PRELIMINARY
AIC3842
Current-Mode PWM Controller
n FEATURES
n DESCRIPTION
l Low Start-Up Current (300µA Typical)
The AIC3842 control IC provides the features
that are necessary to implement off-line or
DC/DC Converter fixed-frequency current-mode
schemes with a minimum number of external
components. This integrated circuits features
an
undervoltage
lockout
(UVLO)
with
approximately 300µA start-up current, a
precision reference trimmed for accuracy at the
error amplifier input, high gain error amplifier,
current sensing comparator, logic to insure
latched operation, and a totem-pole output
stage designed to source or sink high peak
current. The output stage, suitable for driving N
channel MOSFETs, is low in the off state.
l Internal Precision Reference.
l 500KHz Current-Mode Operation.
l Pulse-by-Pulse Current Limiting.
l Automatic Feed-Forward Compensation.
l Optimized for Off-Line and DC/DC Converters.
l Undervoltage Lockout with Hysteresis.
l Double Pulse Suppression.
l High Current Totem-Pole Output.
n APPLICATIONS
l Off-Line Converter.
l DC/DC Converter.
n TYPICAL APPLICATION CIRCUIT
2
VFB
VCC
7
VCC
+
C2
R3
300pF
51K
FEEDBACK
SIGNAL
C3
1000P
R4
7.5K
C5
0.1µF
1
6
COMP
OUTPUT
8
VREF
GND
4
RT/CT
ISENSE
C1
47µF
INDUCTOR
R1
M1
10
R2
20K
5
3
AIC3842
R5
390
C6
C7
1nF
300P
C4
2.2nF
RS
0.33
Current Mode PWM Control Circuit
n ORDERING INFORMATION
AIC3842CX
ORDER NUMBER
PACKAGE TYPE
N: PLASTIC DIP
S: SMALL OUT LINE
AIC3842CN
(PLASTIC DIP)
PIN CONFIGURATION
COMP
1
VFB 2
ISENSE 3
AIC3842CS
(PLASTIC SO)
RT/CT
4
Analog Integrations Corporation
4F, 9, Industry E. 9th Rd, Science Based Industrial Park, Hsinchu Taiwan, ROC
DS-3842-P1 May 5, 00
TEL: 886-3-5772500
FAX: 886-3-5772510
8 VREF
7 VCC
6 OUTPUT
5 GND
www.analog.com.tw
1
AIC3842
n ABSOLUTE MAXIMUM RATINGS
Supply Voltage (Low Impedance Source )
Supply Voltage (ICC <300mA)
… ..........… … … … .............… … … ........................ 30V
...................… ..… … … … .… … … ..… ................… … … .......... Self Limiting
Output Current ..............… … … … … … … … … ....… … … … ....................… … … .… … … .........… ......... ±1A
Output Energy (Capacitive Load)
...................… ........… … … … … .… .........… … … .… ...................... 5µJ
Analog Inputs (Pins 2, 3) … .....… … … … … … … … ...… ......… … … … .................… … … .... -0.3V to +6.3V
Error Amp Output Sink Current ......… ....… .… … ...… … … … ................… … … .… … … … ............... 10mA
Operation Temperature Range
......… ....… .… … ...… … … … .................… … … … ............... -40°C~85°C
Power Dissipation at TA ≤25°C DIP Package
......… … … … … … … … … ....… … … … ..… … ............... 1W
SOIC Package
Storage Temperature Range
.........… … .… ..… .....… … … … … … … ............. 725mW
.....… … ...… … … … ...........................… … ......… … … …
-65°C to +150°C
Lead Temperature (Soldering, 10 seconds) .....… … ...… … … … ..… … … … … .… … ....… … ........... 300°C
Note 1:
All voltages are with respect to Pin 5.
All currents are positive into the specified terminal.
n TEST CIRCUIT
VREF
RT
Q2
2N2222
R5
4.7K
COMP
Error Amp.
Adjust
R7
4.7K
VREF
R4
100K
VFB
R6
10K
ISENSE
Adjust R3
5K
VCC
A
VCC
ISENSE
OUTPUT
C2
0.1µF
R1
1K
1W
S1
C1
0.1µF
OUTPUT
RT/CT
GND
AIC3842
GROUND
CT
2
AIC3842
n ELECTRICAL CHARACTERISTICS {V
CC
= 15V (see Note 2), Ta=25°C, unless otherwise
specified.}
PARAMETERS
CONDITIONS
MIN.
TYP.
MAX.
UNIT
4.9
5
5.1
V
Reference Section
Output Voltage
IO=1mA
Line Regulation
VCC=12V to 25V
5
20
mV
Load Regulation
IO=1mA to 20mA
5
25
mV
Temperature Coefficient of
0.2
mV/°C
50
µV
5
mV
85
mA
Output Voltage
Output Noise Voltage
f=10Hz to 10KHz
Output Voltage Long-Term
After 1000H at TA=25°C
Drift
Short-Circuit Output Current
Oscillator Section
Oscillator Frequency (see
47
Note 3)
Frequency Change with
Supply Voltage
Frequency Change with
Temperature
VCC=12V to 25V
TA=TLOW to THIGH
Peak-to-Peak Amplitude at
RT/CT
52
57
KHz
0.2
%
5
%
1.7
V
Error Amplifier Section
Feedback Input Voltage
COMP at 2.5V
2.42
2.50
2.58
V
-0.3
µA
90
dB
1
MHz
VCC =12V to 25V
70
dB
Output Sink Current
VFB at 2.7V, COMP at 1.1V
10
mA
Output Source Current
VFB at 2.3V, COMP at 5V
1
mA
High-Level Output Voltage
VFB at 2.3V, RL=15KΩ to GND
6.2
V
Low-Level Output Voltage
VFB at 2.7V, RL=15Ω to VREF
0.8
V
Input Bias Current
Open-Loop Voltage
VO=2V to 4V
Amplification
Gain-Bandwidth Product
Supply Voltage Rejection
Ratio
3
AIC3842
n ELECTRICAL CHARACTERISTICS (Continued)
PARAMETERS
CONDITIONS
MIN.
TYP.
MAX.
UNIT
Current Sense Section
Voltage Amplification
See Note 3 and 4
3
V/V
Current Sense Comparator
COMP at 5V, See Note 3
1
V
70
dB
-2
µA
150
nS
ISOURCE=20mA
13.5
V
ISOURCE =200mA
13.4
V
ISINK=20mA
0.1
V
ISINK=200mA
1.5
V
Rise Time
CL=1nF
50
nS
Fall Time
CL=1nF
50
nS
Threshold
Supply Voltage Rejection
VCC =12V to 25V, See Note 3
Ratio
Input Bias Current
Delay Time to Output
Output Section
High-Level Output Voltage
Low-Level Output Voltage
Undervoltage Lockout Section
Start Threshold Voltage
14.5
16
17.5
V
Minimum Operating Voltage
8.5
10
11.5
V
after Start-Up
Pulse-Width-Modulator Section
Maximum Duty Cycle
96
Minimum Duty Cycle
%
0
%
0.3
0.5
mA
17
mA
Supply Voltage
Start-Up Current
Operating Supply Current
VFB and ISENSE at 0V
12
Limiting Voltage
ICC =25mA
34
V
Note: 2: Adjust VCC above the start threshold before setting it to 15V.
3. These parameters are measured at the trip point of the latch with VFB at 0V.
4. Voltage amplification is measured between ISENSE and COMP with the input changing from 0V to
0.8V.
4
AIC3842
n BLOCK DIAGRAM
VCC
7
5
ZD
34V
UVLO
S/R
GND
8
5V
REF
VREF
5V/50mA
Internal Bias
2.50V
4
VREF
Good
Logic
Q1
6
OSC
RT/CT
Error Amp.
S
2
PWM
Latch
OUTPUT
Q2
R
VFB
1
R2
D2
D1
Z1
1V
COMP
Current
Sense
comparator
3
ISENSE
n PIN DESCRIPTIONS
PIN 1: COMP
- This pin is the error amplifier
PIN 5: GND-
output and is made available for
loop compensation.
PIN 2: VFB
This pin is the combined control
circuitry and power ground.
PIN 6: OUTPUT- This output directly drives the
- This is the inverting input of the
gate of a power MOSFET. Peak
error amplifier. It is normally
currents up to 1A are sourced
connected
and sunk by this pin.
to
the
switching
power supply output through a
resistor divider.
PIN 3: ISENSE - A
voltage
This pin is the positive supply of
the control IC.
proportional
to
inductor current is connected to
this input. The PWM uses this
information
PIN 7: VCC -
to
terminate
the
PIN 8: VREF -
This is the reference output. It
provides charging current for
capacitor CT through resistor RT.
output switch conduction.
PIN 4: RT/CT
- The
oscillator
frequency
and
maximum output duty cycle are
programmed
by
connecting
resistor RT to VREF and capacitor
CT
to
ground.
Operation
to
500KHz is feasible.
5
AIC3842
n APPLICATION INFORMATIONS
l Undervoltage Lockout
There are two separate undervoltage lockout
comparators incorporated to make sure that the IC
is fully functional before the output stage is
enabled. One is for power supply voltage (VCC)
and the other is for reference output voltage
(VREF). Each has a built in hysteresis to prevent
erratic output behavior when their respective
thresholds are crossed. For VCC comparator the
upper and lower thresholds are 16V and 10V,
respectively. For VREF compactor the upper and
lower threshold are 3.6V and 3.4V, respectively.
The large hysteresis and low start up current
(0.3mA) of the AIC3842 make it ideally suited in
off-line converter applications where efficient
bootstrap startup techniques are required. A 34V
zener is connected as a shunt regulator from VCC
to ground. Its purpose is to protect the IC from
excessive voltage that can occur during system
start-up.
Reference Output
The 5.0V reference output is trimmed to ±2.0%
tolerance at TA=25°C. It supplies charging current
to the oscillator timing capacitor and is capable of
providing current in excess of 20mA for powering
additional control system circuitry. In case of
overload, the reference is short-circuit protected at
about 85mA.
Error Amplifier
A fully compensated error amplifier is provided
with inverting input and output externally
accessible. The noninverting input is internally
biased at 2.5V. The converter output voltage is
usually divided down and connected to the
inverting input.
The output of the error amplifier is accessible for
external loop compensation, with an offset at two
diode drops (≅1.4V) and divided by three, before
connected to the inverting input of the current
sense comparator. This guarantees that no drive
pulse appears at the output (pin 6).
Oscillator
The oscillator frequency can be programmed
through the setting of timing components RT and
CT. Capacitor CT is charged from the 5.0V
reference output through RT to about 2.8V and
discharged to about 1.2V by the internal
discharge current. When CT is discharged the
output (pin 6) must be in the low state, thus
producing a controlled amount of output
deadtime. Note that many values of RT and CT
can produce the same frequency but only one
combination will yield a specific output deadtime
at a given frequency.
Current Sense Comparator and PWM
Latch
The output switch of AIC3842 is initiated by the
oscillator and terminated when the peak inductor
current reaches the threshold level established by
the error amplifier output (pin 1). The AIC3842 is
operated at a current mode since the inductor
current is monitored cycle-by-cycle and decides
the duty cycle.
The inductor current is converted to a voltage by
inserting the ground referenced sense resistor RS
in series with the source of output switch M1. This
voltage is monitored by the current sense input
(pin 3) and is compared to a level derived from the
error amplifier output. In the normal operating
conditions the peak inductor current is controlled
by the voltage at pin 1 where
IPK =
V ( pin 1)− 1. 4V
3RS
PWM Latch is used to ensure that only a single
pulse appears at the output during any given
oscillator cycle. However, a narrow spike on the
leading edge of the current waveform can usually
be observed and may cause the power supply to
exhibit an instability when the output is tightly
loaded.
Output Switch
The AIC3842 contains a single totem-pole output
stage that was specifically designed for direct
drive of power MOSFET. If any undervoltage
lockout is deetected, internal circuitry will keep the
output switch in a sinking current mode, no
external pull down resistor is needed.
6
AIC3842
n PHYSICAL DIMENSIONS
l 8 LEAD PLASTIC SO (unit: mm)
D
SYMBOL
MIN
MAX
A
1.35
1.75
A1
0.10
0.25
B
0.33
0.51
C
0.19
0.25
D
4.80
5.00
E
3.80
4.00
H
E
e
e
A
A1
C
B
1.27(TYP)
H
5.80
6.20
L
0.40
1.27
L
l 8 LEAD PLASTIC DIP (unit: mm)
D
E1
E
A2
A1
C
L
SYMBOL
MIN
MAX
A1
0.381
—
A2
2.92
4.96
b
0.35
0.56
C
0.20
0.36
D
9.01
10.16
E
7.62
8.26
E1
6.09
7.12
e
eB
b
e
2.54 (TYP)
eB
—
10.92
L
2.92
3.81
7