo
Spec_H
Spec_L
1.7
1.68
1.66
1.64
1.62
1.6
0
10
20
30
40
50
60
I (A)
Fig. 9 - Measured output drooping characteristics of the 60A design.
The efficiency of the design is depends on the MOSFET being used and thermal management requirements of
controlling the PCB temperature and the MOSFET junction temperature. The following efficiency curve is corresponding
to 4mOhm bottom FET, while the top FET has 12 mOhm Rdson.
Efficency (%)
95.00
90.00
85.00
80.00
75.00
70.00
65.00
0
10
20
30
40
50
60
70
I_out(A)
Fig. 10 - Typical efficiency curve for 12 mOhm top FETs and 4 Ohm bottom FETs.
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SC2434
POWER MANAGEMENT
Applications Information (Cont.)
PRELIMINARY
The typical phase node voltage and the output voltage ripple waveform is shown in Fig. 11 under 60A full load
operation, where one can see the output ripple is very small and even with a frequency three times of the switching
frequency.
Fig. 11 - The typical phase node voltage and the output voltage ripple waveform under 60A full load operation.
The typical gate waveform for the top and bottom MOSFETs is also shown here, well-controlled dead time is
demonstrated which ensures high efficiency operation of the VR.
Ch2: HS Gate
Ch3: Phase Node
Ch4: LS Gate
Fig. 12 - The typical gate waveform for the top and bottom MOSFETs.
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SC2434
POWER MANAGEMENT
Applications Information (Cont.)
The transient response for a maximum load step changes (10A to 60A) is shown in Fig. 14, where one can see that
accurate drooping will help to reduce the amount of output capacitance needed. Please notice that using more
multilayer ceramic capacitors for better high frequency decoupling can reduce the narrow voltage spikes.
Output Voltage
Load Current
Fig. 13 - Transient response and the test condition:
Step Load from 10A to 60A
Output Capacitors: 14 units of 560uF OSCON caps, 38 units of 10uF ceramic caps
Ch1: Output Voltage
Ch4: Output Current (1A = 27.5mV di/dt = 370A/uS)
Meet Intel P-4 spec
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