TL2844 [TI]
CURRENT-MODE PWM CONTROLLERS; 电流模式PWM控制器
| 型号： | TL2844 |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | CURRENT-MODE PWM CONTROLLERS |
| 文件： | 总12页 (文件大小：173K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038E – JANUARY 1989 – REVISED DECEMBER 1999
D PACKAGE
(TOP VIEW)
Optimized for Off-Line and dc-to-dc
Converters
Low Start-Up Current (<1 mA)
COMP
NC
REF
13 NC
1
2
3
4
5
6
7
14
Automatic Feed-Forward Compensation
Pulse-by-Pulse Current Limiting
Enhanced Load-Response Characteristics
Undervoltage Lockout With Hysteresis
Double-Pulse Suppression
12
11
10
9
VFB
V
CC
NC
VC
ISENSE
NC
OUTPUT
GND
8
POWER GROUND
RT/CT
High-Current Totem-Pole Output
Internally Trimmed Bandgap Reference
500-kHz Operation
NC – No internal connection
D-8 OR P PACKAGE
(TOP VIEW)
Error Amplifier With Low Output
Resistance
COMP
VFB
ISENSE
RT/CT
REF
1
2
3
4
8
7
6
5
Designed to Be Interchangeable With
UC2842 and UC3842 Series
V
CC
OUTPUT
GND
description
The TL284x and TL384x series of control
integrated circuits provide the features that are
necessary to implement off-line or dc-to-dc fixed-frequency current-mode control schemes with a minimum
number of external components. Some of the internally implemented circuits are an undervoltage lockout
(UVLO), featuring a start-up current of less than 1 mA, and a precision reference trimmed for accuracy at the
error amplifier input. Other internal circuits include logic to ensure latched operation, a pulse-width modulation
(PWM)comparator(whichalsoprovidescurrent-limitcontrol), andatotem-poleoutputstagedesignedtosource
or sink high-peak current. The output stage, suitable for driving N-channel MOSFETs, is low when it is in the
off state.
Major differences between members of these series are the UVLO thresholds and maximum duty-cycle ranges.
TypicalUVLOthresholdsof16V(on)and10V(off)ontheTLx842andTLx844devicesmakethemideallysuited
to off-line applications. The corresponding typical thresholds for the TLx843 and TLx845 devices are 8.4 V (on)
and 7.6 V (off). The TLx842 and TLx843 devices can operate to duty cycles approaching 100%. A duty-cycle
range of 0 to 50% is obtained by the TLx844 and TLx845 by the addition of an internal toggle flip-flop, which
blanks the output off every other clock cycle.
The TL284x-series devices are characterized for operation from –40°C to 85°C. The TL384x-series devices are
characterized for operation from 0°C to 70°C.
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
Copyright 1999, Texas Instruments Incorporated
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
1
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038E – JANUARY 1989 – REVISED DECEMBER 1999
AVAILABLE OPTIONS
PACKAGED DEVICES
SMALL OUTLINE SMALL OUTLINE PLASTIC DIP
CHIP FORM
(Y)
T
A
(D)
(D-8)
(P)
TL3842D
TL3843D
TL3844D
TL3845D
TL3842D-8
TL3843D-8
TL3844D-8
TL3845D-8
TL3842P
TL3843P
TL3844P
TL3845P
TL3842Y
TL3843Y
TL3844Y
TL3845Y
0°C to 70°C
TL2842D
TL2843D
TL2844D
TL2845D
TL2842D-8
TL2843D-8
TL2844D-8
TL2845D-8
TL2842P
TL2843P
TL2844P
TL2845P
–
–
–
–
–40°C to 85°C
The D and D-8 packages are available taped and reeled. Add the suffix R to the device type (i.e.,
TL3842DR or TL3842DR-8). Chip forms are tested at 25°C.
functional block diagram
12
V
CC
34 V NOM
9
UVLO
5-V REF
EN
14
11
REF
+
–
GND
Internal
Bias
V
Good
Logic
ref
VC
7
10
8
RT/CT
OSC
OUTPUT
†
T
POWER
GROUND
Error
Amplifier
S
R
2R
+
–
3
1
5
PWM
Latch
VFB
R
1 V
Current-
Sense
Comparator
COMP
ISENSE
†
The toggle flip-flop is present only in TL2844, TL2845, TL3844, and TL3845.
Pin numbers shown are for the D Package.
2
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038E – JANUARY 1989 – REVISED DECEMBER 1999
†
absolute maximum ratings over operating free-air temperature range (unless otherwise noted)
Supply voltage (see Note 1) (I
< 30 mA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Self limiting
CC
Analog input voltage range, V (VFB and ISENSE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to 6.3 V
I
Output voltage, V (OUTPUT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 V
O
Input voltage, V , (VC, D package only) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 V
I
Supply current, I
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 mA
CC
Output current, I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±1 A
O
Error amplifier output sink current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 mA
Package thermal impedance, θ (see Notes 2 and 3): D package . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86°C/W
JA
D-8 package . . . . . . . . . . . . . . . . . . . . . . . . . . 97°C/W
P package . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85°C/W
Virtual junction temperature range, T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to 150°C
J
Output energy (capacitive load) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 µJ
Lead temperature, 1,6 mm (1/16 inch) from case for 10 seconds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260°C
Storage temperature range, T
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –65°C to 150°C
stg
†
Stresses beyond those listed under “absolute maximum ratings” may cause permanent damage to the device. These are stress ratings only, and
functional operation of the device at these or any other conditions beyond those indicated under “recommended operating conditions” is not
implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
NOTES: 1. All voltages are with respect to the device GND terminal.
2. Maximum power dissipation is a function of T (max), θ , and T . The maximum allowable power dissipation at any allowable
J
JA
A
ambient temperature is P = (T (max) – T )/θ . Operating at the absolute maximum T of 150°C can impact reliability.
D
J
A
JA
J
3. The package thermal impedance is calculated in accordance with JESD 51.
recommended operating conditions
MIN NOM
MAX
30
UNIT
V
‡
Supply voltage, V
and VC
CC
Input voltage, V , RT/CT
0
0
5.5
5.5
30
V
I
Input voltage, V , VFB and ISENSE
V
I
Output voltage, V , OUTPUT
0
V
O
‡
Output voltage, V , POWER GROUND
–0.1
1
V
O
Supply current, externally limited, I
CC
25
mA
mA
mA
kHz
°C
Average output current, I
200
–20
500
125
85
O
Reference output current, I
O(ref)
Oscillator frequency, f
osc
100
Operating virtual junction temperature, T
0
–40
0
J
TL284x
TL384x
Operating free-air temperature, T
A
°C
70
‡
These recommended voltages for V and POWER GROUND apply only to the D package.
C
3
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038E – JANUARY 1989 – REVISED DECEMBER 1999
electrical characteristics over recommended operating free-air temperature range, V = 15 V (see
CC
Note 4), R = 10 kΩ, C = 3.3 nF (unless otherwise specified)
T
T
reference section
TL284x
TL384x
PARAMETER
TEST CONDITIONS
UNIT
†
†
MIN TYP
MAX
5.05
20
MIN TYP
MAX
5.1
20
Output voltage
Line regulation
Load regulation
I
= 1 mA,
T
A
= 25°C
4.95
5
6
6
4.9
5
6
6
V
O
V
= 12 V to 25 V
CC
= 1 mA to 20 mA
mV
mV
I
O
25
25
Temperature coefficient
of output voltage
0.2
0.4
5.1
0.2
0.4 mV/°C
Output voltage
with worst-case variation
V
= 12 V to 25 V,
I
= 1 mA to 20 mA
4.9
4.82
–30
5.18
V
CC
f = 10 Hz to 10 kHz,
After 1000 h at T = 25°C
O
Output noise voltage
T
= 25°C
50
5
50
5
µV
mV
mA
A
Output-voltage long-term drift
Short-circuit output current
25
25
A
–30
–100
–180
–100
–180
†
All typical values are at T = 25°C.
A
NOTE 4: Adjust V
above the start threshold before setting it to 15 V.
CC
oscillator section
TL284x
TL384x
PARAMETER
TEST CONDITIONS
= 25°C
UNIT
†
†
MIN TYP
MAX
57
MIN TYP
MAX
Oscillator frequency (see Note 5)
Frequency change with supply voltage
Frequency change with temperature
Peak-to-peak amplitude at RT/CT
T
47
52
2
47
52
2
57
kHz
A
V
= 12 V to 25 V
10
10 Hz/kHz
Hz/kHz
V
CC
50
1.7
50
1.7
†
All typical values are at T = 25°C.
A
NOTES: 4. Adjust V
above the start threshold before setting it to 15 V.
CC
5. Output frequency equals oscillator frequency for the TLx842 and TLx843. Output frequency is one-half oscillator frequency for the
TLx844 and TLx845.
error-amplifier section
TL284x
TL384x
PARAMETER
TEST CONDITIONS
COMP at 2.5 V
UNIT
†
†
MIN TYP
MAX
2.55
–1
MIN TYP
MAX
2.58
–2
Feedback input voltage
Input bias current
2.45
2.50
–0.3
90
2.42
2.50
–0.3
90
V
µA
dB
MHz
dB
mA
mA
V
Open-loop voltage amplification
Gain-bandwidth product
Supply-voltage rejection ratio
Output sink current
V
O
= 2 V to 4 V
65
0.7
60
65
0.7
60
1
1
V
CC
= 12 V to 25 V
70
70
VFB at 2.7 V,
VFB at 2.3 V,
VFB at 2.3 V,
VFB at 2.7 V,
COMP at 1.1 V
COMP at 5 V
2
6
2
6
Output source current
High-level output voltage
Low-level output voltage
–0.5
5
–0.8
6
–0.5
5
–0.8
6
R
R
= 15 kΩ to GND
= 15 kΩ to GND
L
L
0.7
1.1
0.7
1.1
V
†
All typical values are at T = 25°C.
A
NOTE 4: Adjust V
above the start threshold before setting it to 15 V.
CC
4
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038E – JANUARY 1989 – REVISED DECEMBER 1999
electrical characteristics over recommended operating free-air temperature range, V = 15 V (see
CC
Note 4), R = 10 kΩ, C = 3.3 nF (unless otherwise specified) (continued)
T
T
current-sense section
TL284x
TL384x
PARAMETER
TEST CONDITIONS
UNIT
†
†
MIN TYP
2.85
MAX
3.13
1.1
MIN TYP
2.85
MAX
3.15
1.1
Voltage amplification
See Notes 6 and 7
COMP at 5 V,
= 12 V to 25 V, See Note 6
3
1
3
1
V/V
V
Current-sense comparator threshold
Supply-voltage rejection ratio
Input bias current
See Note 6
0.9
0.9
V
CC
70
–2
70
–2
dB
µA
ns
–10
300
–10
300
Delay time to output
150
150
†
All typical values are at T = 25°C.
A
NOTES: 4. Adjust V
above the start threshold before setting it to 15 V.
CC
6. These parameters are measured at the trip point of the latch, with VFB at 0 V.
7. Voltage amplification is measured between ISENSE and COMP, with the input changing from 0 V to 0.8 V.
output section
TL284x
TL384x
PARAMETER
High-level output voltage
Low-level output voltage
TEST CONDITIONS
= –20 mA
UNIT
V
†
†
MIN TYP
MAX
MIN TYP
MAX
I
I
I
I
13
12
13.5
13.5
0.1
1.5
50
13
12
13.5
13.5
0.1
1.5
50
OH
OH
OL
OL
= –200 mA
= 20 mA
= 200 mA
= 1 nF,
0.4
2.2
0.4
2.2
V
Rise time
Fall time
C
C
T
A
= 25°C
= 25°C
150
150
150
150
ns
ns
L
L
= 1 nF,
T
A
50
50
†
All typical values are at T = 25°C.
A
NOTE 4: Adjust V
above the start threshold before setting it to 15 V.
CC
undervoltage-lockout section
TL284x
TL384x
PARAMETER
UNIT
V
†
†
MIN TYP
MAX
17
MIN TYP
MAX
17.5
9
TLx842, TLx844
TLx843, TLx845
TLx842, TLx844
TLx843, TLx845
15
7.8
9
16
8.4
10
14.5
7.8
8.5
7
16
8.4
10
Start threshold voltage
9
11
11.5
8.2
Minimum operating voltage after startup
V
7
7.6
8.2
7.6
†
All typical values are at T = 25°C.
A
NOTE 4: Adjust V
above the start threshold before setting it to 15 V.
CC
pulse-width-modulator section
TL284x
TL384x
PARAMETER
UNIT
†
†
MIN TYP
MAX
MIN TYP
MAX
TLx842, TLx843
TLx844, TLx845
95%
46%
97% 100%
95%
46%
97% 100%
Maximum duty cycle
Minimum duty cycle
48%
50%
0
48%
50%
0
†
All typical values are at T = 25°C.
A
NOTE 4: Adjust V
above the start threshold before setting it to 15 V.
CC
5
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038E – JANUARY 1989 – REVISED DECEMBER 1999
electrical characteristics over recommended operating free-air temperature range, V = 15 V (see
CC
Note 4), R = 10 kΩ, C = 3.3 nF (unless otherwise specified) (continued)
T
T
supply voltage
TL284x
TL384x
PARAMETER
TEST CONDITIONS
UNIT
†
†
MIN TYP
MAX
1
MIN TYP
MAX
1
Start-up current
0.5
11
0.5
11
mA
mA
V
Operating supply current
Limiting voltage
VFB and ISENSE at 0 V
17
17
I
= 25 mA
34
34
CC
†
All typical values are at T = 25°C.
A
NOTE 4: Adjust V
above the start threshold before setting it to 15 V.
CC
electricalcharacteristics,V =15V(seeNote4),R =10kΩ,C =3.3nF,T =25°C(unlessotherwise
CC
T
T
A
specified)
reference section
TL384xY
PARAMETER
TEST CONDITIONS
UNIT
MIN
TYP
5
MAX
Output voltage
Line regulation
Load regulation
I
= 1 mA
V
mV
O
V
= 12 V to 25 V
CC
= 1 mA to 20 mA
6
I
O
6
mV
Temperature coefficient of output voltage
Output noise voltage
0.2
50
5
mV/°C
µV
f = 10 Hz to 10 kHz
After 1000 h at T = 25°C
Output-voltage long-term drift
Short-circuit output current
mV
A
–100
mA
NOTE 4: Adjust V
above the start threshold before setting it to 15 V.
CC
oscillator section
TL384xY
PARAMETER
TEST CONDITIONS
UNIT
MIN
TYP
52
2
MAX
Oscillator frequency (see Note 5)
Frequency change with supply voltage
Frequency change with temperature
Peak-to-peak amplitude at RT/CT
kHz
Hz/kHz
Hz/kHz
V
V
CC
= 12 V to 25 V
5
1.7
NOTES: 4. Adjust V
above the start threshold before setting it to 15 V.
CC
5. Output frequency equals oscillator frequency for the TLx842 and TLx843. Output frequency is one-half oscillator frequency for the
TLx844 and TLx845.
6
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038E – JANUARY 1989 – REVISED DECEMBER 1999
electricalcharacteristics,V =15V(seeNote4),R =10kΩ,C =3.3nF,T =25°C(unlessotherwise
CC
T
T
A
specified) (continued)
error-amplifier section
PARAMETER
TL384xY
TYP
2.50
–0.3
90
TEST CONDITIONS
COMP at 2.5 V
UNIT
MIN
MAX
Feedback input voltage
Input bias current
V
µA
dB
MHz
dB
mA
mA
V
Open-loop voltage amplification
Gain-bandwidth product
Supply-voltage rejection ratio
Output sink current
V
V
= 2 V to 4 V
O
1
= 12 V to 25 V
70
CC
VFB at 2.7 V,
VFB at 2.3 V,
VFB at 2.3 V,
VFB at 2.7 V,
COMP at 1.1 V
COMP at 5 V
6
Output source current
High-level output voltage
Low-level output voltage
–0.8
6
R
R
= 15 kΩ to GND
= 15 kΩ to GND
L
L
0.7
V
NOTE 4: Adjust V
above the start threshold before setting it to 15 V.
CC
current-sense section
TL384xY
PARAMETER
TEST CONDITIONS
UNIT
MIN
TYP
3
MAX
Voltage amplification
See Notes 6 and 7
COMP at 5 V,
= 12 V to 25 V,
V/V
V
Current-sense comparator threshold
Supply-voltage rejection ratio
Input bias current
See Note 6
See Note 6
1
V
CC
70
–2
150
dB
µA
ns
Delay time to output
NOTES: 4. Adjust V
above the start threshold before setting it to 15 V.
CC
6. These parameters are measured at the trip point of the latch, with VFB at 0 V.
7. Voltage amplification is measured between ISENSE and COMP, with the input changing from 0 V to 0.8 V.
output section
TL384xY
TYP
13.5
13.5
0.1
PARAMETER
TEST CONDITIONS
= –20 mA
UNIT
V
MIN
MAX
I
I
I
I
OH
OH
OL
OL
High-level output voltage
Low-level output voltage
= –200 mA
= 20 mA
= 200 mA
= 1 nF
V
1.5
Rise time
Fall time
C
C
50
ns
ns
L
L
= 1 nF
50
NOTE 4: Adjust V
above the start threshold before setting it to 15 V.
CC
undervoltage-lockout section
TL384xY
TYP
16
PARAMETER
UNIT
V
MIN
MAX
TL3842Y, TL3844Y
TL3843Y, TL3845Y
TL3842Y, TL3844Y
TL3843Y, TL3845Y
Start threshold voltage
8.4
10
Minimum operating voltage after startup
V
7.6
NOTE 4: Adjust V
above the start threshold before setting it to 15 V.
CC
7
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038E – JANUARY 1989 – REVISED DECEMBER 1999
electricalcharacteristics,V =15V(seeNote4),R =10kΩ,C =3.3nF,T =25°C(unlessotherwise
CC
T
T
A
specified) (continued)
pulse-width-modulator section
TL384xY
TYP
PARAMETER
UNIT
MIN
MAX
TL3842Y, TL3843Y
TL3844Y, TL3845Y
97%
Maximum duty cycle
48%
NOTE 4: Adjust V
above the start threshold before setting it to 15 V.
CC
supply voltage
TL384xY
TYP
0.5
PARAMETER
TEST CONDITIONS
UNIT
MIN
MAX
1
Start-up current
mA
mA
V
Operating supply current
Limiting voltage
VFB and ISENSE at 0 V
= 25 mA
11
17
I
34
CC
above the start threshold before setting it to 15 V.
NOTE 4: Adjust V
CC
8
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038E – JANUARY 1989 – REVISED DECEMBER 1999
APPLICATION INFORMATION
2.5 V
0.5 mA
Error
Amplifier
+
–
VFB
Z
i
COMP
Z
f
NOTE A: Error amplifier can source or sink up to 0.5 mA.
Figure 1. Error-Amplifier Configuration
Error
I
Amplifier
S
(see Note A)
2R
+
–
R
1 V
Current-Sense
Comparator
COMP
R
f
ISENSE
R
C
f
S
GND
NOTE A: Peak current (I ) is determined by the formula:
S
1 V
RS
IS max
(
)
A small RC filter formed by resistor R and capacitor C may be required to suppress switch transients.
f
f
Figure 2. Current-Sense Circuit
REF
R
T
(see Note A)
RT/CT
GND
C
T
1.72
RTCT
NOTE A: For R > 5 kΩ:
f
T
Figure 3. Oscillator Section
9
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038E – JANUARY 1989 – REVISED DECEMBER 1999
APPLICATION INFORMATION
TIMING RESISTANCE
vs
DEAD TIME
vs
FREQUENCY
TIMING CAPACITANCE
100
40
100
40
C
= 10 nF
V
R
T
A
= 15 V
≥ 5 kΩ
= 25°C
T
CC
T
C = 4.7 nF
T
C
= 22 nF
T
C
= 1 nF
T
10
4
C
= 22 nF
T
10
4
C
= 47 nF
T
1
C
= 100 nF
T
0.4
V
T
A
= 15 V
CC
= 25°C
1
100
0.1
1 k
10 k
100 k
1 M
0
4
10
40
100
f - Frequency - Hz
C
– Timing Capacitance – nF
T
Figure 4
Figure 5
open-loop laboratory test fixture
In the open-loop laboratory test fixture shown in Figure 6, high peak currents associated with loads necessitate
careful grounding techniques. Timing and bypass capacitors should be connected close to the GND terminal
in a single-point ground. The transistor and 5-kΩ potentiometer sample the oscillator waveform and apply an
adjustable ramp to the ISENSE terminal.
REF
R
T
A
V
CC
4.7 kΩ
2N2222
DUT
REF
100 kΩ
COMP
VFB
0.1 µF
1 kΩ
Error Amplifier
Adjust
V
CC
TL284x
TL384x
0.1 µF
1 kΩ, 1 W
ISENSE
RT/CT
OUTPUT
5 kΩ
OUTPUT
GND
4.7 kΩ
ISENSE
Adjust
GND
C
T
Figure 6. Open-Loop Laboratory Test Fixture
10
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038E – JANUARY 1989 – REVISED DECEMBER 1999
APPLICATION INFORMATION
shutdown technique
The PWM controller (see Figure 7) can be shut down by two methods: either raise the voltage at ISENSE above
1 V or pull the COMP terminal below a voltage two diode drops above ground. Either method causes the output
of the PWM comparator to be high (refer to block diagram). The PWM latch is reset dominant so that the output
remains low until the next clock cycle after the shutdown condition at the COMP or ISENSE terminal is removed.
In one example, an externally latched shutdown can be accomplished by adding an SCR that resets by cycling
V
below the lower UVLO threshold. At this point, the reference turns off, allowing the SCR to reset.
CC
1 kΩ
REF
COMP
Shutdown
ISENSE
330 Ω
500 Ω
Shutdown
To Current-Sense
Resistor
Figure 7. Shutdown Techniques
A fraction of the oscillator ramp can be resistively summed with the current-sense signal to provide slope
compensation for converters requiring duty cycles over 50% (see Figure 8). Note that capacitor C forms a filter
with R2 to suppress the leading-edge switch spikes.
REF
0.1 µF
R
T
RT/CT
C
T
R1
ISENSE
R2
ISENSE
C
R
SENSE
Figure 8. Slope Compensation
11
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
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