SEMICONDUCTOR
AS384x
Current Mode Controller
Description
The AS3842 family of control ICs provide pin-for-pin replacement of the
industry standard UC3842 series of devices. The devices are
redesigned to provide significantly improved tolerances in power sup-
ply manufacturing. The 2.5 V reference has been trimmed to 1.0%
tolerance. The oscillator discharge current is trimmed to provide guar-
anteed duty cycle clamping rather than specified discharge current.
The circuit is more completely specified to guarantee all parameters
impacting power supply manufacturing tolerances.
In addition, the oscillator and flip-flop sections have been enhanced to
provide additional performance. The R
T
/C
T
pin now doubles as a syn-
chronization input that can be easily driven from open collector/open
drain logic outputs. This sync input is a high impedance input and can
easily be used for externally clocked systems. The new flip-flop topol-
ogy allows the duty cycle on the AS3844/5 to be guaranteed between
49 and 50%. The AS3843/5 requires less than 0.5 mA of start-up cur-
rent over the full temperature range.
Features
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2.5 V bandgap reference trimmed to
1.0% and temperature-compensated
Standard temperature range extended
to 105¡C
AS3842/3 oscillations trimmed for
precision duty cycle clamp
AS3844/5 have exact 50% max duty
cycle clamp
Advanced oscillator design simplifies
synchronization
Improved specs on UVLO and
hysteresis provide more predictable
start-up and shutdown
Improved 5 V regulator provides better
AC noise immunity
Guaranteed performance with current
sense pulled below ground
Top view
Pin Configuration Ñ
PDIP (N)
COMP
V
FB
I
SENSE
R
T
/C
T
1
2
3
4
8
7
6
5
8L SOIC (D8)
V
REG
V
CC
OUT
GND
COMP
V
FB
I
SENSE
R
T
/C
T
1
2
3
4
8
7
6
5
14L SOIC (D14)
V
REG
V
CC
OUT
GND
COMP
NC
V
FB
NC
I
SENSE
NC
R
T
/C
T
1
2
3
4
5
6
7
14
V
REG
13
NC
12
V
CC
11
V
C
10
OUT
9
8
PWR G
GND
Ordering Information
AS384X
Circuit Type:
Current Mode Controller (See Table A)
Package Style
D8 = 8 Pin Plastic SOIC
D14 = 14 Pin Plastic SOIC
N = 8 Pin Plastic DIP
D8
13
Packaging Option:
T = Tube
13 = Tape and Reel (13" Reel Dia)
Table A
Model
AS3842
AS3843
AS3844
AS3845
V
CC(min)
10
7.6
10
7.6
V
CC(on)
16
8.4
16
8.4
Duty Cycle
Typ.
97%
97%
49.5%
49.5%
I
CC
0.5 mA
0.3 mA
0.5 mA
0.3 mA
ASTEC Semiconductor
1
AS384x
Functional Block Diagram
(5.0 V)
1
COMP
+
2
V
FB
Ð
ERROR AMP
(1.0 V)
R
2R
Current Mode Controller
5V
REGULATOR
(2.5 V)
REF OK
(5.0 V)
8
V
REG
7
V
CC
(4 V)
UVLO
(6 V)
PWM
COMPARATOR
3
CURRENT
SENSE
(3.0 V)
(5 V)
Ð
+
FF
S
R
PWM LOGIC
6
OUTPUT
Pin Function Description
Pin Number Function
1
2
3
4
COMP
V
FB
Current
Sense
R
T
/C
T
Description
This pin is the error amplifier output. Typically used to provide loop compensation to
maintain V
FB
at 2.5 V.
Inverting input of the error amplifier. The non-inverting input is a trimmed 2.5 V
bandgap reference.
A voltage proportional to inductor current is connected to the input. The PWM uses
this information to terminate the gate drive of the output.
Oscillator frequency and maximum output duty cycle are set by connecting a resistor
(R
T
) to V
REG
and a capacitor (C
T
) to ground. Pulling this pin to ground or to V
REG
will
accomplish a synchronization function.
Circuit common ground, power ground, and IC substrate.
This output is designed to directly drive a power MOSFET switch. This output can sink
or source peak currents up to 1A. The output for the AS3844/5 switches at one-half
the oscillator frequency.
Positive supply voltage for the IC.
This 5 V regulated output provides charging current for the capacitor C
T
through the
resistor R
T
.
2
5
6
GND
Output
7
8
V
CC
V
REG
ASTEC Semiconductor
+
(0.6 V)
+
4
R
T
/C
T
(1.3 V)
+
Ð
FF
S
R
OSCILLATOR
OVER
TEMPERATURE
T
CLK [3842/43]
FF
5
GND
Ð
Ð
CLK
÷
2 [3844/45]
Figure 1. Block Diagram of the AS3842/3/4/5
Current Mode Controller
Absolute Maximum Ratings
Parameter
Symbol
Rating
AS384x
Unit
Supply Voltage (I
CC
< 30 mA)
Supply Voltage (Low Impedance Source)
Output Current
Output Energy (Capacitive Load)
Analog Inputs (Pin 2, Pin 3)
Error Amp Sink Current
Maximum Power Dissipation
V
CC
V
CC
I
OUT
Self-Limiting
30
±1
5
Ð0.3 to 30
10
V
V
A
µJ
V
mA
750 mW
1000 mW
950 mW
¡C
¡C
¡C
¡C
Maximum Junction Temperature
Operating Temperature
Storage Temperature Range
Lead Temperature, Soldering 10 Seconds
P
D
8L SOIC
8L PDIP
14L SOIC
T
J
T
STG
T
L
150
0 to 150
Ð65 to 150
300
Stresses greater than those listed under ABSOLUTE MAXIMUM RATINGS may cause permanent damage to the device. This is a
stress rating only and functional operation of the device at these or any other conditions above indicated in the operational sections
of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect reliability.
Recommended Conditions
Parameter
Supply Voltage
AS3842,4
AS3843,5
Oscillator
f
OSC
Symbol
V
CC
15
10
50 to 500
V
V
kHz
Rating
Unit
Typical Thermal Resistances
Package
8L PDIP
8L SOIC
14L SOIC
θ
JA
95¡C/W
175¡C/W
130¡C/W
θ
JC
50¡C/W
45¡C/W
35¡C/W
Typical Derating
10.5 mW/¡C
5.7 mW/¡C
7.7 mW/¡C
ASTEC Semiconductor
3
AS384x
Electrical Characteristics
Current Mode Controller
Electrical characteristics are guaranteed over full junction temperature range (0 to 105¡C). Ambient temperature must be derated
based on power dissipation and package thermal characteristics. The conditions are: V
CC
= 15 V, R
T
= 10 k½, and C
T
= 3.3 nF, unless
otherwise stated. To override UVLO, V
CC
should be raised above 17 V prior to test.
Parameter
5 V Regulator
Output Voltage
Line Regulation
Load Regulation
Temperature Stability
1
Total Output Variation
1
Long-term Stability
1
Output Noise Voltage
Short Circuit Current
2.5 V Internal Reference
Nominal Voltage
Line Regulation
Load Regulation
Temperature Stability
1
Total Output Variation
1
Long-term Stability
Oscillator
Initial Accuracy
Voltage Stability
Temperature Stability
1
Amplitude
Upper Trip Point
Lower Trip Point
Sync Threshold
Discharge Current
Duty Cycle Limit
TC
f
f
OSC
V
H
V
L
V
SYNC
I
D
R
T
= 680 ½, C
T
= 5.3 nF, T
J
= 25¡C
400
7.5
46
f
OSC
T
J
= 25¡C
12 V ² V
CC
² 25 V
T
MIN
² T
J
² T
MAX
V
RT/CT
peak-to-peak
47
52
0.2
5
1.6
2.9
1.3
600
8.7
50
800
9.5
52
57
1
kHz
%
%
V
V
V
mV
mA
%
1
Symbol
Test Condition
Min.
Typ.
Max.
Unit
V
REG
PSRR
T
J
= 25¡C, I
REG
= 1 mA
12 ² V
CC
² 25 V
1 ² I
REG
² 20 mA
4.95
5.00
2
2
0.2
5.05
10
10
0.4
5.15
V
mV
mV
mV/¡C
V
mV
µV
TC
REG
Line, load, temperature
Over 1,000 hrs at 25¡C
V
NOISE
I
SC
V
FB
PSRR
T = 25¡C; I
REG
= 1 mA
12 V ² V
CC
² 25 V
1 ² I
REG
² 20 mA
TC
VFB
Line, load, temperature
Over 1,000 hrs at 125¡C
2.450
10 Hz ² f ² 100 kHz, T
J
= 25¡C
30
4.85
5
50
100
25
180
mA
2.475
2.500
2
2
0.1
2.500
2
2.525
5
5
0.2
2.550
12
V
mV
mV
mV/¡C
V
mV
ASTEC Semiconductor
4
Current Mode Controller
Electrical Characteristics
(contÕd)
AS384x
Electrical characteristics are guaranteed over full junction temperature range (0 to 105¡C). Ambient temperature must be derated
based on power dissipation and package thermal characteristics. The conditions are: V
CC
= 15 V, R
T
= 10 k½, and C
T
= 3.3 nF, unless
otherwise stated. To override UVLO, V
CC
should be raised above 17 V prior to test.
Parameter
Error Amplifier
Input Voltage
Input Bias Current
Voltage Gain
Transconductance
Unity Gain Bandwidth
1
Power Supply Rejection Ratio
Output Sink Current
Output Source Current
Output Swing High
Output Swing Low
Current Sense Comparator
Transfer Gain
2,3
I
SENSE
Level Shift
2
Maximum Input Signal
2
Power Supply Rejection Ratio
Input Bias Current
Propagation Delay to Output
1
Output
Output Low Level
V
OL
V
OL
Output High Level
1
Symbol
Test Condition
Min.
Typ.
Max.
Unit
V
FB
I
BIAS
A
VOL
G
m
GBW
PSRR
I
COMPL
I
COMPH
V
COMPH
V
COMPL
AV
CS
V
LS
PSRR
I
BIAS
t
PD
T
J
= 25¡C
2 ² V
COMP
² 4 V
2.475
2.500
Ð0.1
2.525
Ð1
1
V
µA
dB
mA/mV
MHz
dB
mA
mA
V
65
90
1
0.8
12 ² V
CC
² 25 V
V
FB
= 2.7 V, V
COMP
= 1.1 V
V
FB
= 2.3 V, V
COMP
= 5 V
V
FB
= 2.3 V, R
L
= 15 k½ to Ground
V
FB
= 2.7 V, R
L
= 15 k½ to Pin 8
Ð0.2 ² V
SENSE
² 0.8 V
V
SENSE
= 0 V
V
COMP
= 5 V
12 ² V
CC
² 25 V
0.9
2.85
60
2
0.5
5
1.2
70
6
0.8
5.5
0.7
1.1
V
3.0
1.5
1
70
Ð1
85
3.15
V/V
V
1.1
V
dB
Ð10
150
µA
ns
I
SINK
= 20 mA
I
SINK
= 200 mA
I
SOURCE
= 20 mA
I
SOURCE
= 200 mA
C
L
= 1 nF
C
L
= 1 nF
3842/4
3843/5
15
7.8
9
7.0
13
12
0.1
1.5
13.5
13.5
50
50
0.4
2.2
V
V
V
V
V
OH
V
OH
t
R
t
F
V
CC
(on)
V
CC
(min)
V
OUV
T
OT
Rise Time
Fall Time
1
150
150
ns
ns
Housekeeping
Start-up Threshold
16
8.4
10
7.6
1.5
125
17
9.0
11
8.2
2.0
V
V
V
V
V
¡C
Minimum Operating Voltage
After Turn On
Output Low Level in UV State
Over-Temperature Shutdown
4
3842/4
3843/5
I
SINK
= 20 mA, V
CC
= 6 V
ASTEC Semiconductor
5
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