DATASHEET

ISL8105, ISL8105A
®
Data Sheet
April 15, 2010
+5V or +12V Single-Phase Synchronous
Buck Converter PWM Controller with
Integrated MOSFET Gate Drivers
The ISL8105, ISL8105A is a simple single-phase PWM
controller for a synchronous buck converter. It operates from
+5V or +12V bias supply voltage. With integrated linear
regulator, boot diode, and N-Channel MOSFET gate drivers,
the ISL8105, ISL8105A reduces external component count and
board space requirements. These make the IC suitable for a
wide range of applications.
Utilizing voltage-mode control, the output voltage can be
precisely regulated to as low as 0.6V. The 0.6V internal
reference features a maximum tolerance of ±1.0% over the
commercial temperature range, and ±1.5% over the
industrial temperature range. Two fixed oscillator frequency
versions are available; 300kHz (ISL8105 for high efficiency
applications) and 600kHz (ISL8105A for fast transient
applications).
The ISL8105, ISL8105A features the capability of safe
start-up with pre-biased load. It also provides overcurrent
protection by monitoring the ON-resistance of the
bottom-side MOSFET to inhibit PWM operation
appropriately. During start-up interval, the resistor connected
to BGATE/BSOC pin is employed to program overcurrent
protection condition. This approach simplifies the
implementation and does not deteriorate converter
efficiency.
FN6306.5
Features
• Operates from +5V or +12V Bias Supply Voltage
- 1.0V to 12V Input Voltage Range (up to 20V possible
with restrictions; see “Input Voltage Considerations” on
page 9)
- 0.6V to VIN Output Voltage Range
• 0.6V Internal Reference Voltage
- ±1.0% Tolerance Over the Commercial Temperature
Range (0°C to +70°C)
- ±1.5% Tolerance Over the Industrial Temperature
Range (-40°C to +85°C).
• Integrated MOSFET Gate Drivers that Operate from
VBIAS (+5V to +12V)
- Bootstrapped High-side Gate Driver with Integrated
Boot Diode
- Drives N-Channel MOSFETs
• Simple Voltage-Mode PWM Control
- Traditional Dual Edge Modulation
• Fast Transient Response
- High-Bandwidth Error Amplifier
- Full 0% to 100% Duty Cycle
• Fixed Operating Frequency
- 300kHz for ISL8105
- 600kHz for ISL8105A
• Fixed Internal Soft-Start with Pre-biased Load Capability
• Lossless, Programmable Overcurrent Protection
- Uses Bottom-side MOSFET’s rDS(ON)
Pinouts
ISL8105, ISL8105A
(10 LD 3X3 DFN)
TOP VIEW
• Enable/Disable Function Using COMP/EN Pin
• Output Current Sourcing and Sinking Currents
BOOT
1
10 LX
• Pb-Free (RoHS Compliant)
TGATE
2
9
COMP/EN
Applications
N/C
3
8
FB
GND
4
7
N/C
BGATE/BSOC
5
6
VBIAS
GND
• 5V or 12V DC/DC Regulators
• Industrial Power Systems
• Telecom and Datacom Applications
ISL8105, ISL8105A
(8 LD SOIC)
TOP VIEW
• Test and Measurement Instruments
• Distributed DC/DC Power Architecture
BOOT 1
8
LX
TGATE 2
7
COMP/EN
3
6
FB
BGATE/BSOC 4
5
VBIAS
GND
1
• Point of Load Modules
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2005-2007, 2010. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
ISL8105, ISL8105A
Ordering Information
PART NUMBER
(Note)
PART
MARKING
SWITCHING
FREQUENCY
(kHz)
TEMPERATURE
RANGE
(°C)
PACKAGE
(Pb-Free)
PKG.
DWG. #
ISL8105CRZ*
5CRZ
300
0 to +70
10 Ld DFN
L10.3x3C
ISL8105IBZ*
8105 IBZ
300
-40 to +85
8 Ld SOIC
M8.15
ISL8105IRZ*
5IRZ
300
-40 to +85
10 Ld DFN
L10.3x3C
ISL8105ACRZ*
05AZ
600
0 to +70
10 Ld DFN
L10.3x3C
ISL8105AIBZ*
8105 AIBZ
600
-40 to +85
8 Ld SOIC
M8.15
ISL8105AIRZ*
5AIZ
600
-40 to +85
10 Ld DFN
L10.3x3C
ISL8105AEVAL1Z
Evaluation Board
*Add “-T” suffix for tape and reel. Please refer to TB347 for details on reel specifications.
NOTE: These Intersil Pb-free plastic packaged products employ special Pb-free material sets, molding compounds/die attach materials, and 100%
matte tin plate plus anneal (e3 termination finish, which is RoHS compliant and compatible with both SnPb and Pb-free soldering operations).
Intersil Pb-free products are MSL classified at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J
STD-020.
Typical Application Diagram
VIN
+1V TO +12V
VBIAS
+5V OR +12V
CHF
CBULK
CDCPL
VBIAS
BOOT
COMP/EN
CBOOT
TGATE
C1
C2
ISL8105
R2
Q1
LOUT
VOUT
LX
COUT
FB
Q2
BGATE/BSOC
GND
RBSOC
C3
R3
R1
R0
2
FN6306.5
April 15, 2010
Block Diagram
VBIAS
DBOOT
3
SAMPLE
AND
HOLD
INTERNAL
REGULATOR
POR AND
SOFT-START
+
BOOT
OC
TGATE
COMPARATOR
5V INT.
LX
20kΩ
PWM
TO
COMPARATOR
BGATE/BSOC
0.6V
INHIBIT
+
-
+
-
ERROR
PWM
GATE
CONTROL
LOGIC
VBIAS
AMP
FB
DIS
BGATE/BSOC
5V INT.
0.4V
+
20μA
DIS
OSCILLATOR
COMP/EN
FIXED 300kHZ OR 600kHz
GND
ISL8105, ISL8105A
21.5μA
FN6306.5
April 15, 2010
ISL8105, ISL8105A
Absolute Maximum Ratings
Thermal Information
Bias Voltage, VBIAS . . . . . . . . . . . . . . . . . . . . GND - 0.3V to +15.0V
Boot Voltage, VBOOT . . . . . . . . . . . . . . . . . . . GND - 0.3V to +36.0V
TGATE Voltage, VTGATE . . . . . . . . . . . VLX - 0.3V to VBOOT + 0.3V
BGATE/BSOC Voltage, VBGATE/BSOC . .GND - 0.3 to VBIAS + 0.3V
LX Voltage, VLX . . . . . . . . . . . . . . . . . .GND - 0.3V to VBOOT + 0.3V
Upper Driver Supply Voltage, VBOOT - VLX . . . . . . . . . . . . . . . .15V
Clamp Voltage, VBOOT - VBIAS . . . . . . . . . . . . . . . . . . . . . . . . . .24V
FB, COMP/EN Voltage . . . . . . . . . . . . . . . . . . . . . GND - 0.3V to 6V
Thermal Resistance
θJA (°C/W)
θJC (°C/W)
SOIC Package (Note 1) . . . . . . . . . . . .
95
N/A
DFN Package (Notes 1, 2) . . . . . . . . . .
44
5.5
Maximum Junction Temperature
(Plastic Package) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +150°C
Maximum Storage Temperature Range . . . . . . . . . .-65°C to +150°C
Pb-Free Reflow Profile. . . . . . . . . . . . . . . . . . . . . . . . .see link below
http://www.intersil.com/pbfree/Pb-FreeReflow.asp
Recommended Operating Conditions
Bias Voltage, VBIAS . . . . . +5V ±10%, +12V ±20%, or 6.5V to 14.4V
Ambient Temperature Range
ISL8105C, ISL8105AC . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
ISL8105I, ISL8105AI. . . . . . . . . . . . . . . . . . . . . . .-40°C to +85°C
Junction Temperature Range. . . . . . . . . . . . . . . . . .-40°C to +125°C
CAUTION: Do not operate at or near the maximum ratings listed for extended periods of time. Exposure to such conditions may adversely impact product reliability and
result in failures not covered by warranty.
NOTES:
1. θJA is measured in free air with the component mounted on a high effective thermal conductivity test board with “direct attach” features. See
Tech Brief TB379.
2. For θJC, the “case temp” location is the center of the exposed metal pad on the package underside.
Electrical Specifications
Recommended Operating Conditions, Unless Otherwise Noted. Parameters with MIN and/or MAX limits are
100% tested at +25°C, unless otherwise specified. Temperature limits established by characterization and are
not production tested.
PARAMETER
SYMBOL
TEST CONDITIONS
MIN
TYP
MAX
UNITS
4
5.2
7
mA
0.375
0.4
0.425
V
ISL8105C
270
300
330
kHz
ISL8105I
240
300
330
kHz
ISL8105AC
540
600
660
kHz
ISL8105AI
510
600
660
kHz
INPUT SUPPLY CURRENTS
Shutdown VBIAS Supply Current
IVBIAS_S
VBIAS = 12V; Disabled
DISABLE
Disable Threshold (COMP/EN pin)
VDISABLE
OSCILLATOR
Nominal Frequency Range
fOSC
fOSC
ΔVOSC
Ramp Amplitude (Note 3)
1.5
VP-P
POWER-ON RESET
Rising VBIAS Threshold
VPOR_R
3.9
4.1
4.3
V
VBIAS POR Threshold Hysteresis
VPOR_H
0.30
0.35
0.40
V
REFERENCE
Nominal Reference Voltage
0.6
VREF
Reference Voltage Tolerance
V
ISL8105C (0°C to +70°C)
-1.0
+1.0
%
ISL8105I (-40°C to +85°C)
-1.5
+1.5
%
ERROR AMPLIFIER
DC Gain (Note 3)
Unity Gain-Bandwidth (Note 3)
Slew Rate (Note 3)
GAINDC
96
dB
UGBW
20
MHz
SR
9
V/µs
3.0
Ω
GATE DRIVERS
TGATE Source Resistance
RTG-SRCh
4
VBIAS = 14.5V, 50mA Source Current
FN6306.5
April 15, 2010
ISL8105, ISL8105A
Electrical Specifications
Recommended Operating Conditions, Unless Otherwise Noted. Parameters with MIN and/or MAX limits are
100% tested at +25°C, unless otherwise specified. Temperature limits established by characterization and are
not production tested. (Continued)
PARAMETER
SYMBOL
TEST CONDITIONS
MIN
TYP
MAX
UNITS
TGATE Source Resistance
RTG-SRCl
VBIAS = 4.25V, 50mA Source Current
3.5
Ω
TGATE Sink Resistance
RTG-SNKh
VBIAS = 14.5V, 50mA Source Current
2.7
Ω
TGATE Sink Resistance
RTG-SNKl
VBIAS = 4.25V, 50mA Source Current
2.7
Ω
BGATE Source Resistance
RBG-SRCh
VBIAS = 14.5V, 50mA Source Current
2.4
Ω
BGATE Source Resistance
RBG-SRCl
VBIAS = 4.25V, 50mA Source Current
2.75
Ω
BGATE Sink Resistance
RBG-SNKh
VBIAS = 14.5V, 50mA Source Current
2.0
Ω
BGATE Sink Resistance
RBG-SNKl
VBIAS = 4.25V, 50mA Source Current
2.1
Ω
OVERCURRENT PROTECTION (OCP)
BSOC Current Source
IBSOC
ISL8105C; BGATE/BSOC Disabled
19.5
21.5
23.5
µA
ISL8105I; BGATE/BSOC Disabled
18.0
21.5
23.5
µA
NOTE:
3. Limits established by characterization and are not production tested.
Functional Pin Description (SOIC, DFN)
BOOT (SOIC Pin 1, DFN Pin 1)
This pin provides ground referenced bias voltage to the
top-side MOSFET driver. A bootstrap circuit is used to create
a voltage suitable to drive an N-Channel MOSFET (equal to
VBIAS minus the on-chip BOOT diode voltage drop), with
respect to LX.
TGATE (SOIC Pin 2, DFN Pin 2)
VBIAS (SOIC Pin 5, DFN Pin 6)
This pin provides the bias supply for the ISL8105, as well as
the bottom-side MOSFET's gate and the BOOT voltage for
the top-side MOSFET's gate. An internal 5V regulator will
supply bias if VBIAS rises above 6.5V (but the BGATE/BSOC
and BOOT will still be sourced by VBIAS). Connect a well
decoupled +5V or +12V supply to this pin.
FB (SOIC Pin 6, DFN Pin 8)
Connect this pin to the gate of top-side MOSFET; it provides
the PWM-controlled gate drive. It is also monitored by the
adaptive shoot-through protection circuitry to determine
when the top-side MOSFET has turned off.
This pin is the inverting input of the internal error amplifier.
Use FB, in combination with the COMP/EN pin, to
compensate the voltage-control feedback loop of the
converter. A resistor divider from the output to GND is used
to set the regulation voltage.
GND (SOIC Pin 3, DFN Pin 4)
COMP/EN (SOIC Pin 7, DFN Pin 9)
This pin represents the signal and power ground for the IC.
Tie this pin to the ground island/plane through the lowest
impedance connection available.
This is a multiplexed pin. During soft-start and normal converter
operation, this pin represents the output of the error amplifier.
Use COMP/EN, in combination with the FB pin, to compensate
the voltage-control feedback loop of the converter.
BGATE/BSOC (SOIC Pin 4, DFN Pin 5)
Connect this pin to the gate of the bottom-side MOSFET; it
provides the PWM-controlled gate drive (from VBIAS). This
pin is also monitored by the adaptive shoot-through
protection circuitry to determine when the lower MOSFET
has turned off.
During a short period of time following Power-On Reset
(POR) or shut-down release, this pin is also used to
determine the current limit threshold of the converter.
Connect a resistor (RBSOC) from this pin to GND. See
“Overcurrent Protection (OCP)” on page 7 for equations. An
overcurrent trip cycles the soft-start function, after two
dummy soft-start time-outs. Some of the text describing the
BGATE function may leave off the BSOC part of the name,
when it is not relevant to the discussion.
5
Pulling COMP/EN low (VDISABLE = 0.4V nominal) will
disable (shut-down) the controller, which causes the
oscillator to stop, the BGATE and TGATE outputs to be held
low, and the soft-start circuitry to re-arm. The external
pull-down device will initially need to overcome maximum of
5mA of COMP/EN output current. However, once the IC is
disabled, the COMP output will also be disabled, so only a
20µA current source will continue to draw current.
When the pull-down device is released, the COMP/EN pin
will start to rise at a rate determined by the 20µA charging up
the capacitance on the COMP/EN pin. When the COMP/EN
pin rises above the VDISABLE trip point, the ISL8105 will
begin a new initialization and soft-start cycle.
FN6306.5
April 15, 2010
ISL8105, ISL8105A
LX (SOIC Pin 8, DFN Pin 10)
Connect this pin to the source of the top-side MOSFET and
the drain of the bottom-side MOSFET. It is used as the sink
for the TGATE driver and to monitor the voltage drop across
the bottom-side MOSFET for overcurrent protection. This pin
is also monitored by the adaptive shoot-through protection
circuitry to determine when the top-side MOSFET has turned
off.
N/C (DFN Only; Pin3, Pin 7)
These two pins in the DFN package are No Connect.
Functional Description
time, the BGATE/BSOC pin is initialized by disabling the
BGATE driver and drawing BSOC (nominal 21.5µA) through
RBSOC. This sets up a voltage that will represent the BSOC
trip point. At t2, there is a variable time period for the OCP
sample and hold operation (0ms to 3.4ms nominal; the
longer time occurs with the higher overcurrent setting). The
sample and hold uses a digital counter and DAC to save the
voltage, so the stored value does not degrade, for as long as
the VBIAS is above VPOR. See “Overcurrent Protection
(OCP)” on page 7 for more details on the equations and
variables. Upon the completion of sample and hold at t3, the
soft-start operation is initiated, and the output voltage ramps
up between t4 and t5.
Initialization (POR and OCP Sampling)
BGATE
STARTS
SWITCHING
Figure 1 shows a start-up waveform of ISL8105. The
Power-ON-Reset (POR) function continually monitors the
bias voltage at the VBIAS pin. Once the rising POR
threshold is exceeded 4V (VPOR nominal), the POR function
initiates the Overcurrent Protection (OCP) sample and hold
operation (while COMP/EN is ~1V). When the sampling is
complete, VOUT begins the soft-start ramp.
COMP/EN
VOUT
BGATE/BSOC
VBIAS
3.4ms
t0 t1
3.4ms
0ms to 3.4ms
t2 t3 t4
t5
VOUT
~4V POR
VCOMP/EN
FIGURE 2. BGATE/BSOC AND SOFT-START OPERATION
Soft-Start and Pre-Biased Outputs
FIGURE 1. POR AND SOFT-START OPERATION
If the COMP/EN pin is held low during power-up, the
initialization will be delayed until the COMP/EN is released
and its voltage rises above the VDISABLE trip point.
Figure 2 shows a typical power-up sequence in more detail.
The initialization starts at t0, when either VBIAS rises above
VPOR, or the COMP/EN pin is released (after POR). The
COMP/EN will be pulled up by an internal 20µA current
source, but the timing will not begin until the COMP/EN
exceeds the VDISABLE trip point (at t1). The external
capacitance of the disabling device, as well as the
compensation capacitors, will determine how quickly the
20µA current source will charge the COMP/EN pin. With
typical values, it should add a small delay compared to the
soft-start times. The COMP/EN will continue to ramp to ~1V.
From t1, there is a nominal 6.8ms delay, which allows the
VBIAS pin to exceed 6.5V (if rising up towards 12V), so that
the internal bias regulator can turn on cleanly. At the same
6
Functionally, the soft-start internally ramps the reference on
the non-inverting terminal of the error amp from 0V to 0.6V in
a nominal 6.8ms. The output voltage will thus follow the
ramp, from zero to final value, in the same 6.8ms (the actual
ramp seen on the VOUT will be less than the nominal time),
due to some initialization timing, between t3 and t4).
The ramp is created digitally, so there will be 64 small
discrete steps. There is no simple way to change this ramp
rate externally, and it is the same for either frequency
version of the IC (300kHz or 600kHz).
After an initialization period (t3 to t4), the error amplifier
(COMP/EN pin) is enabled, and begins to regulate the
converter's output voltage during soft-start. The oscillator's
triangular waveform is compared to the ramping error
amplifier voltage. This generates LX pulses of increasing
width that charge the output capacitors. When the internally
generated soft-start voltage exceeds the reference voltage
(0.6V), the soft-start is complete and the output should be in
regulation at the expected voltage. This method provides a
rapid and controlled output voltage rise; there is no large
inrush current charging the output capacitors. The entire
start-up sequence from POR typically takes up to 17ms; up
FN6306.5
April 15, 2010
ISL8105, ISL8105A
Overcurrent Protection (OCP)
VOUT OVER-CHARGED
PRE-BIASED
VVOUT
OUTPRE-BIASED
NORMAL
VV
OUTNORMAL
OUT
t0
t1t1
t2
t2
FIGURE 3. SOFT-START WITH PRE-BIAS
to 10.2ms for the delay and OCP sample and 6.8ms for the
soft-start ramp.
Figure 3 shows the normal curve in blue; initialization begins
at t0, and the output ramps between t1 and t2. If the output is
pre-biased to a voltage less than the expected value, as
shown by the red curve, the ISL8105, ISL8105A will detect
that condition. Neither MOSFET will turn on until the
soft-start ramp voltage exceeds the output; VOUT starts
seamlessly ramping from there. If the output is pre-biased to
a voltage above the expected value, as in the gray curve,
neither MOSFET will turn on until the end of the soft-start, at
which time it will pull the output voltage down to the final
value. Any resistive load connected to the output will help
pull down the voltage (at the RC rate of the R of the load and
the C of the output capacitance).
If the VIN for the synchronous buck converter is from a
different supply that comes up after VBIAS, the soft-start
would go through its cycle, but with no output voltage ramp.
When VIN turns on, the output would follow the ramp of the
VIN from zero up to the final expected voltage (at close to
100% duty cycle, with COMP/EN pin >4V). If VIN is too fast,
there may be excessive inrush current charging the output
capacitors (only the beginning of the ramp, from zero to
VOUT matters here). If this is not acceptable, then consider
changing the sequencing of the power supplies, or sharing
the same supply, or adding sequencing logic to the
COMP/EN pin to delay the soft-start until the VIN supply is
ready (see “Input Voltage Considerations” on page 9).
If the IC is disabled after soft-start (by pulling COMP/EN pin
low), and then enabled (by releasing the COMP/EN pin),
then the full initialization (including OCP sample) will take
place. However, there is no new OCP sampling during
overcurrent retries. If the output is shorted to GND during
soft-start, the OCP will handle it, as described in the next
section.
The overcurrent function protects the converter from a
shorted output by using the bottom-side MOSFET's
on-resistance, rDS(ON), to monitor the current. A resistor
(RBSOC) programs the overcurrent trip level (see “Typical
Application Diagram” on page 2). This method enhances the
converter's efficiency and reduces cost by eliminating a
current sensing resistor. If overcurrent is detected, the output
immediately shuts off, it cycles the soft-start function in a
hiccup mode (2 dummy soft-start time-outs, then up to one
real one) to provide fault protection. If the shorted condition
is not removed, this cycle will continue indefinitely.
Following POR (and 6.8ms delay), the ISL8105, ISL8105A
initiates the Overcurrent Protection sample and hold
operation. The BGATE driver is disabled to allow an internal
21.5µA current source to develop a voltage across RBSOC.
The ISL8105, ISL8105A samples this voltage (which is
referenced to the GND pin) at the BGATE/BSOC pin, and
holds it in a counter and DAC combination. This sampled
voltage is held internally as the Overcurrent Set Point, for as
long as power is applied, or until a new sample is taken after
coming out of a shut-down.
The actual monitoring of the bottom-side MOSFET's
on-resistance starts 200ns (nominal) after the edge of the
internal PWM logic signal (that creates the rising external
BGATE signal). This is done to allow the gate transition
noise and ringing on the LX pin to settle out before
monitoring. The monitoring ends when the internal PWM
edge (and thus BGATE) goes low. The OCP can be detected
anywhere within the above window.
If the regulator is running at high TGATE duty cycles (around
75% for 600kHz or 87% for 300kHz operation), then the
BGATE pulse width may not be wide enough for the OCP to
properly sample the rDS(ON). For those cases, if the BGATE
is too narrow (or not there at all) for 3 consecutive pulses,
then the third pulse will be stretched and/or inserted to the
425ns minimum width. This allows for OCP monitoring every
third pulse under this condition. This can introduce a small
pulse-width error on the output voltage, which will be
corrected on the next pulse; and the output ripple voltage will
have an unusual 3-clock pattern, which may look like jitter. If
the OCP is disabled (by choosing a too-high value of
RBSOC, or no resistor at all), then the pulse stretching
feature is also disabled. Figure 4 illustrates the BGATE pulse
width stretching, as the width gets smaller.
If the output is shorted to GND during soft-start, the OCP will
handle it, as described in the next section.
7
FN6306.5
April 15, 2010
ISL8105, ISL8105A
MOSFETs is typically in the 20mV to 120mV ballpark
(500Ω to 3000Ω). If the voltage drop across RBSOC is set
too low, that can cause almost continuous OCP tripping and
retry. It would also be very sensitive to system noise and
inrush current spikes, so it should be avoided. The maximum
usable setting is around 0.2V across RBSOC (0.4V across
the MOSFET); values above that might disable the
protection. Any voltage drop across RBSOC that is greater
than 0.3V (0.6V MOSFET trip point) will disable the OCP.
The preferred method to disable OCP is simply to remove
the resistor, which will be detected as no OCP.
BGATE > 425ns
BGATE = 425ns
Note that conditions during power-up or during a retry may
look different than normal operation. During power-up in a
12V system, the IC starts operation just above 4V; if the
supply ramp is slow, the soft-start ramp might be over well
before 12V is reached. So with bottom-side gate drive
voltages, the rDS(ON) of the MOSFETs will be higher during
power-up, effectively lowering the OCP trip. In addition, the
ripple current will likely be different at lower input voltage.
BGATE < 425ns
Another factor is the digital nature of the soft-start ramp. On
each discrete voltage step, there is in effect a small load
transient, and a current spike to charge the output
capacitors. The height of the current spike is not controlled; it
is affected by the step size of the output, the value of the
output capacitors, as well as the IC error amp compensation.
So it is possible to trip the overcurrent with inrush current, in
addition to the normal load and ripple considerations.
BGATE << 425ns
FIGURE 4. BGATE PULSE STRETCHING
The overcurrent function will trip at a peak inductor current
(IPEAK) determined by Equation 1:
2 × I BSOC × R BSOC
I PEAK = -----------------------------------------------------r DS ( ON )
(EQ. 1)
where IBSOC is the internal BSOC current source (21.5µA
typical). The scale factor of 2 doubles the trip point of the
MOSFET voltage drop, compared to the setting on the
RBSOC resistor. The OC trip point varies in a system mainly
due to the MOSFET's rDS(ON) variations (over process,
current and temperature). To avoid overcurrent tripping in
the normal operating load range, find the RBSOC resistor
from Equation 1 with:
1. The maximum rDS(ON) at the highest junction
temperature
2. The minimum IBSOC from the specification table
( ΔI )
3. Determine IPEAK for IPEAK > IOUT(MAX) + ---------- , where
2
ΔI is the output inductor ripple current.
For an equation for the ripple current, see “Output Inductor
Selection” on page 13.
The range of allowable voltages detected (2*IBSOC*RBSOC)
is 0mV to 475mV; but the practical range for typical
8
Figure 5 shows the output response during a retry of an
output shorted to GND. At time t0, the output has been
turned off, due to sensing an overcurrent condition. There
are two internal soft-start delay cycles (t1 and t2) to allow the
MOSFETs to cool down, to keep the average power
dissipation in retry at an acceptable level. At time t2, the
output starts a normal soft-start cycle, and the output tries to
ramp. If the short is still applied, and the current reaches the
BSOC trip point any time during soft-start ramp period, the
output will shut off and return to time t0 for another delay
cycle. Thus, the retry period is two dummy soft-start cycles
plus one variable one (which depends on how long it takes to
trip the sensor each time). Figure 5 also shows an example
where the output gets about half-way up before shutting
down; therefore, the retry (or hiccup) time will be around
17ms. The minimum should be nominally 13.6ms and the
maximum 20.4ms. If the short condition is finally removed,
the output should ramp up normally on the next t2 cycle.
Starting up into a shorted load looks the same as a retry into
that same shorted load. In both cases, OCP is always
enabled during soft-start; once it trips, it will go into retry
(hiccup) mode. The retry cycle will always have two dummy
time-outs, plus whatever fraction of the real soft-start time
passes before the detection and shutoff; at that point, the
logic immediately starts a new two dummy cycle time-out.
FN6306.5
April 15, 2010
ISL8105, ISL8105A
There is an internal 5V regulator for bias; it turns on between
5.5 and 6.5V. Some of the delay after POR is there to allow a
typical power supply to ramp-up past 6.5V before the
soft-start ramps begins. This prevents a disturbance on the
output, due to the internal regulator turning on or off. If the
transition is slow (not a step change), the disturbance should
be minimal. So while the recommendation is to not have the
output enabled during the transition through this region, it
may be acceptable. The user should monitor the output for
their application to see if there is any problem.
INTERNAL SOFT-START RAMP
VOUT
6.8ms
6.8ms
6.8ms
t1
t1
t0
0ms TO 6.8ms
t2
FIGURE 5. OVERCURRENT RETRY OPERATION
Output Voltage Selection
The output voltage can be programmed to any level between
the 0.6V internal reference, up to the VBIAS supply. The
ISL8105, ISL8105A can run at near 100% duty cycle at zero
load, but the rDS(ON) of the top-side MOSFET will effectively
limit it to something less as the load current increases. In
addition, the OCP (if enabled) will also limit the maximum
effective duty cycle.
An external resistor divider is used to scale the output
voltage relative to the internal reference voltage, and feed it
back to the inverting input of the error amp. See “Typical
Application Diagram” on page 2 for more detail; R1 is the
upper resistor; ROFFSET (shortened to R0 below) is the
lower one. The recommended value for R1 is 1kΩ to 5kΩ
(±1% for accuracy) and then ROFFSET is chosen according
to Equations 2 and 3. Since R1 is part of the compensation
circuit (see “Feedback Compensation” on page 11), it is
often easier to change ROFFSET to change the output
voltage; that way the compensation calculations do not need
to be repeated. If VOUT = 0.6V, then ROFFSET can be left
open. Output voltages less than 0.6V are not available.
( R1 + R0 )
V OUT = 0.6V • -------------------------R0
(EQ. 2)
R 1 • 0.6V
R 0 = ---------------------------------V OUT – 0.6V
(EQ. 3)
Input Voltage Considerations
The “Typical Application Diagram” on page 2 shows a
standard configuration where VBIAS is either 5V (±10%) or
12V (±20%); in each case, the gate drivers use the VBIAS
voltage for BGATE and BOOT/TGATE. In addition, VBIAS is
allowed to work anywhere from 6.5V up to the 14.4V
maximum. The VBIAS range between 5.5V and 6.5V is
NOT allowed for long-term reliability reasons, but
transitions through it to voltages above 6.5V are acceptable.
The VIN to the top-side MOSFET can share the same supply
as VBIAS but can also run off a separate supply or other
sources, such as outputs of other regulators. If VBIAS
powers up first, and the VIN is not present by the time the
initialization is done, then the soft-start will not be able to
ramp the output, and the output will later follow part of the
VIN ramp when it is applied. If this is not desired, then
change the sequencing of the supplies, or use the
COMP/EN pin to disable VOUT until both supplies are ready.
Figure 6 shows a simple sequencer for this situation. If
VBIAS powers up first, Q1 will be off, and R3 pulling to VBIAS
will turn Q2 on, keeping the ISL8105, ISL8105A in shutdown.
When VIN turns on, the resistor divider R1 and R2
determines when Q1 turns on, which will turn off Q2 and
release the shut-down. If VIN powers up first, Q1 will be on,
turning Q2 off; so the ISL8105, ISL8105A will start-up as
soon as VBIAS comes up. The VDISABLE trip point is 0.4V
nominal, so a wide variety of NFET's or NPN's or even some
logic IC's can be used as Q1 or Q2; but Q2 must be low
leakage when off (open-drain or open-collector) so as not to
interfere with the COMP output. Q2 should also be placed
near the COMP/EN pin.
The VIN range can be as low as ~1V (for VOUT as low as the
0.6V reference). It can be as high as 20V (for VOUT just
below VIN). There are some restrictions for running high VIN
voltage.
The first consideration for high VIN is the maximum BOOT
voltage of 36V. The VIN (as seen on LX) + VBIAS (boot
voltage - the diode drop) + any ringing (or other transients)
on the BOOT pin must be less than 36V. If VIN is 20V, that
limits VBIAS + ringing to 16V.
The second consideration for high VIN is the maximum
(BOOT - VBIAS) voltage; this must be less than 24V. Since
BOOT = VIN + VBIAS + ringing, that reduces to (VIN + ringing)
VIN
R1
R2
VBIAS
R3
TO COMP/EN
Q1
Q2
FIGURE 6. SEQUENCER CIRCUIT
9
FN6306.5
April 15, 2010
ISL8105, ISL8105A
must be <24V. So based on typical circuits, a 20V maximum
VIN is a good starting assumption; the user should verify the
ringing in their particular application.
Another consideration for high VIN is duty cycle. Very low
duty cycles (such as 20V in to 1.0V out, for 5% duty cycle)
require component selection compatible with that choice
(such as low rDS(ON) bottom-side MOSFET, and a good LC
output filter). At the other extreme (for example, 20V in to
12V out), the top-side MOSFET needs to be low rDS(ON). In
addition, if the duty cycle gets too high, it can affect the
overcurrent sample time. In all cases, the input and output
capacitors and both MOSFETs must be rated for the
voltages present.
Application Guidelines
Layout Considerations
As in any high-frequency switching converter, layout is very
important. Switching current from one power device to
another can generate voltage transients across the
impedances of the interconnecting bond wires and circuit
traces. These interconnecting impedances should be
minimized by using wide, short printed circuit traces. The
critical components should be located as close together as
possible using ground plane construction or single point
grounding.
VIN
Switching Frequency
ISL8105
TGATE
Q1
LO
VOUT
LX
CIN
BGATE
Q2
LOAD
The switching frequency is either a fixed 300kHz or 600kHz,
depending on the part number chosen (ISL8105 is 300kHz;
ISL8105A is 600kHz; the generic name “ISL8105” may apply
to either in the rest of this document, except when choosing
the frequency). However, all of the other timing mentioned
(POR delay, OCP sample, soft-start, etc.) is independent of
the clock frequency (unless otherwise noted).
CO
PGND
BOOT Refresh
In the event that the TGATE is on for an extended period of
time, the charge on the boot capacitor can start to sag,
raising the rDS(ON) of the top-side MOSFET. The ISL8105
has a circuit that detects a long TGATE on-time (nominal
100µs), and forces the BGATE to go higher for one clock
cycle, which will allow the boot capacitor some time to
recharge. Separately, the OCP circuit has a BGATE pulse
stretcher (to be sure the sample time is long enough), which
can also help refresh the boot. But if OCP is disabled (no
current sense resistor), the regular boot refresh circuit will
still be active.
Current Sinking
The ISL8105 incorporates a MOSFET shoot-through
protection method which allows a converter to sink current
as well as source current. Care should be exercised when
designing a converter with the ISL8105 when it is known that
the converter may sink current.
When the converter is sinking current, it is behaving as a
boost converter that is regulating its input voltage. This
means that the converter is boosting current into the VIN rail.
If there is nowhere for this current to go, such as to other
distributed loads on the VIN rail, through a voltage limiting
protection device, or other methods, the capacitance on the
VIN bus will absorb the current. This situation will allow
voltage level of the VIN rail (also LX) to increase. If the
voltage level of the LX is increased to a level that exceeds
the maximum voltage rating of the ISL8105, then the IC will
experience an irreversible failure and the converter will no
longer be operational. Ensuring that there is a path for the
current to follow other than the capacitance on the rail will
prevent this failure mode.
10
RETURN
FIGURE 7. PRINTED CIRCUIT BOARD POWER AND
GROUND PLANES OR ISLANDS
Figure 7 shows the critical power components of the
converter. To minimize the voltage overshoot/undershoot,
the interconnecting wires indicated by heavy lines should be
part of ground or power plane in a printed circuit board. The
components shown in Figure 8 should be located as close
together as possible. Please note that the capacitors CIN
and CO each represent numerous physical capacitors.
Locate the ISL8105 within three inches of the MOSFETs, Q1
and Q2. The circuit traces for the MOSFETs’ gate and
source connections from the ISL8105 must be sized to
handle up to 1A peak current.
Proper grounding of the IC is important for correct operation
in noisy environments. The GND pin should be connected to
a large copper fill under the IC which is subsequently
connected to board ground at a quiet location on the board,
typically found at an input or output bulk (electrolytic)
capacitor.
Figure 8 shows the circuit traces that require additional
layout consideration. Use single point and ground plane
construction for the circuits shown. Locate the resistor,
RBSOC, close to the BGATE/BSOC pin as the internal BSOC
current source is only 21.5µA.
FN6306.5
April 15, 2010
ISL8105, ISL8105A
+VIN
BOOT
Q1
CBOOT
VOUT
BGATE/BSOC
VBIAS
Q2
LOAD
+VBIAS
RBSOC
LO
LX
ISL8105
GND
C2
CO
COMP
R2
-
R1
FB
CVBIAS
E/A
+
GND
VREF
FIGURE 8. PRINTED CIRCUIT BOARD SMALL SIGNAL
LAYOUT GUIDELINES
Minimize the loop from any pulldown transistor connected to
COMP/EN pin to reduce antenna effect. Provide local
decoupling between VBIAS and GND pins as described
earlier. Locate the capacitor, CBOOT, as close as practical to
the BOOT and LX pins. All components used for feedback
compensation (not shown) should be located as close to the
IC as practical.
VIN
PWM
CIRCUIT
VOSC
TGATE
HALF-BRIDGE
DRIVE
Figure 9 highlights the voltage-mode control loop for a
synchronous-rectified buck converter, applicable to the
ISL8105 circuit. The output voltage (VOUT) is regulated to
the reference voltage, VREF, level. The error amplifier output
(COMP pin voltage) is compared with the oscillator (OSC)
triangle wave to provide a pulse-width modulated wave with
an amplitude of VIN at the LX node. The PWM wave is
smoothed by the output filter (L and C). The output filter
capacitor bank’s equivalent series resistance is represented
by the series resistor ESR.
The modulator transfer function is the small-signal transfer
function of VOUT /VCOMP. This function is dominated by a DC
gain, given by dMAXVIN /VOSC, and shaped by the output filter,
with a double pole break frequency at FLC and a zero at FCE .
For the purpose of this analysis, C and ESR represent the total
output capacitance and its equivalent series resistance.
(EQ. 4)
The compensation network consists of the error amplifier
(internal to the ISL8105) and the external R1 to R3, C1 to C3
components. The goal of the compensation network is to
provide a closed loop transfer function with high 0dB crossing
frequency (F0; typically 0.1 to 0.3 of fSW) and adequate phase
margin (better than +45°).
Phase margin is the difference between the closed loop
phase at F0dB and +180°.
11
L
DCR
LX
BGATE
This section highlights the design considerations for a
voltage-mode controller requiring external compensation. To
address a broad range of applications, a type-3 feedback
network is recommended (see Figure 9).
1
F CE = --------------------------------2π ⋅ C ⋅ ESR
VOUT
OSCILLATOR
Feedback Compensation
1
F LC = --------------------------2π ⋅ L ⋅ C
C3
R3
C1
ISL8105
C
ESR
EXTERNAL CIRCUIT
FIGURE 9. VOLTAGE-MODE BUCK CONVERTER
COMPENSATION DESIGN
Equations 5 through 8 that relate the compensation network’s
poles, zeros and gain to the components (R1 , R2 , R3 , C1 , C2 ,
and C3) in Figure 9. Use the following guidelines for locating
the poles and zeros of the compensation network:
1. Select a value for R1 (1kΩ to 10kΩ, typically). Calculate
value for R2 for desired converter bandwidth (F0). If
setting the output voltage to be equal to the reference set
voltage as shown in Figure 9, the design procedure can
be followed as presented in Equation 5.
V OSC ⋅ R 1 ⋅ F 0
R 2 = --------------------------------------------d MAX ⋅ V IN ⋅ F LC
(EQ. 5)
2. Calculate C1 such that FZ1 is placed at a fraction of the FLC,
at 0.1 to 0.75 of FLC (to adjust, change the 0.5 factor to
desired number). The higher the quality factor of the output
filter and/or the higher the ratio FCE/FLC, the lower the FZ1
frequency (to maximize phase boost at FLC).
1
C 1 = ----------------------------------------------2π ⋅ R 2 ⋅ 0.5 ⋅ F LC
(EQ. 6)
3. Calculate C2 such that FP1 is placed at FCE.
C1
C 2 = -------------------------------------------------------2π ⋅ R 2 ⋅ C 1 ⋅ F CE – 1
(EQ. 7)
4. Calculate R3 such that FZ2 is placed at FLC. Calculate C3
such that FP2 is placed below fSW (typically, 0.5 to 1.0
times fSW). fSW represents the regulator’s switching
frequency. Change the numerical factor to reflect desired
placement of this pole. Placement of FP2 lower in
FN6306.5
April 15, 2010
ISL8105, ISL8105A
FZ1 FZ2
MODULATOR GAIN
COMPENSATION GAIN
CLOSED LOOP GAIN
OPEN LOOP E/A GAIN
FP1
GAIN
frequency helps reduce the gain of the compensation
network at high frequency, in turn reducing the HF ripple
component at the COMP pin and minimizing resultant
duty cycle jitter.
FP2
R1
R 3 = -------------------f SW
----------–1
F LC
(EQ. 8)
R2
20 log ⎛ --------⎞
⎝ R1⎠
1
C 3 = ----------------------------------------------2π ⋅ R 3 ⋅ 0.7 ⋅ f SW
d MAX ⋅ V
IN
20 log --------------------------------V
OSC
0
d MAX ⋅ V IN
1 + s ( f ) ⋅ ESR ⋅ C
G MOD ( f ) = ------------------------------ ⋅ ----------------------------------------------------------------------------------------------------------2
V OSC
1 + s ( f ) ⋅ ( ESR + DCR ) ⋅ C + s ( f ) ⋅ L ⋅ C
1 + s ( f ) ⋅ R2 ⋅ C1
G FB ( f ) = ---------------------------------------------------- ⋅
s ( f ) ⋅ R1 ⋅ ( C1 + C2 )
(EQ. 9)
COMPENSATION BREAK FREQUENCY EQUATIONS
1
F P1 = --------------------------------------------C1 ⋅ C2
2π ⋅ R 2 ⋅ --------------------C1 + C2
1
F Z2 = ------------------------------------------------2π ⋅ ( R 1 + R 3 ) ⋅ C 3
1
F P2 = ------------------------------2π ⋅ R 3 ⋅ C 3
(EQ. 10)
Figure 10 shows an asymptotic plot of the DC/DC converter’s
gain vs frequency. The actual modulator gain has a high gain
peak dependent on the quality factor (Q) of the output filter,
which is not shown. Using the above guidelines should yield a
compensation gain similar to the curve plotted. The open loop
error amplifier gain bounds the compensation gain. Check the
compensation gain at FP2 against the capabilities of the error
amplifier. The closed loop gain, GCL, is constructed on the
log-log graph of Figure 10 by adding the modulator gain,
GMOD (in dB), to the feedback compensation gain, GFB (in
dB). This is equivalent to multiplying the modulator transfer
function and the compensation transfer function and then
plotting the resulting gain.
A stable control loop has a gain crossing with close to a
-20dB/decade slope and a phase margin greater than +45°.
Include worst case component variations when determining
phase margin. The mathematical model presented makes a
number of approximations and is generally not accurate at
12
LOG
fLC
fCE
f0
FREQUENCY
FIGURE 10. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN
frequencies approaching or exceeding half the switching
frequency. When designing compensation networks, select
target crossover frequencies in the range of 10% to 30% of
the switching frequency, fSW.
Output Capacitor Selection
where, s ( f ) = 2π ⋅ f ⋅ j
1
F Z1 = ------------------------------2π ⋅ R 2 ⋅ C 1
GCL
GMOD
Component Selection Guidelines
1 + s ( f ) ⋅ ( R1 + R3 ) ⋅ C3
------------------------------------------------------------------------------------------------------------------------⎛ C1 ⋅ C2 ⎞ ⎞
⎛
( 1 + s ( f ) ⋅ R 3 ⋅ C 3 ) ⋅ ⎜ 1 + s ( f ) ⋅ R 2 ⋅ ⎜ ---------------------⎟ ⎟
⎝ C 1 + C 2⎠ ⎠
⎝
G CL ( f ) = G MOD ( f ) ⋅ G FB ( f )
GFB
LOG
It is recommended that a mathematical model is used to plot
the loop response. Check the loop gain against the error
amplifier’s open-loop gain. Verify phase margin results and
adjust as necessary. The equations in Equation 9, describe
the frequency response of the modulator (GMOD), feedback
compensation (GFB) and closed-loop response (GCL):
An output capacitor is required to filter the output and supply
the load transient current. The filtering requirements are a
function of the switching frequency and the ripple current.
The load transient requirements are a function of the slew
rate (di/dt) and the magnitude of the transient load current.
These requirements are generally met with a mix of
capacitors and careful layout.
Modern microprocessors produce transient load rates above
1A/ns. High frequency capacitors initially supply the transient
and slow the current load rate seen by the bulk capacitors.
The bulk filter capacitor values are generally determined by
the ESR (effective series resistance) and voltage rating
requirements rather than actual capacitance requirements.
High frequency decoupling capacitors should be placed as
close to the power pins of the load as physically possible. Be
careful not to add inductance in the circuit board wiring that
could cancel the usefulness of these low inductance
components. Consult with the manufacturer of the load on
specific decoupling requirements. For example, Intel
recommends that the high frequency decoupling for the
Pentium Pro be composed of at least forty (40) 1.0mF
ceramic capacitors in the 1206 surface-mount package.
Follow on specifications have only increased the number
and quality of required ceramic decoupling capacitors.
Use only specialized low-ESR capacitors intended for
switching-regulator applications for the bulk capacitors. The
bulk capacitor’s ESR will determine the output ripple voltage
and the initial voltage drop after a high slew-rate transient. An
aluminum electrolytic capacitor's ESR value is related to the
case size with lower ESR available in larger case sizes.
FN6306.5
April 15, 2010
ISL8105, ISL8105A
The output inductor is selected to meet the output voltage
ripple requirements and minimize the converter’s response
time to the load transient. The inductor value determines the
converter’s ripple current and the ripple voltage is a function
of the ripple current. The ripple voltage and current are
approximated by Equation 11:
ΔVOUT= ΔI x ESR
(EQ. 11)
Increasing the value of inductance reduces the ripple current
and voltage. However, the large inductance values reduce
the converter’s response time to a load transient.
One of the parameters limiting the converter’s response to a
load transient is the time required to change the inductor
current. Given a sufficiently fast control loop design, the
ISL8105 will provide either 0% or 100% duty cycle in response
to a load transient. The response time is the time required to
slew the inductor current from an initial current value to the
transient current level. During this interval the difference
between the inductor current and the transient current level
must be supplied by the output capacitor. Minimizing the
response time can minimize the output capacitance required.
The response time to a transient is different for the
application of load and the removal of load. Equation 12
gives the approximate response time interval for application
and removal of a transient load:
L O × I TRAN
t RISE = -------------------------------V IN – V OUT
L O × I TRAN
t FALL = ------------------------------V OUT
(EQ. 12)
where:
ITRAN is the transient load current step
tRISE is the response time to the application of load
tFALL is the response time to the removal of load
With a lower input source such as 1.8V or 3.3V, the worst
case response time can be either at the application or
removal of load and dependent upon the output voltage
setting. Be sure to check both of these equations at the
minimum and maximum output levels for the worst case
response time.
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use small ceramic
13
0.50
0.5Io
0.40
0.30
0.25Io
0.20
Output Inductor Selection
V IN - V OUT V OUT
ΔI = -------------------------------- • ---------------FS x L
V IN
0.60
KICM
However, the equivalent series inductance (ESL) of these
capacitors increases with case size and can reduce the
usefulness of the capacitor to high slew-rate transient loading.
Unfortunately, ESL is not a specified parameter. Work with
your capacitor supplier and measure the capacitor’s
impedance with frequency to select a suitable component. In
most cases, multiple electrolytic capacitors of small case size
perform better than a single large case capacitor.
Δ I = 0Io
0.10
0.00
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
DUTY CYCLE (D)
FIGURE 11. INPUT-CAPACITOR CURRENT MULTIPLIER FOR
SINGLE-PHASE BUCK CONVERTER
capacitors for high frequency decoupling and bulk capacitors
to supply the current needed each time Q1 turns on. Place the
small ceramic capacitors physically close to the MOSFETs
and between the drain of Q1 and the source of Q2.
The important parameters for the bulk input capacitor are the
voltage rating and the RMS current rating. For reliable
operation, select the bulk capacitor with voltage and current
ratings above the maximum input voltage and largest RMS
current required by the circuit. The capacitor voltage rating
should be at least 1.25x greater than the maximum input
voltage and a voltage rating of 1.5x is a conservative
guideline. The RMS current rating requirement for the input
capacitor of a buck regulator is approximately as shown in
Equation 13..
I IN, RMS =
I2
2 ( D – D2 ) + Δ
-------- D
IO
12
VO
D = ---------VIN
OR
(EQ. 13)
I IN, RMS = K ICM • I O
For a through-hole design, several electrolytic capacitors
(Panasonic HFQ series or Nichicon PL series or Sanyo
MV-GX or equivalent) may be needed. For surface mount
designs, solid tantalum capacitors can be used, but caution
must be exercised with regard to the capacitor surge current
rating. These capacitors must be capable of handling the
surge-current at power-up. The TPS series, available from
AVX, and the 593D, available series from Sprague, are both
surge current tested.
MOSFET Selection/Considerations
The ISL8105 requires 2 N-Channel power MOSFETs. These
should be selected based upon rDS(ON), gate supply
requirements, and thermal management requirements.
In high-current applications, the MOSFET power dissipation,
package selection and heatsink are the dominant design
factors. The power dissipation includes two loss
FN6306.5
April 15, 2010
ISL8105, ISL8105A
components: conduction loss and switching loss. The
conduction losses are the largest component of power
dissipation for both the top and the bottom-side MOSFETs.
These losses are distributed between the two MOSFETs
according to duty factor. The switching losses seen when
sourcing current will be different from the switching losses
seen when sinking current. When sourcing current, the
top-side MOSFET realizes most of the switching losses. The
bottom-side switch realizes most of the switching losses
when the converter is sinking current (see Equation 14).
These equations assume linear voltage current transitions
and do not adequately model power loss due to the reverse
recovery of the upper and lower MOSFET’s body diode. The
gate-charge losses are dissipated by the ISL8105 and do not
heat the MOSFETs. However, large gate charge increases
the switching interval, tSW, which increases the MOSFET
switching losses. Ensure that both MOSFETs are within their
maximum junction temperature at high ambient temperature
by calculating the temperature rise according to package
thermal-resistance specifications. A separate heatsink may
be necessary depending upon MOSFET power, package
type, ambient temperature and air flow.
complementary MOSFET is turned on. Also avoid MOSFETs
with excessive switching times; the circuitry is expecting
transitions to occur in under 50ns or so.
Bootstrap Considerations
Figure 12 shows the top-side gate drive (BOOT pin) supplied
by a bootstrap circuit from VBIAS. The boot capacitor,
CBOOT, develops a floating supply voltage referenced to the
LX pin. The supply is refreshed to a voltage of VBIAS less
the boot diode drop (VD) each time the lower MOSFET, Q2,
turns on. Check that the voltage rating of the capacitor is
above the maximum VBIAS voltage in the system. A 16V
rating should be sufficient for a 12V system. A value of 0.1µF
is typical for many systems driving single MOSFETs.
+VBIAS
+
VD
-
+1V TO +12V
BOOT
CBOOT
ISL8105
Q1
TGATE
VG-S ≈ VBIAS - VD
LX
Losses while Sourcing Current
2
1
P TOP = Io × r DS ( ON ) × D + --- ⋅ Io × V IN × t SW × f S
2
+VBIAS
PBOTTOM = Io2 x rDS(ON) x (1 - D)
BGATE
+
Q2
Losses while Sinking Current
NOTE:
VG-S ≈ VBIAS
PTOP = Io2 x rDS(ON) x D
2
1
P BOTTOM = Io × r DS ( ON ) × ( 1 – D ) + --- ⋅ Io × V IN × t SW × f S
2
GND
FIGURE 12. UPPER GATE DRIVE - BOOTSTRAP OPTION
(EQ. 14)
Where:
D is the duty cycle = VOUT / VIN,
tSW is the combined switch ON and OFF time, and
fS is the switching frequency.
When operating with a 12V power supply for VBIAS (or down
to a minimum supply voltage of 6.5V), a wide variety of
NMOSFETs can be used. Check the absolute maximum
VGS rating for both MOSFETs; it needs to be above the
highest VBIAS voltage allowed in the system; that usually
means a 20V VGS rating (which typically correlates with a
30V VDS maximum rating). Low threshold transistors
(around 1V or below) are not recommended for the reasons
explained in the next paragraph.
For 5V-only operation, given the reduced available gate bias
voltage (5V), logic-level transistors should be used for both
N-MOSFETs. Look for rDS(ON) ratings at 4.5V. Caution
should be exercised with devices exhibiting very low
VGS(ON) characteristics. The shoot-through protection
present aboard the ISL8105 may be circumvented by these
MOSFETs if they have large parasitic impedances and/or
capacitances that would inhibit the gate of the MOSFET from
being discharged below its threshold level before the
14
If VBIAS is 12V, but VIN is lower (such as 5V), then another
option is to connect the BOOT pin to 12V and remove the
BOOT capacitor (although, you may want to add a local
capacitor from BOOT to GND). This will make the TGATE
VGS voltage equal to (12V - 5V = 7V). That should be high
enough to drive most MOSFETs, and low enough to improve
the efficiency slightly. Do NOT leave the BOOT pin open,
and try to get the same effect by driving BOOT through
VBIAS and the internal diode; this path is not designed for
the high current pulses that will result.
For low VBIAS voltage applications where efficiency is very
important, an external BOOT diode (in parallel with the
internal one) may be considered. The external diode drop
has to be lower than the internal one. The resulting higher
VG-S of the top-side FET will lower its rDS(ON). The modest
gain in efficiency should be balanced against the extra cost
and area of the external diode.
For information on the Application circuit, including a
complete Bill-of-Materials and circuit board description, can
be found in Application Note AN1258.
http://www.intersil.com/data/an/AN1258.pdf
FN6306.5
April 15, 2010
ISL8105, ISL8105A
Dual Flat No-Lead Plastic Package (DFN)
L10.3x3C
2X
0.10 C A
A
10 LEAD DUAL FLAT NO-LEAD PLASTIC PACKAGE
D
MILLIMETERS
2X
0.10 C B
E
SYMBOL
MIN
NOMINAL
MAX
NOTES
A
0.85
0.90
0.95
-
A1
-
-
0.05
-
A3
6
INDEX
AREA
b
0.20 REF
0.20
D
TOP VIEW
B
D2
//
A
C
SEATING
PLANE
D2
6
INDEX
AREA
0.08 C
7
8
D2/2
1
2.33
2.38
2.43
7, 8
1.69
7, 8
3.00 BSC
1.59
e
1.64
-
0.50 BSC
-
k
0.20
-
-
-
L
0.35
0.40
0.45
8
N
10
2
Nd
5
3
NOTES:
1. Dimensioning and tolerancing conform to ASME Y14.5-1994.
NX k
2. N is the number of terminals.
3. Nd refers to the number of terminals on D.
E2
E2/2
4. All dimensions are in millimeters. Angles are in degrees.
5. Dimension b applies to the metallized terminal and is measured
between 0.15mm and 0.30mm from the terminal tip.
NX L
N
N-1
6. The configuration of the pin #1 identifier is optional, but must be
located within the zone indicated. The pin #1 identifier may be
either a mold or mark feature.
NX b
e
(Nd-1)Xe
REF.
BOTTOM VIEW
5
0.10 M C A B
(A1)
9 L
5
7. Dimensions D2 and E2 are for the exposed pads which provide
improved electrical and thermal performance.
8. Nominal dimensions are provided to assist with PCB Land
Pattern Design efforts, see Intersil Technical Brief TB389.
CL
NX (b)
5, 8
Rev. 1 4/06
2
(DATUM A)
8
0.30
3.00 BSC
E
E2
A3
SIDE VIEW
(DATUM B)
0.10 C
0.25
-
9. COMPLIANT TO JEDEC MO-229-WEED-3 except for
dimensions E2 & D2.
e
SECTION "C-C"
C C
TERMINAL TIP
FOR ODD TERMINAL/SIDE
15
FN6306.5
April 15, 2010
ISL8105, ISL8105A
Small Outline Plastic Packages (SOIC)
M8.15 (JEDEC MS-012-AA ISSUE C)
N
8 LEAD NARROW BODY SMALL OUTLINE PLASTIC PACKAGE
INDEX
AREA
H
0.25(0.010) M
B M
INCHES
E
SYMBOL
-B-
1
2
3
L
SEATING PLANE
-A-
A
D
h x 45°
-C-
e
A1
B
0.25(0.010) M
C
0.10(0.004)
C A M
MIN
MAX
MIN
MAX
NOTES
A
0.0532
0.0688
1.35
1.75
-
A1
0.0040
0.0098
0.10
0.25
-
B
0.013
0.020
0.33
0.51
9
C
0.0075
0.0098
0.19
0.25
-
D
0.1890
0.1968
4.80
5.00
3
E
0.1497
0.1574
3.80
4.00
4
e
α
B S
0.050 BSC
1.27 BSC
-
H
0.2284
0.2440
5.80
6.20
-
h
0.0099
0.0196
0.25
0.50
5
L
0.016
0.050
0.40
1.27
6
N
α
NOTES:
MILLIMETERS
8
0°
8
8°
0°
7
8°
1. Symbols are defined in the “MO Series Symbol List” in Section 2.2 of
Publication Number 95.
Rev. 1 6/05
2. Dimensioning and tolerancing per ANSI Y14.5M-1982.
3. Dimension “D” does not include mold flash, protrusions or gate burrs.
Mold flash, protrusion and gate burrs shall not exceed 0.15mm (0.006
inch) per side.
4. Dimension “E” does not include interlead flash or protrusions. Interlead flash and protrusions shall not exceed 0.25mm (0.010 inch) per
side.
5. The chamfer on the body is optional. If it is not present, a visual index
feature must be located within the crosshatched area.
6. “L” is the length of terminal for soldering to a substrate.
7. “N” is the number of terminal positions.
8. Terminal numbers are shown for reference only.
9. The lead width “B”, as measured 0.36mm (0.014 inch) or greater
above the seating plane, shall not exceed a maximum value of
0.61mm (0.024 inch).
10. Controlling dimension: MILLIMETER. Converted inch dimensions
are not necessarily exact.
All Intersil U.S. 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, software 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
16
FN6306.5
April 15, 2010
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