AN9974: Using the ISL6430 in DC-DC Converters for Microprocessors in Gateway Applications

Using the ISL6430 in DC-DC Converters
for Microprocessors in Gateway Applications
TM
Application Note
September 2001
AN9974
Introduction
Intersil ISL6430
Today’s high-performance microprocessors present many
challenges to their power source. High power consumption,
low bus voltages, and fast load changes are the principal
characteristics which have led to the need for a switch-mode
DC-DC converter local to the microprocessor. A common
requirement of these and similar processors are decreasing
supply voltages as the processor clock frequency increases.
The Intersil ISL6430 is a voltage-mode controller with many
functions needed for high-performance processors. Figure 1
shows a simple block diagram of the ISL6430. The device
contains a high-performance error amplifier, a high-accuracy
reference, a programmable free-running oscillator, and
overcurrent protection circuitry. The ISL6430 has two
MOSFET drivers for use in synchronous-rectified buck
converters. A more complete description of the parts can be
found in their data sheets.
The Intersil ISL6430 pulse-width modulator (PWM)
controllers are targeted specifically for DC-DC converters
powering high-performance microprocessors with varying
core voltage requirements.
ISL6430EVAL1
This device provides a cost-effective solutions for point-ofuse switch-mode, DC-DC converters for many applications.
This application note details the use of the ISL6430 in
DC-DC converters for high-performance microprocessors
with a fixed core voltage.
VCC
MONITOR AND
PROTECTION
EN
BOOT
OSC
UGATE
ISL6430
PHASE
REF
FB
PVCC
+
-
LGATE
+
90
PGND
GND
COMP
NOT PRESENT
(PINS NC)
ON HIP6007
FIGURE 1. BLOCK DIAGRAM OF ISL6430
EFFICIENCY (%)
RT
Efficiency
Figure 2 displays the ISL6430EVAL1 efficiency versus load
current for both 5V and 12V inputs with 100 linear feet per
minute (LFM) of airflow. For a given output voltage and load,
the efficiency is lower at higher input voltages. This is due
primarily to higher MOSFET switching losses and is
displayed in Figure 2.
OCSET
SS
The ISL6430EVAL1 is a synchronous buck converter
capable of providing up to 9A of current at a fixed 2.5V
output voltages. Simple resistor value changes allow for
outputs as low as 1.3V. The schematic and bill-of-materials
for this design can be found in the appendix.
VIN = 5V
85
VIN = 12V
80
75
2
4
6
LOAD CURRENT (A)
8
10
FIGURE 2. ISL6430EVAL1 EFFICIENCY vs LOAD
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 9974
OC Protection
2.50V
VOUT
50mV/DIV
COMP
2V/DIV
0V
IL
5A/DIV
The ISL6430EVAL1 has lossless overcurrent (OC)
protection. This is accomplished via the current-sense
function, which senses converter load current by monitoring
the drop across the upper MOSFET (Q1 in the schematics).
By selecting the appropriate value of the OCSET resistor
(R6), an overcurrent protection scheme is employed without
the cost and power loss associated with an external currentsense resistor. See the Over-Current Protection section of
the ISL6430 data sheet for details on the design procedure
for the OCSET resistor.
Customization of Reference Designs
0A
TIME (10ms/DIV)
FIGURE 3. ISL6430EVAL1 TRANSIENT RESPONSE
WITH V IN = 12V
The ISL6430EVAL1 reference design provides solutions for
microprocessors with current demands of up to 9A. One
basic design is employed to meet many different
applications. The evaluation board can be powered from
+5V or +12V and a synchronous buck topology may be
employed. Employing one basic design for numerous
applications involves some trade-offs. These trade-offs are
discussed below to help the user optimize for a given
application.
Control Loop Bandwidth/Transient Response
VOUT
20mV/DIV
IL
1A/DIV
TIME (1ms/DIV)
FIGURE 4. ISL6430EVAL1 OUTPUT VOLTAGE RIPPLE
Transient Response
Figure 3 shows a laboratory oscillogram of the
ISL6430EVAL1 in response to a 0-9A load transient
application. The output voltage responds rapidly and is
within 1% of its nominal value in less than 15µs. Check the
Feedback Compensation section of the data sheet for details
on loop compensation design. Table 1 details simulated
closed-loop bandwidth and phase margin for both reference
boards at both +5V and +12V input sources.
Table 1 shows how the control loop characteristics vary with
line voltage and topology. The line voltage determines the
amount of DC gain, which directly affects the modulator
(control-to-output) transfer function. The topology (standard
buck or synchronous buck) is important because we have
chosen to use a larger output inductor for the standard buck
(HIP6005) design. This lowers the boundary of continuous
conduction mode (ccm) and discontinuous conduction mode
(dcm) operation. Staying in ccm at light loads can have an
adverse affect on transient response of the converter. The
ISL6430EVAL1 design will not go into dcm operation
because the lower MOSFET conducts current even at light
or zero load conditions.
From Table 1, we see that the highest control loop bandwidth
is with VIN = 12V. The transient response of the converter for
this case is shown in Figure 3. The other cases have slower
responding loops and can be improved with value changes
in the compensation components. Table 2 details suggested
changes and the improved control loop characteristics for
the three applications with slower control loops.
TABLE 2. MODIFICATIONS TO CONTROL LOOP
ISL6430EVAL1
VIN = 5V
VIN = 12V
R5
30.1K
no change
C14
no change
no change
f0dB
47kHz
61kHz
ϕMARGIN
53o
62o
TABLE 1. CONTROL LOOP CHARACTERISTICS
ISL6430EVAL1
VIN = 5V
VIN = 12V
f0dB
27kHz
61kHz
ϕMARGIN
72o
62o
2
Application Note 9974
Ripple Voltage
∆V OUT = ∆IL • ESR
VIN = 5V, RFP25N05
VIN = 5V, RFP45N06
90
EFFICIENCY (%)
The amount of ripple voltage on the output of the DC-DC
converter varies with input voltage, switching frequency,
output inductor, and output capacitors. For a fixed switching
frequency and output filter, the voltage ripple increases with
higher input voltage. The ripple content of the output voltage
can be estimated with the following simple equation:
85
VIN = 12V, RFP25N05
80
VIN = 12V, RFP45N06
75
where
V OUT
( VIN – VOUT ) • ---------------- • Ts
VIN
∆IL = ----------------------------------------------------------------------L OU T
ESR = equivalent series resistance of output capacitors
2
4
6
LOAD CURRENT (A)
8
10
FIGURE 5. ISL6430EVAL1 EFFICIENCY WITH EITHER
RFP25N05 MOSFETs OR RFP45N06 MOSFETs
Ts = switching period (1/Fs)
Conclusion
LOUT = output inductance
The ISL6430EVAL1 is a DC-DC converter reference design
for microprocessors with fixed core voltages and current
requirements of up to 9A. In addition, the design can be
modified for applications with different requirements. The
printed circuit board is laid out to accommodate the
necessary components for operation at currents up to 15A.
Therefore, for equivalent output ripple performance at VIN =
12V as at 5V, the output filter or switching frequency must
change. Assuming 200kHz operation is desired, either the
output inductor value should increase or the number of
parallel output capacitors should increase (to decrease the
effective ESR).
Increased Output Power Capability
The ISL6430EVAL1 printed circuit board is laid out with
flexibility to increase the power level of the DC-DC converter
beyond 9A. Locations for additional input capacitors and
output capacitors are provided. In conjunction with higher
current MOSFETs, Schottky rectifiers, and inductors, the
evaluation board can be tailored for applications requiring
upwards of 15A. The ISL6430 data sheet’s Component
Selection Guidelines section helps the user with the design
issues for these applications. Of course, the ISL6430EVAL1
can be modified for more cost-effective solutions at lower
currents as well.
MOSFET Selection
As a supplement to the data sheets’ application information
on MOSFET Selection Considerations, this section shows
graphically that a larger, lower rDS(ON) MOSFET does not
always improve converter efficiency. Figure 8 shows that
smaller RFP25N05 MOSFETs are more efficient over most
of the line and load range than larger RFP45N06 MOSFETs.
The RFP25N05 (used on the ISL6430/7EVAL1) has a
rDS(ON) equal to 47mΩ (maximum at 25oC) versus 28mΩ
for the RFP45N06. In comparison to the RFP25N05, the
RFP45N06 MOSFETs increased switching losses are
greater than its decreased conduction losses at load
currents up to about 7A with a 5V input and about 9A with a
12V input.
4-3
References
For Intersil documents available on the web, see
http://www.intersil.com/
Application Note 9974
12VCC
VIN
C1-3
3 x 680µF
C17-18
2 x 1µF
1206
RTN
C12
1µF
1206
R7
10K
C19
VCC
EN 6
SS 3
ENABLE
CR1
4148
1000pF
14
2 OCSET
MONITOR AND
PROTECTION
R6
3.01k
PHASE
TP2
10 BOOT
RT 1
Q1
C13
0.1µF
U1
R1
SPARE
FB 5
C14
33pF
C15
0.01µF
C16
15K
Q2
12 LGATE
+
+
-
R5
SPARE
R3
1K
VOUT
13 PVCC
11 PGND
4
R2
1K
L1
8 PHASE
ISL6430
REF
C20
0.1µF
9 UGATE
OSC
GND
JP1
COMP
TP1
R4
SPARE
FIGURE 6. ISL6430EVAL1 SCHEMATIC
4
C6-9
4 x 1000µF
RTN
7
COMP
CR2
MBR
340
Application Note 9974
Bill of Materials for ISL6430EVAL1
PART NUMBER
DESCRIPTION
PACKAGE
QTY
REF
VENDOR
25MV680GX
680µF, 25V Aluminum Capacitor
Radial 10 x 22
3
C1 - C3
Sanyo
6MV1000GX
1000µF, 6.3V Aluminum Capacitor
Radial 8 x 20
4
C6 - C9
Sanyo
1206YZ105MAT1A
1.0µF, 16V, X7S Ceramic Capacitor
1206
3
C12, C17-C18
AVX
1000pF Ceramic
1nF, X7R Ceramic Capacitor
0805
1
C19
Various
0.1uF Ceramic
0.1µF, 25V X7R Ceramic Capacitor
0805
2
C13, C20
AVX/Panasonic
0.01uF Ceramic
0.01µF, X7R Ceramic Capacitor
0805
1
C15
Various
33pF Ceramic
33pF, X7R Ceramic Capacitor
0805
1
C14
Various
Spare
Spare Ceramic Capacitor
0805
1N4148
Rectifier 75V
DO35
1
CR1
Various
MBR340
3A, 40V, Schottky
Axial
1
CR2
Motorola
CTX09-13313-X1
PO343
5.3µH, 12A Inductor
T50-52B Core, 10 Turns of 16 AWG Wire
Wound Toroid
1
L1
Coiltronics
Pulse
RFP25N05
47mΩ, 50V MOSFET
TO220
2
Q1, Q2
Intersil
ISL6430
Synchronous Rectified Buck Controller
SOIC-14
1
U1
Intersil
10kΩ
10kΩ, 5% 0.1W, Resistor
0805
1
R7
Various
Spare
Spare 0.1W, Resistor
0805
15kΩ
15kΩ, 5%, 0.1W, Resistor
0805
1
R5
Various
1kΩ
1kΩ, 5%, 0.1W, Resistor
0805
2
R2-R3
Various
3.01kΩ
3.01kΩ, 1%, 0.1W, Resistor
0805
1
R6
Various
576802B00000
TO-220 Clip-on Heatsink
2
1514-2
Terminal Post
6
VIN, 12VCC,
VOUT, RTN
Keystone
1314353-00
Scope Probe Test Point
1
VOUT
Tektronics
SPCJ-123-01
Test Point
3
ENABLE, TP1,
TP2
Jolo
4-5
C16
R1,R4
AAVID
Application Note 9974
All Intersil products are manufactured, assembled and tested utilizing ISO9000 quality systems.
Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design and/or specifications at any time without notice.
Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable.
However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its
use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com
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