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AS3844 Datasheet Preview

AS3844 Datasheet

Current Mode Controller

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SEMICONDUCTOR
AS384x
Current Mode Controller
Features
¥ 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
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 RT/CT 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.
Pin Configuration Ñ Top view
PDIP (N)
8L SOIC (D8)
14L SOIC (D14)
COMP 1
VFB 2
ISENSE 3
RT/CT 4
8 VREG
7 VCC
6 OUT
5 GND
COMP 1
VFB 2
ISENSE 3
RT/CT 4
Ordering Information
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
AS384X D8 13
8 VREG
7 VCC
6 OUT
5 GND
COMP 1
NC 2
VFB 3
NC 4
ISENSE 5
NC 6
RT/CT 7
14 VREG
13 NC
12 VCC
11 VC
10 OUT
9 PWR G
8 GND
Packaging Option:
T = Tube
13 = Tape and Reel (13" Reel Dia)
Table A
Model
AS3842
AS3843
AS3844
AS3845
VCC(min)
10
7.6
10
7.6
VCC(on)
16
8.4
16
8.4
Duty Cycle
Typ.
97%
97%
49.5%
49.5%
ICC
0.5 mA
0.3 mA
0.5 mA
0.3 mA
ASTEC Semiconductor
1




ASTEC

AS3844 Datasheet Preview

AS3844 Datasheet

Current Mode Controller

No Preview Available !

AS384x
Current Mode Controller
Functional Block Diagram
1
COMP
2
VFB
+
Ð
ERROR AMP
(5.0 V)
(2.5 V)
2R
(1.0 V)
R
5V
REGULATOR
(5.0 V)
REF OK
(4 V)
8
VREG
7
VCC
UVLO
(6 V)
3
CURRENT
SENSE
4
RT/CT
PWM
COMPARATOR
(5 V)
(3.0 V)
(1.3 V)
(0.6 V)
Ð
+
Ð
+
Ð
+
Ð
+
FF
S PWM LOGIC
R
OSCILLATOR
FF
S
R
CLK ÷ 2 [3844/45]
CLK [3842/43]
FF
T
OVER
TEMPERATURE
Figure 1. Block Diagram of the AS3842/3/4/5
6
OUTPUT
5
GND
Pin Function Description
Pin Number Function
Description
1
COMP
This pin is the error amplifier output. Typically used to provide loop compensation to
maintain VFB at 2.5 V.
2 VFB Inverting input of the error amplifier. The non-inverting input is a trimmed 2.5 V
bandgap reference.
3 Current A voltage proportional to inductor current is connected to the input. The PWM uses
Sense
this information to terminate the gate drive of the output.
4
RT/CT
Oscillator frequency and maximum output duty cycle are set by connecting a resistor
(RT) to VREG and a capacitor (CT) to ground. Pulling this pin to ground or to VREG will
accomplish a synchronization function.
5
GND
Circuit common ground, power ground, and IC substrate.
6
Output
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.
7 VCC Positive supply voltage for the IC.
8
VREG
This 5 V regulated output provides charging current for the capacitor CT through the
resistor RT.
ASTEC Semiconductor
2


Part Number AS3844
Description Current Mode Controller
Maker ASTEC
Total Page 20 Pages
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