SG1540/SG2540/SG3540
OFF-LINE START-UP CONTROLLER
DESCRIPTION
The SG1540 is an integrated circuit designed to efficiently provide
start-up power from a high-voltage DC bus to a PWM control circuit
in a switching power supply. When used on the primary side, it
reduces start-up current to less than 1mA and allows any standard
PWM control circuit to be used as a primary-side controller. When
used to power a controller on the secondary side, it efficiently
eliminates the need for a heavy 50/60Hz line transformer with its
associated low frequency magnetic fields.
The circuit consists of three sections: a micropower bandgap
comparator/power switch referenced to 2.5 volts which isolates the
start-up capacitor from its load; a high frequency square-wave
oscillator with 200mA totem-pole output for driving an isolation
transformer; and a second bandgap comparator with latching
crowbar to protect against overvoltage faults while starting or
running.
The SG1540 is specified for operation over the full military ambient
temperature range of -55°C to 125°C. The SG2540 is character-
ized for the industrial range of -25°C to 85°C, and the SG3540 is
designed for the commercial range of 0°C to 70°C.
FEATURES
•
Useable with primary and secondary side PWM
controllers
•
Micropower comparator / switch
- Internal 2.5V bandgap reference
- 50mA power switch
•
Squarewave oscillator
- 500Hz to 200KHz operation
- 200mA totem pole outputs
•
Eliminates bulky, expensive 50/60 Hz transformer
•
Minimizes high voltage bleeder current
•
Programmable start-up voltage and hysteresis
•
Internal and programmable overvoltage crowbar
latch
•
Available in 8 pin DIP, 10 pin flat pack, and 16 pin
widebody SOIC
HIGH RELIABILITY FEATURES - SG1540
♦
Available to MIL-STD - 883
♦
LMI level "S" processing available
BLOCK DIAGRAM
4/90 Rev 1.1 2/94
Copyright
©
1994
1
11861 Western Avenue
∞
Garden Grove, CA 92841
(714) 898-8121
∞
FAX: (714) 893-2570
L
INFINITY
Microelectronics Inc.
SG1540/SG2540/SG3540
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Supply Voltage (+V
IN
) ........................................................ +37V
DC Output Current, Continuous (V
OUT
) ........................... 100mA
AC Output Current, Continuous ..................................... 200mA
Analog Inputs (Start and Overvoltage) ................. -0.3V to 6.0V
Analog Input Currents (V>8V) .......................................... 10mA
Overvoltage Crowbar Current, Continuous ..................... 50mA
Note 1. Values beyond which damage may occur.
Overvoltage Crowbar Energy (½CV²) ................................. 8mJ
Operating Junction Temperature
Hermetic (Y Package) ................................................ 150°C
Plastic (M, DW Packages) .......................................... 150°C
Storage Temperature Range ............................ -65°C to 150°C
Lead Temperature (Soldering, 10 Seconds) .................. 300°C
THERMAL DATA
Y Package:
Thermal Resistance-
Junction to Case
,
θ
JC
.................. 50°C/W
Thermal Resistance-
Junction to Ambient
,
θ
JA
............ 130°C/W
M Package:
Thermal Resistance-
Junction to Case
,
θ
JC
.................. 60°C/W
Thermal Resistance-
Junction to Ambient
,
θ
JA
............. 95°C/W
DW Package:
Thermal Resistance-
Junction to Case
,
θ
JC
.................. 40°C/W
Thermal Resistance-
Junction to Ambient
,
θ
JA
............. 95°C/W
Note A. Junction Temperature Calculation: T
J
= T
A
+ (P
D
x
θ
JA
).
Note B. The above numbers for
θ
JC
are maximums for the limiting
thermal resistance of the package in a standard mount-
ing configuration. The
θ
JA
numbers are meant to be
guidelines for the thermal performance of the device/pc-
board system. All of the above assume no ambient
airflow.
RECOMMENDED OPERATING CONDITIONS
(Note 2)
Supply Voltage Range ............................................... 7V to 35V
DC Output Current, Continuous ................................ 0 to 50mA
AC Output Current, Continuous ............................. 0 to 100 mA
Oscillator Frequency Range ........................... 1KHz to 400KHz
Timing Resistor Range ...................................... 2KΩ to 150KΩ
Note 2. Range over which the device is functiona
l.
Timing Capacitor Range ........................................ 1nF to 20µF
Operating Ambient Temperature Range
SG1540 ........................................................... -55°C to 125°C
SG2540 ............................................................. -25°C to 85°C
SG3540 ................................................................ 0°C to 70°C
ELECTRICAL CHARACTERISTICS
(Unless otherwise specified, these specifications apply over the operating ambient temperatures for SG1540 with -55°C
≤
T
A
≤
125°C, SG2540 with
-25°C
≤
T
A
≤
85°C, SG3540 with 0°C
≤
T
A
≤
70°C, and +V
IN
= 15V. A 0.1µF high frequency bypass capacitor is recommended on V
IN
. Low duty cycle
testing techniques are used which maintains junction and case temperatures equal to the ambient temperature.)
Parameter
Start-up Section
Start Current I
CC
Operating Current
Start Threshold
Start Bias Current
Start Clamp Voltage
Hysteresis ON Voltage
DC Output Section
V
OUT
Voltage
Short Circuit Current
Test Conditions
V
START
= 0 to 2.37V
V
TIMING
= +V
IN
;
V
OUT
Open
F
OSC
= 50KHz, V
OUT
and AC
OUT
Open
V
PIN 1
= 0 to 5V
I
PIN 1
= 1mA
I
PIN 4
= 100µA
I
SOURCE
= 10mA
I
SOURCE
= 50mA
V
OUT
= 0V
SG1540/2540/3540
Units
Min. Typ. Max.
0.2
0.3
3
2.37
2.50
0.1
6
7
0.1
12.5
12.0
50
13.5
13.0
100
0.4
0.6
6
2.63
1
8
0.2
mA
mA
mA
V
µA
V
V
V
V
mA
225
4/90 Rev 1.1 2/94
Copyright
©
1994
2
11861 Western Avenue
∞
Garden Grove, CA 92841
(714) 898-8121
∞
FAX: (714) 893-2570
L
INFINITY
Microelectronics Inc.
SG1540/SG2540/SG3540
ELECTRICAL CHARACTERISTICS
(continued)
Parameter
Oscillator Section
(Note 3)
Initial Accuracy
Voltage Stability
Temperature Stability
(Note 4)
Oscillator Minimum Frequency
Oscillator Maximum Frequency
AC Output Section
HIGH Output Voltage
LOW Output Voltage
Squarewave Duty Cycle
AC Output Risetime
AC Output Falltime
Overvoltage Crowbar Section
Overvoltage Threshold
Overvoltage Bias Current
Overvoltage Clamp Voltage
+V
IN
Overvoltage Threshold
SCR ON Voltage
SCR Holding Current
T
J
= 25°C
+V
IN
= 12 to 18V
R
T
= 17.8K, C
T
= .068µF
R
T
= 1.5K, C
T
= 470pF
I
SOURCE
= 20mA
I
SOURCE
= 100mA
I
SINK
= 20mA
I
SINK
= 100mA
C
L
= 1000pF
C
L
= 1000pF
2.37
V
O.V.
= 0 to 2.37V
I
O.V.
= 1mA
I
VIN
= 35mA
V
O.V.
= 0
6
37
0.15
Test Conditions
SG1540/2540/3540 Units
Min. Typ. Max.
46
50
5
2
54
12
5
1
KHz
%
%
KHz
KHz
V
V
V
V
%
µS
µS
V
µA
V
V
V
mA
400
12.5
12.0
13.5
13.0
0.2
1.2
50
0.3
0.1
2.50
0.1
7
42
9
0.35
45
0.3
2.0
55
0.6
0.2
2.63
1
8
44
12
0.55
Note 3. F
OSC
= 50KHz, R
T
= 3.48K C
T
= 4.7nF unless otherwise specified.
Note 4. These parameters, although guaranteed, are not tested in production.
CHARACTERISTIC CURVES
FIGURE 1 - SATURATION VOLTAGE (DC OUT PIN) VS. OUTPUT CURRENT
FIGURE 2 - OSCILLATOR FREQUENCY VS. R
T
AND C
T
4/90 Rev 1.1 2/94
Copyright
©
1994
3
11861 Western Avenue
∞
Garden Grove, CA 92841
(714) 898-8121
∞
FAX: (714) 893-2570
L
INFINITY
Microelectronics Inc.
SG1540/SG2540/SG3540
APPLICATION INFORMATION
FIGURE 3 - EFFICIENT PRIMARY SIDE START-UP
PRIMARY SIDE START-UP
When the design goal is efficient start-up for a control PWM referenced to the primary side of the power transformer, the configuration
in Figure 3 is recommended. An energy storage capacitor C
START
is trickle-charged from the 300-400 Volt DC bus by resistor R
START
.
The value of R
START
is chosen to provide a constant 1mA charging current, allowing the use of a ½ watt resistor. As the voltage on
C
START
ramps up from zero, the only load current is the standby current of the SG1540 and that of the divider network R1-R3. (Connecting
the TIMING pin to +V
IN
disables the internal power oscillator and forces the circuitry into a micropower standby model. Since the input
bias current at the START pin is 1µA maximum, a divider current of 100µA is adequate).
When the voltage at the START pin reaches +2.5 Volts, the hysteresis transistor turns off, overdriving the START pin. The V
OUT
pin
is switched to the HIGH state, providing power to the PWM control circuit. As energy flows out of the START capacitor, its voltage
decays; but it remains connected to the PWM circuit until the dropout voltage is reached (V
START
- V
HYSTERESIS
). The bootstrap winding
on the power transformer and rectifier diode D5 prevent this from happening. As the PWM control circuit becomes active, the power
transistor begins to switch, providing operating current to the PWM circuit through the SG1540.
RESISTOR CALCULATIONS
Given that V
START
and V
DROPOUT
have been chosen, and that the divider current at start-up is 100µA, then the values for R1 through R3
are calculated as follows:
1. For simplification, let X =
V
START
- 2.5
2.5
V
DROPOUT
-2.5
2.5
2. Then,R1 = 2.5 x 10
4
* X
R2 = R1/Y
and
R3 =
R1 * R2
X * R2 - R1
[1]
[2]
[3]
and Y =
DESIGN EXAMPLE
Suppose we have a power MOSFET device, and so want to start at +18 volts and drop out at +12 volts.
Then
X = 6.20
and Y = 3.80
R1 = 2.5 x 10
4
* 6.2 = 155K
R2 = 1.5 x 10
5
/3.8 = 39.5K
R3 =
1.5 x 10
5
* 3.9 x 10
4
= 63.7K
6.2 * 3.9 x 10
4
- 1.5 x 10
5
(Choose 150K)
(Choose 39K)
(Choose 62K)
Therefore
4/90 Rev 1.1 2/94
Copyright
©
1994
4
11861 Western Avenue
∞
Garden Grove, CA 92841
(714) 898-8121
∞
FAX: (714) 893-2570
L
INFINITY
Microelectronics Inc.
SG1540/SG2540/SG3540
APPLICATIONS INFORMATION
(continued)
The voltage waveform at +V
IN
is shown in Figure 4 with these resistor values and with C
START
= 3µF. Notice that two tries are required
before the +15 volt bootstrap winding becomes active.
FIGURE 4 - STARTUP VOLTAGE WAVEFORM
FIGURE 5 - SECONDARY-SIDE START-UP WITHOUT A LINE TRANSFORMER
4/90 Rev 1.1 2/94
Copyright
©
1994
5
11861 Western Avenue
∞
Garden Grove, CA 92841
(714) 898-8121
∞
FAX: (714) 893-2570
L
INFINITY
Microelectronics Inc.
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