Intersil ISL8845AMUZ High performance industry standard single-ended current mode pwm controller Datasheet

ISL8840A, ISL8841A, ISL8842A,
ISL8843A, ISL8844A, ISL8845A
®
Data Sheet
April 18, 2007
High Performance Industry Standard
Single-Ended Current Mode PWM
Controller
FN6320.3
Features
• 1A MOSFET gate driver
The ISL884xA is a high performance drop-in replacement for
the popular 28C4x and 18C4x PWM controllers suitable for a
wide range of power conversion applications including
boost, flyback, and isolated output configurations. Its fast
signal propagation and output switching characteristics
make this an ideal product for existing and new designs.
Features include 30V operation, low operating current, 90μA
start-up current, adjustable operating frequency to 2MHz,
and high peak current drive capability with 20ns rise and fall
times.
• 90μA start-up current, 125μA maximum
• 35ns propagation delay current sense to output
• Fast transient response with peak current mode control
• 30V operation
• Adjustable switching frequency to 2MHz
• 20ns rise and fall times with 1nF output load
• Trimmed timing capacitor discharge current for accurate
deadtime/maximum duty cycle control
• 1.5MHz bandwidth error amplifier
PART NUMBER
RISING UVLO
MAX. DUTY CYCLE
ISL8840A
7.0
100%
• Tight tolerance voltage reference over line, load and
temperature
ISL8841A
7.0
50%
• ±3% current limit threshold
ISL8842A
14.4V
100%
ISL8843A
8.4V
100%
• Pb-free plus anneal available and ELV, WEEE, RoHS
Compliant
ISL8844A
14.4V
50%
ISL8845A
8.4V
50%
Applications
• Telecom and datacom power
• Wireless base station power
Pinout
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
(8 LD SOIC, MSOP)
TOP VIEW
• File server power
• Industrial power systems
• PC power supplies
8 VREF
• Isolated buck and flyback regulators
FB 2
7 VDD
• Boost regulators
CS 3
6 OUT
RTCT 4
5 GND
COMP 1
1
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, 2006, 2007. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
Ordering Information (Continued)
Ordering Information
PART
NUMBER*
ISL8840AABZ
(See Note)
PART
MARKING
8840 AABZ
TEMP.
PACKAGE PKG.
RANGE (°C) (Pb-free) DWG. #
-40 to +105 8 Ld SOIC
M8.15
PART
NUMBER*
PART
MARKING
TEMP.
PACKAGE PKG.
RANGE (°C) (Pb-free) DWG. #
ISL8843AMBZ 8843 AMBZ
(See Note)
-55 to +125 8 Ld SOIC
M8.15
ISL8840AAUZ 40AAZ
(See Note)
-40 to +105 8 Ld MSOP M8.118
ISL8843AMUZ 43AMZ
(See Note)
-55 to +125 8 Ld MSOP M8.118
ISL8840AMBZ 8840 AMBZ
(See Note)
-55 to +125 8 Ld SOIC
ISL8844AABZ
(See Note)
-40 to +105 8 Ld SOIC
ISL8840AMUZ 40AMZ
(See Note)
-55 to +125 8 Ld MSOP M8.118
ISL8844AAUZ 44AAZ
(See Note)
-40 to +105 8 Ld MSOP M8.118
ISL8841AABZ
(See Note)
-40 to +105 8 Ld SOIC
ISL8844AMBZ 8844 AMBZ
(See Note)
-55 to +125 8 Ld SOIC
8841 AABZ
M8.15
M8.15
8844 AABZ
M8.15
M8.15
ISL8841AAUZ 41AAZ
(See Note)
-40 to +105 8 Ld MSOP M8.118
ISL8844AMUZ 44AMZ
(See Note)
-55 to +125 8 Ld MSOP M8.118
ISL8841AMBZ 8841 AMBZ
(See Note)
-55 to +125 8 Ld SOIC
ISL8845AABZ
(See Note)
-40 to +105 8 Ld SOIC
ISL8841AMUZ 41AMZ
(See Note)
-55 to +125 8 Ld MSOP M8.118
ISL8845AAUZ 45AAZ
(See Note)
-40 to +105 8 Ld MSOP M8.118
ISL8842AABZ
(See Note)
-40 to +105 8 Ld SOIC
ISL8845AMBZ 8845 AMBZ
(See Note)
-55 to +125 8 Ld SOIC
-55 to +125 8 Ld MSOP M8.118
8842 AABZ
M8.15
M8.15
8845 AABZ
M8.15
M8.15
ISL8842AAUZ 42AAZ
(See Note)
-40 to +105 8 Ld MSOP M8.118
ISL8845AMUZ 45AMZ
(See Note)
ISL8842AMBZ 8842 AMBZ
(See Note)
-55 to +125 8 Ld SOIC
ISL8842AMUZ 42AMZ
(See Note)
-55 to +125 8 Ld MSOP M8.118
ISL8843AABZ
(See Note)
-40 to +105 8 Ld SOIC
*Add “-T” suffix for tape and reel.
NOTE: Intersil Pb-free plus anneal products employ special Pb-free
material sets; molding compounds/die attach materials and 100%
matte tin plate termination finish, which are 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.
8843 AABZ
ISL8843AAUZ 43AAZ
(See Note)
M8.15
M8.15
-40 to +105 8 Ld MSOP M8.118
2
FN6320.3
April 18, 2007
Functional Block Diagram
VDD
ENABLE
VDD OK
VREF FAULT
+
VREF
UV COMPARATOR
GND
2.5V
A
4.65V
4.80V
+-
3
+-
A = 0.5
PWM
COMPARATOR
+-
CS
100mV
2R
1.1V
CLAMP
+
-
FB
VF TOTAL = 1.15V
ERROR
AMPLIFIER
+
-
ONLY
ISL8841A/
ISL8844A/
ISL8845A
R
Q
T
COMP
Q
OUT
S Q
36k
R Q
RESET
DOMINANT
VREF
100k
2.9V
1.0V
ON
150k
OSCILLATOR
COMPARATOR
<10ns
+
RTCT
8.4mA
ON
CLOCK
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
+
-
VREF
VREF
5.00V
START/STOP
UV COMPARATOR
FN6320.3
April 18, 2007
Typical Application - 48V Input Dual Output Flyback
CR5
+3.3V
C21
T1
+ C16
R21
VIN+
+1.8V
C4
CR4
4
C2
C17
CR2
C5
+
C22
+
C20
C19
RETURN
CR6
R1
36V TO 75V
R16
R17
C6
C1
C3
R18
R19
U2
Q1
C14
R4
R22
C13
R15
U3
VIN-
R27
R20
U4
R26
COMP
VREF
CS
V DD
FB
OUT
RTCT
GND
ISL884xA
R6
R10
CR1
Q3
C12
VR1
C8
R13
C11
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
R3
+ C15
FN6320.3
April 18, 2007
Typical Application - Boost Converter
R8
C10
CR1
L1
VIN+
+VOUT
C2
C3
5
RETURN
R4
Q1
R5
C9
C1
R1
R2
U1
ISL884xA
FB
CS
C4
R7
VREF
COMP
RTCT
VIN+
VDD
OUT
GND
R3
C5
C7
VIN-
C6
C8
R6
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
+
FN6320.3
April 18, 2007
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
Absolute Maximum Ratings
Thermal Information
Supply Voltage, VDD . . . . . . . . . . . . . . . . . . . . . GND -0.3V to +30V
OUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GND -0.3V to VDD + 0.3V
Signal Pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GND -0.3V to 6.0V
Peak GATE Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1A
ESD Classification
Human Body Model (Per JESD22-A114C.01) . . . . . . . . . . .2000V
Machine Model (Per EIA/JESD22-A115-A) . . . . . . . . . . . . . .200V
Charged Device Model (Per JESD22-C191-A) . . . . . . . . . .1000V
Thermal Resistance (Typical, Note 1)
θJA (°C/W)
SOIC Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
100
MSOP Package . . . . . . . . . . . . . . . . . . . . . . . . . . . .
130
Maximum Junction Temperature . . . . . . . . . . . . . . .-55°C to +150°C
Maximum Storage Temperature Range . . . . . . . . . .-65°C to +150°C
Maximum Lead Temperature (Soldering 10s) . . . . . . . . . . . . +300°C
(SOIC, MSOP - Lead Tips Only)
Operating Conditions
Temperature Range
ISL884xAAxZ . . . . . . . . . . . . . . . . . . . . . . . . . . . .-40°C to +105°C
ISL884xAMxZ. . . . . . . . . . . . . . . . . . . . . . . . . . . .-55°C to +125°C
Supply Voltage Range (Typical, Note 2)
ISL884xA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9V to 30V
CAUTION: Stresses above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the
device at these or any other conditions above those indicated in the operational sections of this specification is not implied.
+150°C max junction temperature is intended for short periods of time to prevent shortening the lifetime. Constantly operated at +150°C may shorten the life of the part.
NOTES:
1. θJA is measured with the component mounted on a high effective thermal conductivity test board in free air. See Technical Brief TB379 for details.
2. All voltages are with respect to GND.
Electrical Specifications
ISL884xAA - Recommended operating conditions unless otherwise noted. Refer to Block Diagram and
Typical Application schematic onpage 3 and page 4. VDD = 15V, RT = 10kΩ, CT = 3.3nF, TA = -40 to +105°C
(Note 3). Typical values are at TA = +25°C
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
START Threshold (ISL8840A, ISL8841A)
6.5
7.0
7.5
V
START Threshold (ISL8843A, ISL8845A)
8.0
8.4
9.0
V
13.3
14.3
15.3
V
STOP Threshold (ISL8840A, ISL8841A)
6.1
6.6
6.9
V
STOP Threshold (ISL8843A, ISL8845A)
7.3
7.6
8.0
V
STOP Threshold (ISL8842A, ISL8844A)
8.0
8.8
9.6
V
Hysteresis (ISL8840A, ISL8841A)
-
0.4
-
V
Hysteresis (ISL8843A, ISL8845A)
-
0.8
-
V
Hysteresis (ISL8842A, ISL8844A)
-
5.4
-
V
-
90
125
μA
UNDERVOLTAGE LOCKOUT
START Threshold (ISL8842A, ISL8844A)
(Note 6)
Startup Current, IDD
VDD < START Threshold
Operating Current, IDD
(Note 4)
-
2.9
4.0
mA
Operating Supply Current, ID
Includes 1nF GATE loading
-
4.75
5.5
mA
REFERENCE VOLTAGE
Overall Accuracy
Over line (VDD = 12V to 30V), load,
temperature
4.925
5.000
5.050
V
Long Term Stability
TA = +125°C, 1000 hours (Note 5)
-
5
-
mV
-20
-
-
mA
5
-
-
mA
Current Limit, Sourcing
Current Limit, Sinking
CURRENT SENSE
Input Bias Current
VCS = 1V
-1.0
-
1.0
μA
CS Offset Voltage
VCS = 0V (Note 5)
95
100
105
mV
COMP to PWM Comparator Offset Voltage
VCS = 0V (Note 5)
0.80
1.15
1.30
V
6
FN6320.3
April 18, 2007
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
Electrical Specifications
ISL884xAA - Recommended operating conditions unless otherwise noted. Refer to Block Diagram and
Typical Application schematic onpage 3 and page 4. VDD = 15V, RT = 10kΩ, CT = 3.3nF, TA = -40 to +105°C
(Note 3). Typical values are at TA = +25°C (Continued)
PARAMETER
TEST CONDITIONS
Input Signal, Maximum
Gain, ACS = ΔVCOMP/ΔVCS
0 < VCS < 910mV, VFB = 0V
CS to OUT Delay
MIN
TYP
MAX
UNITS
0.97
1.00
1.03
V
2.5
3.0
3.5
V/V
-
35
55
ns
ERROR AMPLIFIER
Open Loop Voltage Gain
(Note 5)
60
90
-
dB
Unity Gain Bandwidth
(Note 5)
1.0
1.5
-
MHz
Reference Voltage
VFB = VCOMP
2.475
2.500
2.530
V
FB Input Bias Current
VFB = 0V
-1.0
-0.2
1.0
μA
COMP Sink Current
VCOMP = 1.5V, VFB = 2.7V
1.0
-
-
mA
COMP Source Current
VCOMP = 1.5V, VFB = 2.3V
-0.4
-
-
mA
COMP VOH
VFB = 2.3V
4.80
-
VREF
V
COMP VOL
VFB = 2.7V
0.4
-
1.0
V
PSRR
Frequency = 120Hz, VDD = 12V to
30V (Note 5)
60
80
-
dB
OSCILLATOR
Frequency Accuracy
Initial, TA = +25°C
48
51
53
kHz
Frequency Variation with VDD
TA= +25°C, (f30V - f10V)/f30V
-
0.2
1.0
%
Temperature Stability
(Note 5)
-
-
5
%
Amplitude, Peak to Peak
Static Test
-
1.75
-
V
RTCT Discharge Voltage (Valley Voltage)
Static Test
-
1.0
-
V
Discharge Current
RTCT = 2.0V
6.5
7.8
8.5
mA
OUTPUT
Gate VOH
VDD to OUT, IOUT = -200mA
-
1.0
2.0
V
Gate VOL
OUT to GND, IOUT = 200mA
-
1.0
2.0
V
Peak Output Current
COUT = 1nF (Note 5)
-
1.0
-
A
Rise Time
COUT = 1nF (Note 5)
-
20
40
ns
Fall Time
COUT = 1nF (Note 5)
-
20
40
ns
GATE VOL UVLO Clamp Voltage
VDD = 5V, ILOAD = 1mA
-
-
1.2
V
PWM
Maximum Duty Cycle
(ISL8840A, ISL8842A, ISL8843A)
COMP = VREF
94.0
96.0
-
%
Maximum Duty Cycle
(ISL8841A, ISL8844A, ISL8845A)
COMP = VREF
47.0
48.0
-
%
Minimum Duty Cycle
COMP = GND
-
-
0
%
NOTES:
3. Specifications at -40°C and +105°C are guaranteed by +25°C test with margin limits.
4. This is the VDD current consumed when the device is active but not switching. Does not include gate drive current.
5. These parameters, although guaranteed, are not 100% tested in production.
6. Adjust VDD above the start threshold and then lower to 15V.
7
FN6320.3
April 18, 2007
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
Electrical Specifications
ISL884xAM - Recommended operating conditions unless otherwise noted. Refer to Block Diagram and
Typical Application schematic. VDD = 15V, RT = 10kΩ, CT = 3.3nF, TA = -55 to +125°C (Note 7), Typical values
are at TA = +25°C
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
START Threshold (ISL8840A, ISL8841A)
6.5
7.0
7.5
V
START Threshold (ISL8843A, ISL8845A)
8.0
8.4
9.0
V
13.3
14.3
15.3
V
STOP Threshold (ISL8840A, ISL8841A)
6.1
6.6
6.9
V
STOP Threshold (ISL8843A, ISL8845A)
7.3
7.6
8.0
V
STOP Threshold (ISL8842A, ISL8844A)
8.0
8.8
9.6
V
Hysteresis (ISL8840A, ISL8841A)
-
0.4
-
V
Hysteresis (ISL8843A, ISL8845A)
-
0.8
-
V
UNDERVOLTAGE LOCKOUT
START Threshold (ISL8842A, ISL8844A)
(Note 10)
Hysteresis (ISL8842A, ISL8844A)
-
5.4
-
V
Startup Current, IDD
VDD < START Threshold
-
90
125
μA
Operating Current, IDD
(Note 8)
-
2.9
4.0
mA
Operating Supply Current, ID
Includes 1nF GATE loading
-
4.75
5.5
mA
REFERENCE VOLTAGE
Overall Accuracy
Over line (VDD = 12V to 30V), load,
temperature
4.900
5.000
5.050
V
Long Term Stability
TA = +125°C, 1000 hours (Note 9)
-
5
-
mV
-20
-
-
mA
5
-
-
mA
-1.0
-
1.0
μA
Current Limit, Sourcing
Current Limit, Sinking
CURRENT SENSE
Input Bias Current
VCS = 1V
CS Offset Voltage
VCS = 0V (Note 9)
95
100
105
mV
COMP to PWM Comparator Offset Voltage
VCS = 0V (Note 9)
0.80
1.15
1.30
V
0.97
1.00
1.03
V
2.5
3.0
3.5
V/V
-
35
60
ns
Input Signal, Maximum
Gain, ACS = ΔVCOMP/ΔVCS
0 < VCS < 910mV, VFB = 0V
CS to OUT Delay
ERROR AMPLIFIER
Open Loop Voltage Gain
(Note 9)
60
90
-
dB
Unity Gain Bandwidth
(Note 9)
1.0
1.5
-
MHz
Reference Voltage
VFB = VCOMP
2.460
2.500
2.535
V
FB Input Bias Current
VFB = 0V
-1.0
-0.2
1.0
μA
COMP Sink Current
VCOMP = 1.5V, VFB = 2.7V
1.0
-
-
mA
COMP Source Current
VCOMP = 1.5V, VFB = 2.3V
-0.4
-
-
mA
COMP VOH
VFB = 2.3V
4.80
-
VREF
V
COMP VOL
VFB = 2.7V
0.4
-
1.0
V
PSRR
Frequency = 120Hz, VDD = 12V to
30V (Note 9)
60
80
-
dB
Frequency Accuracy
Initial, TA = +25°C
48
51
53
kHz
Frequency Variation with VDD
TA = +25°C, (f30V - f10V)/f30V
-
0.2
1.0
%
Temperature Stability
(Note 9)
-
-
5
%
OSCILLATOR
8
FN6320.3
April 18, 2007
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
Electrical Specifications
ISL884xAM - Recommended operating conditions unless otherwise noted. Refer to Block Diagram and
Typical Application schematic. VDD = 15V, RT = 10kΩ, CT = 3.3nF, TA = -55 to +125°C (Note 7), Typical values
are at TA = +25°C (Continued)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Amplitude, Peak to Peak
Static Test
-
1.75
-
V
RTCT Discharge Voltage (Valley Voltage)
Static Test
-
1.0
-
V
Discharge Current
RTCT = 2.0V
6.2
8.0
8.5
mA
OUTPUT
Gate VOH
VDD - OUT, IOUT = -200mA
-
1.0
2.0
V
Gate VOL
OUT - GND, IOUT = 200mA
-
1.0
2.0
V
Peak Output Current
COUT = 1nF (Note 9)
-
1.0
-
A
Rise Time
COUT = 1nF (Note 9)
-
20
40
ns
Fall Time
COUT = 1nF (Note 9)
-
20
40
ns
GATE VOL UVLO Clamp Voltage
VDD = 5V, ILOAD = 1mA
-
-
1.2
V
PWM
Maximum Duty Cycle
(ISL8840A, ISL8842A, ISL8843A)
COMP = VREF
94.0
96.0
-
%
Maximum Duty Cycle
(ISL8841A, ISL8844A, ISL8845A)
COMP = VREF
47.0
48.0
-
%
Minimum Duty Cycle
COMP = GND
-
-
0
%
NOTES:
7. Specifications at -55°C and +125°C are guaranteed by +25°C test with margin limits.
8. This is the VDD current consumed when the device is active but not switching. Does not include gate drive current.
9. These parameters, although guaranteed, are not 100% tested in production.
10. Adjust VDD above the start threshold and then lower to 15V.
Typical Performance Curves
1.001
NORMALIZED VREF
NORMALIZED FREQUENCY
1.01
1.00
0.99
1.000
0.999
0.998
0.997
0.996
0.98
-60 -40 -20
0
20
40
60
80
100 120 140
TEMPERATURE (°C)
FIGURE 1. FREQUENCY vs TEMPERATURE
9
0.995
-60 -40 -20
0
20
40
60
80 100 120 140
TEMPERATURE (°C)
FIGURE 2. REFERENCE VOLTAGE vs TEMPERATURE
FN6320.3
April 18, 2007
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
103
1.001
FREQUENCY (Hz)
NORMALIZED EA REFERENCE
Typical Performance Curves (Continued)
1.000
0.998
0.997
0.996
-60 -40 -20
100pF
100
220pF
330pF
470pF
1.0nF
10
2.2nF
3.3nF
4.7nF
6.8nF
0
20
40
60
80 100 120 140
1
1
TEMPERATURE (°C)
FIGURE 3. EA REFERENCE vs TEMPERATURE
10
RT (kΩ)
FIGURE 4. RESISTANCE FOR CT CAPACITOR VALUES GIVEN
Pin Descriptions
RTCT - This is the oscillator timing control pin. The
operational frequency and maximum duty cycle are set by
connecting a resistor, RT, between VREF and this pin and a
timing capacitor, CT, from this pin to GND. The oscillator
produces a sawtooth waveform with a programmable
frequency range up to 2.0MHz. The charge time, tC, the
discharge time, tD, the switching frequency, f, and the
maximum duty cycle, Dmax, can be approximated from the
following equations:
t C ≈ 0.533 ⋅ RT ⋅ CT
(EQ. 1)
0.008 ⋅ RT – 3.83
t D ≈ – RT ⋅ CT ⋅ In ⎛ --------------------------------------------- ⎞
⎝ 0.008 ⋅ RT – 1.71 ⎠
(EQ. 2)
f = 1 ⁄ (tC + tD)
(EQ. 3)
D = tC ⋅ f
(EQ. 4)
OUT - This is the drive output to the power switching device.
It is a high current output capable of driving the gate of a
power MOSFET with peak currents of 1.0A. This GATE
output is actively held low when VDD is below the UVLO
threshold.
VDD - VDD is the power connection for the device. The total
supply current will depend on the load applied to OUT. Total
IDD current is the sum of the operating current and the
average output current. Knowing the operating frequency, f,
and the MOSFET gate charge, Qg, the average output
current can be calculated from:
I OUT = Qg × f
The formulae have increased error at higher frequencies due
to propagation delays. Figure 4 may be used as a guideline
in selecting the capacitor and resistor values required for a
given frequency.
COMP - COMP is the output of the error amplifier and the
input of the PWM comparator. The control loop frequency
compensation network is connected between the COMP and
FB pins.
FB - The output voltage feedback is connected to the
inverting input of the error amplifier through this pin. The
non-inverting input of the error amplifier is internally tied to a
reference voltage.
CS - This is the current sense input to the PWM comparator.
The range of the input signal is nominally 0V to 1.0V and has
an internal offset of 100mV.
GND - GND is the power and small signal reference ground
for all functions.
10
100
(EQ. 5)
To optimize noise immunity, bypass VDD to GND with a
ceramic capacitor as close to the VDD and GND pins as
possible.
VREF - The 5.00V reference voltage output. +1.0/-1.5%
tolerance over line, load and operating temperature. Bypass
to GND with a 0.1μF to 3.3μF capacitor to filter this output as
needed.
Functional Description
Features
The ISL884xA current mode PWM makes an ideal choice for
low-cost flyback and forward topology applications. With its
greatly improved performance over industry standard parts,
it is the obvious choice for new designs or existing designs
which require updating.
Oscillator
The ISL884xA has a sawtooth oscillator with a
programmable frequency range to 2MHz, which can be
programmed with a resistor from VREF and a capacitor to
GND on the RTCT pin. (Please refer to Figure 4 for the
resistor and capacitance required for a given frequency.)
FN6320.3
April 18, 2007
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
Soft-Start Operation
Soft-start must be implemented externally. One method,
illustrated below, clamps the voltage on COMP.
From the small signal current-mode model [1] it can be
shown that the naturally-sampled modulator gain, Fm,
without slope compensation, is in Equation 6.
1
Fm = -------------------SnTsw
VREF
R1
COMP
Q1
ISL884xA
D1
GND
C1
(EQ. 6)
where Sn is the slope of the sawtooth signal and Tsw is the
duration of the half-cycle. When an external ramp is added,
the modulator gain becomes:
1
1
Fm = --------------------------------------- = ---------------------------( Sn + Se )Tsw
m c SnTsw
(EQ. 7)
where Se is slope of the external ramp and
Se
m c = 1 + ------Sn
(EQ. 8)
FIGURE 5. SOFT-START
The COMP pin is clamped to the voltage on capacitor C1
plus a base-emitter junction by transistor Q1. C1 is charged
from VREF through resistor R1 and the base current of Q1.
At power-up C1 is fully discharged, COMP is at ~0.7V, and
the duty cycle is zero. As C1 charges, the voltage on COMP
increases, and the duty cycle increases in proportion to the
voltage on C1. When COMP reaches the steady state
operating point, the control loop takes over and soft start is
complete. C1 continues to charge up to VREF and no longer
affects COMP. During power down, diode D1 quickly
discharges C1 so that the soft start circuit is properly
initialized prior to the next power on sequence.
Gate Drive
The ISL884xA is capable of sourcing and sinking 1A peak
current. To limit the peak current through the IC, an optional
external resistor may be placed between the totem-pole
output of the IC (OUT pin) and the gate of the MOSFET. This
small series resistor also damps any oscillations caused by
the resonant tank of the parasitic inductances in the traces of
the board and the FET’s input capacitance.
Slope Compensation
For applications where the maximum duty cycle is less than
50%, slope compensation may be used to improve noise
immunity, particularly at lighter loads. The amount of slope
compensation required for noise immunity is determined
empirically, but is generally about 10% of the full scale
current feedback signal. For applications where the duty
cycle is greater than 50%, slope compensation is required to
prevent instability.
Slope compensation may be accomplished by summing an
external ramp with the current feedback signal or by
subtracting the external ramp from the voltage feedback
error signal. Adding the external ramp to the current
feedback signal is the more popular method.
11
The criteria for determining the correct amount of external
ramp can be determined by appropriately setting the
damping factor of the double-pole located at the switching
frequency. The double-pole will be critically damped if the
Q-factor is set to 1, over-damped for Q < 1, and
under-damped for Q > 1. An under-damped condition may
result in current loop instability.
1
Q = ------------------------------------------------π ( m c ( 1 – D ) – 0.5 )
(EQ. 9)
where D is the percent of on time during a switching cycle.
Setting Q = 1 and solving for Se yields
1
1
S e = S n ⎛ ⎛ --- + 0.5⎞ ------------- – 1⎞
⎠1 –D
⎝⎝π
⎠
(EQ. 10)
Since Sn and Se are the on time slopes of the current ramp
and the external ramp, respectively, they can be multiplied
by tON to obtain the voltage change that occurs during tON.
1
1
V e = V n ⎛ ⎛ --- + 0.5⎞ ------------- – 1⎞
⎠1 –D
⎝⎝π
⎠
(EQ. 11)
where Vn is the change in the current feedback signal (ΔI)
during the on time and Ve is the voltage that must be added
by the external ramp.
For a flyback converter, Vn can be solved for in terms of
input voltage, current transducer components, and primary
inductance, yielding
D ⋅ T SW ⋅ V IN ⋅ R CS 1
1
V e = ---------------------------------------------------- ⎛ ⎛ --- + 0.5⎞ ------------- – 1⎞
⎠1–D
⎝⎝π
⎠
Lp
V
(EQ. 12)
where RCS is the current sense resistor, fsw is the switching
frequency, Lp is the primary inductance, VIN is the minimum
input voltage, and D is the maximum duty cycle.
FN6320.3
April 18, 2007
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
The current sense signal at the end of the ON time for CCM
operation is:
( 1 – D ) ⋅ VO ⋅ f ⎞
N S ⋅ R CS ⎛
sw
V CS = ------------------------ ⎜ I O + --------------------------------------------⎟
NP
2L s
⎝
⎠
V
(EQ. 13)
Since the peak current limit threshold is 1.00V, the total
current feedback signal plus the external ramp voltage must
sum to this value when the output load is at the current limit
threshold.
V e + V CS = 1
(EQ. 14)
Substituting Equations 12 and 13 into Equation 14 and
solving for RCS yields
1
R CS = ----------------------------------------------------------------------------------------------------------------------------------------------------1
-+
0.5
⎛
⎞
( 1 – D ) ⋅ V O ⋅ f sw⎞
D ⋅ f sw ⋅ V IN π
Ns ⎛
------------------------------⋅ ⎜ ------------------ – 1⎟ + ------- ⋅ ⎜ I O + --------------------------------------------⎟
⎜ 1–D
⎟ N ⎝
Lp
2L s
⎠
p
⎝
⎠
(EQ. 15)
Adding slope compensation is accomplished in the
ISL884xA using an external buffer transistor and the RTCT
signal. A typical application sums the buffered RTCT signal
with the current sense feedback and applies the result to the
CS pin as shown in Figure 6.
R9
CS
R6
ISL8843
where VCS is the voltage across the current sense resistor,
Ls is the secondary winding inductance, and IO is the output
current at current limit. Equation 13 assumes the voltage
drop across the output rectifier is negligible.
VREF
RTCT
C4
FIGURE 6. SLOPE COMPENSATION
Assuming the designer has selected values for the RC filter
(R6 and C4) placed on the CS pin, the value of R9 required
to add the appropriate external ramp can be found by
superposition.
2.05D ⋅ R 6
V e = ---------------------------R6 + R9
(EQ. 16)
V
The factor of 2.05 in Equation 16 arises from the peak
amplitude of the sawtooth waveform on RTCT minus a
base-emitter junction drop. That voltage multiplied by the
maximum duty cycle is the voltage source for the slope
compensation. Rearranging to solve for R9 yields:
( 2.05D – V e ) ⋅ R 6
R 9 = ---------------------------------------------Ve
Ω
(EQ. 17)
The value of RCS determined in Equation 15 must be
rescaled so that the current sense signal presented at the
CS pin is that predicted by Equation 13. The divider created
by R6 and R9 makes this necessary.
R6 + R9
R′ CS = --------------------- ⋅ R CS
R9
12
(EQ. 18)
FN6320.3
April 18, 2007
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
Example:
Fault Conditions
VIN = 12V
VO = 48V
A Fault condition occurs if VREF falls below 4.65V. When a
Fault is detected OUT is disabled. When VREF exceeds
4.80V, the Fault condition clears, and OUT is enabled.
Ls = 800μH
Ground Plane Requirements
Ns/Np = 10
Careful layout is essential for satisfactory operation of the
device. A good ground plane must be employed. A unique
section of the ground plane must be designated for high di/dt
currents associated with the output stage. VDD should be
bypassed directly to GND with good high frequency
capacitors.
Lp = 8.0μH
IO = 200mA
Switching Frequency, fsw = 200kHz
Duty Cycle, D = 28.6%
References
R6 = 499Ω
Solve for the current sense resistor, RCS, using Equation 15.
RCS = 295mΩ
[1] Ridley, R., “A New Continuous-Time Model for Current
Mode Control”, IEEE Transactions on Power
Electronics, Vol. 6, No. 2, April 1991.
Determine the amount of voltage, Ve, that must be added to
the current feedback signal using Equation 12.
Ve = 92.4mV
Using Equation 17, solve for the summing resistor, R9, from
CT to CS.
R9 = 2.67kΩ
Determine the new value of RCS (R’CS) using Equation 18.
R’CS = 350mΩ
Additional slope compensation may be considered for
design margin. The above discussion determines the
minimum external ramp that is required. The buffer transistor
used to create the external ramp from RTCT should have a
sufficiently high gain (>200) so as to minimize the required
base current. Whatever base current is required reduces the
charging current into RTCT and will reduce the oscillator
frequency.
13
FN6320.3
April 18, 2007
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
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
-
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
α
1. Symbols are defined in the “MO Series Symbol List” in Section 2.2 of
Publication Number 95.
1.27 BSC
H
N
NOTES:
MILLIMETERS
8
0°
8
8°
0°
7
8°
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.
14
FN6320.3
April 18, 2007
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
Mini Small Outline Plastic Packages (MSOP)
M8.118 (JEDEC MO-187AA)
N
8 LEAD MINI SMALL OUTLINE PLASTIC PACKAGE
E1
INCHES
E
-B-
INDEX
AREA
0.20 (0.008)
1 2
A B C
TOP VIEW
4X θ
0.25
(0.010)
R1
R
GAUGE
PLANE
SEATING
PLANE -CA
4X θ
A2
A1
b
-H-
0.10 (0.004)
L
SEATING
PLANE
C
MIN
MAX
MIN
MAX
NOTES
A
0.037
0.043
0.94
1.10
-
A1
0.002
0.006
0.05
0.15
-
A2
0.030
0.037
0.75
0.95
-
b
0.010
0.014
0.25
0.36
9
c
0.004
0.008
0.09
0.20
-
D
0.116
0.120
2.95
3.05
3
E1
0.116
0.120
2.95
3.05
4
0.026 BSC
0.20 (0.008)
C
C
a
SIDE VIEW
CL
E1
0.20 (0.008)
C D
-
0.187
0.199
4.75
5.05
-
L
0.016
0.028
0.40
0.70
6
0.037 REF
N
-A-
0.65 BSC
E
L1
e
D
SYMBOL
e
L1
MILLIMETERS
0.95 REF
8
R
0.003
R1
0
α
-
8
-
0.07
0.003
-
5o
15o
0o
6o
7
-
-
0.07
-
-
5o
15o
-
0o
6o
-B-
Rev. 2 01/03
END VIEW
NOTES:
1. These package dimensions are within allowable dimensions of
JEDEC MO-187BA.
2. Dimensioning and tolerancing per ANSI Y14.5M-1994.
3. Dimension “D” does not include mold flash, protrusions or gate
burrs and are measured at Datum Plane. Mold flash, protrusion
and gate burrs shall not exceed 0.15mm (0.006 inch) per side.
4. Dimension “E1” does not include interlead flash or protrusions
and are measured at Datum Plane. - H - Interlead flash and
protrusions shall not exceed 0.15mm (0.006 inch) per side.
5. Formed leads shall be planar with respect to one another within
0.10mm (0.004) at seating Plane.
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. Dimension “b” does not include dambar protrusion. Allowable
dambar protrusion shall be 0.08mm (0.003 inch) total in excess
of “b” dimension at maximum material condition. Minimum space
between protrusion and adjacent lead is 0.07mm (0.0027 inch).
10. Datums -A -H- .
and - B - to be determined at Datum plane
11. Controlling dimension: MILLIMETER. Converted inch dimensions are for reference only.
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems.
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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
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15
FN6320.3
April 18, 2007
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