LM3842A [HTC]
CURRENT MODE PWM CONTROLLER; 电流模式PWM控制器
| 型号： | LM3842A |
| 厂家： | 
HTC KOREA TAEJIN TECHNOLOGY CO. |
| 描述： | CURRENT MODE PWM CONTROLLER |
| 文件： | 总7页 (文件大小：356K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LM3842A/3A/4A/5A
CURRENT MODE PWM CONTROLLER
FEATURES
8 SOP/ 8 DIP PIN Configulation
Compen
sation
VREF
8
7
6
5
● Automatic feed forward compensation
● Optimized for offline converter
● Double pulse suppression
1
2
Voltage
Feedback
Vcc
Current
Sense
● Current mode operation to 500 KHz
● High gain totem pole output
OUTPUT
3
4
RT/CT
GND
● Internally trimmed bandgap reference
● Undervoltage lockout with hysteresis
● Low start up current :＜ 0.3 mA
ORDERING INFORMATION
Device
Package
8 SOP
8 DIP
LM3842A/3A/4A/5A D
LM3842A/3A/4A/5A N
DESCRIPTION
The LM3842A is fixed frequency current-mode PWM controller. It is specially designed for Off-Line and
DC-to-DC converter applications with minimal external components.
This integrated circuit features a trimmed oscillator for precise duty cycle control, a temperature
compensated reference, high gain error amplifier, current sensing comparator, and a high current
totempole output ideally suited for driving a power MOSFET.
Protection circuitry includes built in under-voltage lockout and current limiting.
SIMPLIFIED BLOCK DIAGRAM
Vcc
5V
Undervoltage
VREF
⑧
Reference
Lockout
R
R
VREF
Undervoltage
Lockout
⑦ Vcc
Oscillator
RT/CT ④
VFB ②
⑥ Output
⑤ Ground
Latching
PWM
+
-
Error
Amplifie
③ Current
Sense
COMP ①
Input
ABSOLUTE MAXIMUM RATINGS (TA= 25℃)
Characteristic
Power Supply Voltage
Output Current
Symbol
Value
Unit
V
Vcc
I
30
± 1
A
VIN
ISINK
PD
Analog Inputes Voltage
Error Amp Output Sink Current
-0.3 to 5.5V
10
V
mA
Power Dissipation
1
W
℃
℃
Storage Temperature Range
Lead Temperature (soldering 5 sec)
Tstg
T
-65 to 150
260
HTC
1
LM3842A/3A/4A/5A
CURRENT MODE PWM CONTROLLER
ELECTRICAL CHARACTERISTIC
(Vcc=15V(Note 1), RT = 10kΩ, CT=3.3nF 0 ≤ TA ≤ 70℃ ; unless otherwise specified)
Characteristic
REFERENCE SECTION
Reference Output Voltage
Line Regulation
Symbol
Test Condition
Min
Typ
Max
Unit
VREF
Vo
Tj = 25℃, IO=1 mA
12V ≤ Vcc ≤ 25V
1mA ≤ Io ≤ 20mA
TA = 25℃
4.90 5.00 5.10
V
2
3
20
mV
mV
Load Regulation
Vo
2.5
Output Short Circuit Current
OSCILLATOR SECTION
Normal Frequency
Isc
-85 -180 mA
FOSC
Sv
Tj = 25℃
47
52
0.2
1.6
57
1
kHz
%
Voltage Stability
12V ≤ Vcc ≤ 25V
Amplitude
Vosc
Vp-p
ERROR AMPLIFIER SECTION
Input B Current
IIB
-0.1
-2
μA
V
VFB
Feedback Input Voltage
Open Loop Voltage Gain
Power Supplier Rejection Ratio
Output Sink Current
Output source Current
Output Voltage High
Output Voltage Low
CURRENT SENSE SECTION
Input Voltage Gain
Vo=2.5V
2.42 2.50 2.58
AVOL
PSRREA
ISI
2V≤ Vo ≤ 4V
65
60
2
90
70
7
dB
dB
mA
mA
V
12V ≤ Vcc ≤ 25V
VFB=2.7V, Vo - 1.1V
V = 2.3V, Vo=5V
ISO
-0.5 -1.0
VOH
VOL
VFB=2.7V, RL=15kΩ to GND
VFB=2.7V, RL=15kΩ to VRGR
5
6
0.8
1.1
V
Av
(Note 2 & 3)
Vo=5V (Note 2)
12V ≤ Vcc ≤ 25V
2.85
0.9
3
1
3.15
1.1
V/V
V
VMAX
PSRRSC
IIB
Maximum Input Signal
Power Supply Rejection Ratio
Input Bias Current
70
-2
dB
μA
-1.0
OUTPUT SECTION
VOL
VOH
Isink = 20mA
Isink = 20mA
Output Voltage Low
0.1
1.5
13.5
13.0
45
0.4
2.2
V
V
Isource = 20mA
Isource = 20mA
Tj = 25℃, CL=1nF
Tj = 25℃, CL=1nF
Output Voltage High
13
12
V
V
T
T
Rise Time
150
150
nS
nS
Fail Time
35
UNDERVOLTAGE LOCKOUT SECTION
Start-up Threshold
3842A / 3844A
3843A / 3845A
3842A / 3844A
3843A / 3845A
14.5
7.8
8.5
7.0
16
8.4
10
17.5
9
V
V
V
V
Vth
Minimum Operating Voltage
(After Turn-On )
11.5
8.2
VCC(MIN)
7.6
TOTAL STANDBY CURRENT
Start-up Current
Ist
ICC
V
0.17
13
0.3
17
mA
mA
V
Vcc=14V
Operating Supply Current
Zener Voltage
38
Icc = 25mA
30
Note: 1. Adjust Vcc above the start threshould before setting at 15V.
2. Parameter measured at trip point of latch with VFB - 0.
ΔV Output Compensation(pin FB)
ΔV Current Sanseinput(pin CS)
V
A
=
;
3. Comparator Gain defined as:
HTC
2
LM3842A/3A/4A/5A
CURRENT MODE PWM CONTROLLER
Fig.1 Open Loop Test Circuit
VREF
RT
2N
A
4.7K
2222
V1
8
7
6
5
COMP
VFB
VREF
1
2
3
4
100K
0.1
1K
E/A
VC
1K/1W
ADJUST
LM3842A
0.1
5K
ISENSE
ISENSE
OUTPUT
GND
ADJUST
OUTPUT
4.7K
RT/C
T
C
T
High peak currents associated with capacitive loads necessitate careful grounding techniques
Timing and bypass capacitors should be connected close to pin 5 in a single point ground.
The transistor and 5KΩ potentiometer are used to sample the oscillator waveform and apply an
adjustable ramp to pin 3.
Fig.2 Under Voltage Lockout
ON/OFF COMMAND
TO REST OF IC
7
ICC
LM3842A
16V
LM3843A/5A
8.4V
< 15mA
< 1mA
VON
VOFF
10V
7.6V
VCC
VOFF VON
During Under-Voltage Lock-Out, the output driver is biased to a high impedance state.
Pin 6 should be shunted to ground with a bleeder resistor to prevent activating the power switch
with output leakage current.
Fig.3 Error Amp Configuration
2.5V
0.5mA
+
VFB
2
1
Zi
-
Zf
COMP
Error amp can source or sink up to 0.5mA
HTC
3
LM3842A/3A/4A/5A
CURRENT MODE PWM CONTROLLER
Fig.4 Current Sense Circuit
ERROR
AMP
2R
IS
R
1V
CURRENT
SENSE
COMP
1
CURRENT
COMPARATOR
SENSE
R
3
C
RS
GND
5
Peak current (IS) is determined by the formula:
1.0V
RS
IS(MAX)
~
~
A small RC filter may be required to suppress switch transients.
Fig.5 Oscillator Waveforms and Maximum Duty Cycle
LARGE RT
V4
SMALL CT
VREF
8
4
5
RT
INTERNAL CLOCK
RT/CT
LARGE RT
SMALL CT
CT
V4
GND
INTERNAL CLOCK
Oscillator timing capacitor, CT, is charged by VREF through RT, and discharged by an internal
current source. During the discharge time, the internal clock signal blanks the output to the
low state. Selection of RT and CT therefore determines both oscillator frequency and maximum
duty cycle. Charge and discharge times are determined by the formulas:
tc 0.55 R C
~
T
T
~
0.0063 RT - 2.7
0.0063 RT - 4
td R C ∫n(
~
)
T
T
Frequency, then, is: f = (tc + td) -1
1.8
RT CT
~
For R >5KΩ, f
T
~
HTC
4
LM3842A/3A/4A/5A
CURRENT MODE PWM CONTROLLER
Fig.6 Shutdown Techniques
1K
VREF
8
COMP
1
ISENSE
3
330Ω
SHUTDOWN
500Ω
SHUTDOWN
TO CURRENT
SENSE-RESISTOR
Shutdown of the LM3842A can be accomplished by two methods; either raise pin 3 above 1V or
pull pin 1 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 will remain low until the next clock cycle after the shoutdown condition at pins 1 and/or 3
is removed. In one example, an externally latched shutdown may be accomplished by adding an
SCR which will be reset by cycling VCC below the lower UVLO threshold. At this point the
reference turns off, allowing the SCR to reset.
Fig.7 Slope Compensation
VREF
8
0.1μF
RT
RT/CT
4
3
LM3842A
CT
R1
ISENSE
R2
ISENSE
RSENCE
C
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%.
Note that capacitor, C, forms a filter with R2 to suppress the leading edge switch spikes.
HTC
5
LM3842A/3A/4A/5A
CURRENT MODE PWM CONTROLLER
Fig.1 Output Dead Time
Fig.2 Timing Resistor vs Frequency
100
30
10
3
80
C =100nF
T
C =200pF
T
50
C =10nF
T
C =100pF
T
C =47nF
T
C =5.0nF
T
C =2.0nF
T
20
C =22nF
T
C =1.0nF
T
C =10nF
T
C =500pF
T
8.0
5.0
C =4.7nF
T
C =2.2nF
T
2.0
0.8
C =10nF
T
10k
20k
50k100k
200k
500k
1.0M
fosc, Frequency (kHz)
100 1k
10k
100k
1M
fosc, Frequency(Hz)
Fig.3 Output Saturation Characteristics
Fig.4 Error Amplifier Open Loop
Gain and Phase Frequency
80
60
40
20
0
0
4
Gain
VCC=15V
-45
-90
-135
-180
TA=25℃
3
Phase
2
SOURCE (V -VOH
CC
)
1
0
SINK (V )
OL
-3
10
-1
2
4
6
8
2
4
6 8
10
Io, Output Load Current (A)
10
100
1K
10K 100K
1M
10M
Fosc, Frequency (Hz)
PIN FUNCTION DESCRIPTION
Pin No.
1
Function
Description
Compensation This pin is the Error Amplifier output and is made available for loop
compensation.
2
3
4
Voltage
Feedback
This is the inverting input of the Error Amplitier. It is normally connected
to the switching power supply output through a resister divider.
Current Sense A voltage proportional to inductor current is connected to this input.
The PWM uses this information to terminate the output switch conduction.
The Oscillator frequency and maximum Output duty cycle are
programmed by connecting resistor RT to VREF and capacitor CT
RT/CT
to ground. Operation to 500kHz is possible.
5
6
GND
Output
This pin is the combined control circuitry and power ground.
This output directly drives the gate of a power MOSFET. Peak currents
up to 1.0A are sourced and sunk by this pin.
7
8
Vcc
VREF
This pin is the positive supply of the control IC.
This is the reference output. It provides charging current for capacitor
CT through resistor RT.
HTC
6
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