an9975

Embedded DC-DC Converters Using the ISL6431
PWM Controller ICs
TM
Application Note
September 2001
AN9975
Introduction
Quick Start Evaluation
The ISL6431 is a voltage mode controller with many
functions that are needed for a multitude of demanding
applications. The ISL6431 a contains a high performance
error amplifier, a high accuracy reference, a fixed 300kHz
internal oscillator and over-current protection circuitry. There
are two MOSFET drivers for use in synchronous-rectified
buck converters. The ISL6431 is also capable of regulating
the output voltage while the DC-DC converter is sinking
current. All these features are packaged in a small, 8 pin
SOIC. More complete descriptions of the ISL6431 can be
found in the datasheets [1].
The input to the ISL6431EVAL1 board will only accept 5V
from a standard power supply. The outputs can be exercised
using either resistive or electronic loads. The shutdown
switch, SW1, will allow the designer to evaluate how the
ISL6431 shuts down and starts up. Pressing the switch will
shut the converter down while releasing it will allow the
converter to restart.
This application note details the ISL6431 in DC-DC
converters for applications requiring a tightly regulated, fixed
output voltage. Any low-cost application requiring a DC-DC
converter can benefit from one of the designs presented in
this application note.
-
PWM
COMPARATOR
+
-
FB
COMP/
OCSET
20mA
UGATE
GATE
CONTROL
LOGIC
VCC
PHASE
ERROR
AMP
+
-
LGATE
+
POR AND
SOFTSTART
INHIBIT
0.8V
+
- OC
COMPARATOR
PWM
SAMPLE
AND
HOLD
BOOT
VCC
Startup
Figure 2 shows a typical startup sequence. Once the input
voltage has exceeded the power-on reset (POR) threshold
level, the IC will begin its start up sequence.
First, the error amplifier is disabled allowing 20µA of current
to be drawn through a resistor tied between V CC and the
COMP/OCSET pin. The voltage that is impressed across the
resistor is then read and sampled at the COMP/OCSET pin,
with respect to VCC. It is then held in internal memory as the
overcurrent set point used in the upper MOSFET RDS(ON)
overcurrent sensing feature.
After the overcurrent set point is established, the error
amplifier is re-enabled and the ISL6431 then initiates its soft
start sequence through the use of an internally generated
soft start ramp. The entire start up procedure typically takes
about 11ms from POR.
OCSET
SAMPLING
SOFTSTART
OSCILLATOR
FIXED 300kHz
GND
FIGURE 1. ISL6431 BLOCK DIAGRAM
VIN
1V/DIV
ISL6431 Reference Design
The ISL6431EVAL1 is an evaluation board that highlights
the operation of the ISL6431 in an embedded application.
The ISL6431EVAL1 is flexible enough to allow for reference
designs with 5A, 10A, and 15A output currents at a fixed
output voltage of 3.3V. A simple resistor change allows the
output to go as low as 0.8V, to as high as the input voltage.
This flexibility allows a power supply designer to easily
modify an existing design to suit almost any relevant
application. The ISL6431EVAL1 DC-DC converter demo
boards are customized to provide up to 15A of current at a
fixed output voltage. The circuit configurations described in
this application note refer to the demo board customized to
15A, unless otherwise noted. The schematic, Bill of Material,
and Board Layout for the ISL6431EVAL1 can be found in the
appendix. Customization of the reference design is
discussed below.
1
VCOMP/OCSET
1V/DIV
VOUT
1V/DIV
TIME (2.5ms/DIV)
FIGURE 2. SYSTEM STARTUP
Shutdown and Restart
DC-DC converters that utilize the ISL6431 may be shut
down by pulling the COMP/OSCET pin below 0.8V. One
method to implement this feature is to connect the drain of a
source-grounded small signal MOSFET to the
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 321-724-7143 | Intersil and Design is a trademark of Intersil Americas Inc.
Copyright © Intersil Americas Inc. 2001, All Rights Reserved
Application Note 9975
for converters that employ a filtering inductor on their inputs.
In these types of converters, all of the RMS current is
supplied by the input capacitors. For converters that do not
employ an input inductor, such as the reference design on
the ISL6431EVAL1 board, the conservative approach may
be too costly. The number of capacitors may be decreased
by assuming that only 50% of the RMS current is supplied by
the input capacitors. This approach will usually provide
enough capacitors to provide proper power decoupling.
COMP/OSCET pin so that an external signal may trigger the
command. The regulator will react to the shutdown
command by pulling both of the MOSFET gates to ground,
thus shutting down the converter.
The ISL6431EVAL1 board demonstrates the shutdown
feature with a push button. Depressing the push button will
disable the converter by pulling the COMP/OSCET pin to
ground. Once released, the converter will follow its normal
start up procedure, which includes the overcurrent set point
programming and soft start.
The voltage rating at maximum ambient temperature should
be 1.25–1.5 times the maximum input voltage. More
conservative approaches can bring the voltage rating up to
two times the maximum input voltage. High frequency
decoupling, which is highly recommended, is implemented
through the use of ceramic capacitors in parallel with the
bulk capacitor filtering.
Reference Design Customization
The ISL6431EVAL1 evaluation board was designed to be as
flexible as possible for the power supply designer. The board
will accommodate either DPAK or D2PAK packaged
MOSFETs in the same footprint and in either the upper or
lower MOSFET locations. This allows for a wide range of
power levels on the same board. The designer may also
implement either a Type II or a Type III compensation
network. Finally, there are component locations in place that
allow the designer to incorporate output voltage droop into
the design.
The reference design utilizes Sanyo 330µF, 6.3V POSCAP
capacitors as the bulk input capacitors (Sanyo Part Number
6TPB330M). Each of these capacitors has a 3ARMS
maximum allowable ripple current rating. The number of
capacitors used in each design, which is shown in Table 1, is
more than sufficient to handle the input RMS current under
the assumption that only 50% of that current is being
supplied by the input capacitance.
Table 1 presents reference values for three different
converter designs that may be used on the ISL6431EVAL1
board. The reference designs are optimized for 5A, 10A and
15A maximum output capabilities. For further customization,
recommended guidelines for component selection are
included below.
MOSFET Selection
The ISL6431EVAL1 board can accommodate multiple
MOSFET package styles. Each placeholder can
accommodate DPAK or D2PAK package connections. The
output loading and the thermal environment ultimately
dictate the MOSFET selected. While many factors are
involved in the selection of the MOSFETs, overall efficiency
of the regulator should be a major contributor. There are
three major aspects of power loss that are associated with
the MOSFET. These are gate drive power losses,
conduction losses and switching losses.
Input Capacitor Selection
The number of input capacitors and their capacitances are
usually determined by their maximum RMS current ratings.
A conservative approach to determining the number and
type of input capacitors is to determine the converter
maximum input RMS current and assume it would all be
supplied by the input capacitors. This approach works well
TABLE 1. ISL6431EVAL1 DESIGN RECOMMENDATIONS
REFERENCE
DESIGN
COMPONENTS
MOSFETS
Q1, Q2
Inductor
Number of Input
Capacitors
Number of Output Capacitors
Number of Decoupling Capacitors
OCSET Resistor
2
MAXIMUM LOAD CURRENT
DESIGN A - 5A
DESIGN B - 10A
DESIGN C - 15A
FAIRCHILD HUF76121D3S FAIRCHILD HUF76129D3S FAIRCHILD HUF76143S3S
L1
6.4µH
Panasonic
ETQP6F6R4HFA
3.2µH
Panasonic
ETQP6F3R2HFA
2.0µH
Panasonic
ETQP6F2R0LFA
C12, C13
1
1
2
C8, C9, C10
1
2
3
C5A, C5B
1
1
2
R5
9.06kΩ
12.7kΩ
6.34kΩ
Application Note 9975
Gate drive power losses result in the power dissipated within
the gate drivers, which are located in the ISL6431. This
power loss is a result of displacing the charge on the gate-tosource capacitance, CGS.
The gate power is frequency dependent and is determined
by the equation:
where
P gate = ( Q gu xV gu + Q gl xV )xF s
gl
Qgu = Upper MOSFET gate charge
Vgu = Upper MOSFET gate voltage
Qgl = Lower MOSFET gate charge
Vgl = Lower MOSFET gate voltage
ISL6431EVAL1 Efficiency
Figure 3 shows the efficiency of the ISL6431EVAL1 for all
three design recommendations: 5A maximum load, 10A
maximum load, and 15A maximum load respectively. As can
be seen in the efficiency curves, the MOSFETs and other
components were selected so as to keep a high efficiency
throughout the load range of the converter.
98%
96%
94%
92%
DESIGN C
15A MAX
90%
Conduction losses are simply I2R losses. Conduction losses
(PCON) can be approximated as:
P CON = R DSU xDx ILOAD
2
P C ON = R DSL x ( 1 – D )xI LOAD
2
DESIGN A
5A MAX
88%
86%
84%
0A
where
5A
10A
15A
FIGURE 3. ISL6431EVAL1 EFFICIENCY
PCON = Upper MOSFET Conduction Loss
PCON= Lower MOSFET Conduction Loss
Output Voltage Programming
RDSU = Upper MOSFET R DS(ON)
Simple resistor value changes allow for outputs as low as
0.8V or as high as the input voltage. The steady state DC
output voltage can be set using the following formula:
RDSL = Lower MOSFET RDS(ON)
D = Duty Cycle
ILOAD= Load Current
where
Note that the RDS(ON) will increase as the junction
temperature of the MOSFET increases. This increase in
impedance should be taken into account when calculating
the conduction losses.
Switching losses are caused by crossover conduction during
the switching interval and by the output capacitance, Coss,
being displaced. Since the dissipation caused by the output
capacitance is very small compared to the loss due to
crossover conduction, it can be ignored.
The equations shown below give a simplified, yet useful,
representation of switching loss for a MOSFET
1
P switch = --- xV in x Iin xtsw xf s
2
where
DESIGN B
10A MAX
Vin = Input Voltage
Iin = Input Current
tsw = MOSFET switching time
fs = Switching Frequency
3
R1
V out = Vref x  1 + --------

R4
Vout = Desired output voltage of converter
Vref = ISL6431 internal reference (0.8V)
The output voltage of the reference designs presented in this
application note is 3.3V.
Lossless Output Voltage Droop with Load
The ISL6431EVAL1 board has unpopulated component
footprints in place to implement output voltage droop. Droop
is an intentional sag in the output voltage that is proportional
to the output current. Although not necessary for proper
circuit operation, utilizing droop allows the dynamic
regulation to be improved by taking advantage of static
regulation requirements and expanding the available
headroom for transient edge output excursions. In practical
applications that are compared to a non-droop
implementation, the droop implementation requires fewer
output capacitors or better regulation with the same type and
number of output capacitors.
By moving the regulation point ahead of the output inductor
(at the PHASE node), droop becomes equal to the average
voltage drop across the output inductor’s DC resistance as
well as any distributed resistance. The droop circuitry is
simply an RC low pass filter placed across the output
inductor. This filter must have the same time constant that
Application Note 9975
On the ISL6431EVAL1 board, this filter is represented by
resistors R6 and R7 and capacitor C7. Resistor R7 can be
used to scale the magnitude of the droop. The output of this
low pass filter is fed directly into the feedback compensation
network of the regulator. The effects of this filter on the
frequency response of the converter is minimal and can be
ignored when evaluating the frequency response of the
converter. To insure symmetric output voltage excursions
about the set voltage in response to load transients, the
output voltage should be programmed to be above the
nominal level by half the calculated droop.
As supplied, the ISL6431EVAL1 does not employ droop. A
successful droop implementation has been designed and
tested for Design C. The DCR of the output inductor is 5mΩ.
This gives a time constant of:
L
OUT
τ = ------------------------------------------------------- = 400µS
DCR + R PARASI TIC
with RPARASITIC = 0Ω
The impedance portion of the time constant calculation for
the RC filter includes the parallel combination of the filter
resistor, R6, and the feedback resistor, R1. A value for R6 is
chosen that will not significantly decrease the impedance
already set by R1. A 16.2kΩ resistor was chosen. This yields
an effective impedance of 2.64kΩ. The filter capacitor, C7, is
then calculated:
τ = Rx C = 2644xC = 400µS
C ~ 0.1µF
Since the regulation point is now located at the phase node,
and a 16.2kΩ resistor is being added to the DC path for
regulation, then the resistor, R4, used to program the output
voltage must be adjusted. The maximum output current for
Design C is 15A and the DCR of the inductor is 5mΩ. This
combination will yield a total droop of 75mV. In order to
center the droop symmetrically about the 3.3V set point, the
no load set point will be 3.375V. The corresponding value for
R4 to accomplish this is 6.04kΩ.
Figure 4shows the output voltage of the converter with the
droop circuitry added.
3.375V
WITHOUT DROOP
3.30V
3.280V
WITH DROOP
OUTPUT
VOLTAGE
the output inductor and its corresponding DCR have. The
design must be careful to include any parasitic impedances
of the PC board if the DCR of the inductor is very low.
0A
15A
OUTPUT CURRENT
FIGURE 4. OUTPUT VOLTAGE DROOP
Component Values for Droop Implementation:
R6 = 16.2kΩ
R7 = Not Populated
C7 = 0.1µF
R4 = 6.04kΩ
With the proper selection of the components used in the RC
filter actoss the inductor, the frequency response of the
system is only minimally affected and the compensation
network does not need to be recalculated.
Output Capacitor Selection
The shape of the output voltage waveform during a load
transient that represents the worst case loading conditions
will ultimately determine the number of output capacitors and
their type. When this load transient is applied to the
converter, most of the energy required by the load is initially
delivered from the output capacitors. This is due to the finite
amount of time required for the inductor current to slew up to
the level of the output current required by the load. This
phenomenon results in a temporary dip in the output voltage.
At the very edge of the transient, the equivalent series
inductance (ESL) of each capacitor induces a spike that
adds on top of the existing voltage drop due to the equivalent
series resistance (ESR)
After the initial spike, attributable to the ESR and ESL of the
capacitors, the output voltage experiences sag. This sag is a
direct consequence of the amount of capacitance on the
output.
During the removal of the same output load, the energy
stored in the inductor is dumped into the output capacitors.
This energy dumping creates a temporary hump in the
output voltage. The hump, as with the sag, can be attributed
to the total amount of capacitance on the output.
4
Application Note 9975
Practically, it can be approximated if an impedance vs.
frequency curve is given for a specific capacitor:
.
∆VESR
VOUT
200mV/DIV
∆VESL
∆VSAG
2
1
ESL = ---- x ( 2xπx fres )
C
where fres is the frequency where the lowest impedance
is achieved (resonant frequency).
IOUT
5A/DIV
dI/dt = 60A/µs
The ESL of the capacitors becomes a concern when
designing circuits that supply power to loads with high rates
of change in the current.
Output Voltage Ripple
TIME = 5µs/DIV
FIGURE 5. TRANSIENT RESPONSE
Figure 5 shows a typical response of the ISL6431EVAL1 to a
load transient. The current slew rate was made large to show
the effects of the parasitic inductances in the output stage.
The amplitudes of the different types of voltage excursions
can be approximated by using the following formulae
∆VESR = ESRxI tran
dI t ran
∆V ESL = ESLx --------------dt
2
Lout xI t ran
∆V SAG = -----------------------------------------------C out x ( V in – Vout )
2
L out x Itran
∆V HUMP = -----------------------------C out x Vout
The amount of ripple voltage on the output of the DC-DC
converter varies with the input voltage, switching frequency,
output inductor, and output capacitors. For a fixed switching
frequency and output filter, the voltage ripple increases with
the input voltage. The ripple content of the output voltage
can be estimated with the following equation:
∆V out = ∆IL xESR
where
V out
( V in – V out )x ----------Vin
∆I L = -----------------------------------------------L out xf s
If the output capacitors have already been selected to meet
certain transient requirements, then the ripple voltage can be
set by the output inductance.
Figure 6 shows the output ripple for Design C.
where
Itran = output load current transient
Cout = total output capacitance
VOUT
10mV/DIV
In a typical converter design, the ESR of the output capacitor
bank dominates the transient response. The ESR and the
ESL are typically the major contributing factors in
determining the output capacitance. The number of output
capacitors can be determined by using the following
equation that relates the ESR and ESL of the capacitors to
the transient load step and the voltage limit (∆Vo):
ESLx∆I t ran
-------------------------------- + ESRxIt ran
dt
Number of Caps = -------------------------------------------------------------------∆V o
If ∆Vsag and/or ∆Vhump are found to be too large for the
output voltage limits, then the amount of capacitance may
need to be increased. In this situation, a trade off between
output inductance and output capacitance may be
necessary.
The ESL of the capacitors, which is an important parameter
in the above equations, is not usually listed in databooks.
5
TIME = 2µs/DIV
FIGURE 6. OUTPUT VOLTAGE RIPPLE
Conclusion
The ISL6431EVAL1 board is a DC-DC converter reference
design that is flexible enough to accommodate a wide range
of output current requirements. The printed circuit board is
laid out to accommodate the necessary components for
operation from low-load levels to 15A of output current.
Application Note 9975
References
For Intersil documents available on the web, see
www.intersil.com/
[1] ISL6431 Data Sheet, Intersil Corporation, Doc. # 9018
ISL6431EVAL1 Schematic
+5V
+
C12,13
C4
GND
C5A,5B
VCC
D1
5
ISL6431
R5
MONITOR
AND
PROTECTION
1
BOOT
C6
2 UGATE
COMP/OCSET 7
Q1
REF
R2
PB1
C2
+
_
C1
R4
4
+
_
FB 6
OSC
L1
8 PHASE
VOUT
LGATE
+
C8-10
Q2
3
+
GND
R6
GND
U1
C11
C7
R1
C3
R7
R3
TABLE 2. ISL6431EVAL1 BILL OF MATERIAL
REF DES
DESCRIPTION
VENDOR
VENDOR P/N
QTY
Various
---
1
C1
470pF Capacitor, 0603
C2
8200pF Capacitor, 0603
Various
---
1
C3
18000pF Capacitor, 0603
Various
---
1
C4,C6,C11
C5A,C5B
C7
C8-10, C12,
C13
0.1µF Capacitor, 0603
Various
---
2
1µF Capacitor, 0805
Various
---
2
---
---
---
Sanyo
6TPB330M
5
1
Not Populated Capacitor, 0603
330µF Capacitor
D1
Diode, 30mA, 30V
L1
2µH Inductor
Digikey
MA732
Panasonic
ETQP6F2ROLFH
1
MOSFET
Fairchild
HUF76143S3S
2
R1
3.16kΩ 1% Resistor, 0603
Various
---
1
R2
10.0kΩ 1% Resistor, 0603
Various
---
1
R3
60.4Ω 1% Resistor, 0603
Various
---
1
R4
1.00kΩ Resistor, 0603
Various
---
1
R5
6.34kΩ Resistor, 0603
Various
---
1
R6
Not Populated Resistor, 0603
---
---
---
R7
0Ω Resistor, 0603
Various
---
1
Pushbutton, miniature
Digikey
P8007S-ND
1
Single Synchronous Buck PWM Controller
Intersil
ISL6431CB
1
Keystone
1514-2
4
Q1, Q2
PB1
U1
TP1,2,3,4
Test Points
6
Application Note 9975
Board Description
Silk Screen
Top Layer
Ground Layer
Power Layer
Bottom Layer
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.
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7
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