LR745N3-G Supertex, LR745N3-G Datasheet - Page 3

• Manufacturer Part Number: LR745N3-G
• Description: Switching Converters, Regulators & Controllers Pwr Sup Start-Up IC
• Manufacturer: Supertex

Specifications of LR745N3-G

• Output Voltage: 25 V
• Output Current: 4 mA
• Input Voltage: 35 V to 450 V
• Operating Temperature Range: - 55 C to + 150 C
• Mounting Style: Through Hole
• Package / Case: TO-92
• Number Of Outputs: 1
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Block Diagram Detailed Description
The Supertex LR745 is a high voltage, switch-mode pow-
er supply start-up circuit which has 3 terminals: V
and V
applied directly at the input V
is monitored by the 2 comparators, COMP1 and COMP2.
An internal reference, V
R3 set the nominal V
13.25V for COMP2.
When a voltage is applied on V
from 0V. When V
will be at a logic high state, keeping the D flip flop in a re-
set state. The output of the D flip flop, Q, will be at logic
low keeping transistor M2 off. The data input for the D flip
flop, D, is internally connected to a logic high. As V
comes greater than 7.0V, COMP1 will change to a logic low
state. V
rent source of typically 3.0mA output will charge an external
storage capacitor. As V
of COMP2 will then switch from a logic high to a logic low
state. The D flip flop’s output does not change state since its
clock input is designed to trigger only on a rising edge, logic
low to logic high transition. When there is no load connected
to the output, the output voltage will continue to increase
until it reaches 21.5V, which is the zener voltage minus the
threshold voltage of transistor M1. The zener voltage is typi-
cally 23V, and the threshold voltage of M1 is typically 1.5V.
The zener diode is biased by resistor R4.
V
nal load greater than the internal constant current source,
which is the case when the PWM IC starts up. When V
falls below 13.25V, the output of COMP2 will switch from a
logic low to a logic high. The output of COMP2 will clock in a
logic 1 into the D flip flop, causing the D flip flop’s output, Q,
to switch from a logic low to a logic high. Transistor M2 will
then be turned on pulling the gate of transistor M1 to ground,
thereby turning transistor M1 off. Transistor M1 will remain
off as long as V
below 7.0V, COMP1 will reset the D flip flop, thereby turning
transistor M2 off and transistor M1 back on.
OUT
will start to decrease when it is connected to an exter-
OUT
OUT
. An input voltage range of 35 - 450VDC can be
will continue to increase, and the constant cur-
OUT
OUT
is greater than 7.0V. Once V
is less than 7.0V, the output of COMP1
OUT
OUT
REF
trip points of 7.0V for COMP1 and
reaches above 13.25V, the output
, and resistor divider R1, R2, and
IN
pin. The output voltage, V
IN
, V
●
OUT
1235 Bordeaux Drive, Sunnyvale, CA 94089
will start to ramp up
OUT
decreases
IN
, GND,
OUT
OUT
be-
OUT
,
3
Typical Application
Figure 1 shows a simplified typical configuration of a switch-
mode power supply, SMPS, using the Supertex LR745 in the
start-up circuit.
The LR745’s V
PWM IC, Unitrode part #UC3844. An auxiliary winding on
the transformer is used to generate a V
er the PWM IC after start-up. The LR745 is used to supply
power for the PWM IC only during start-up. After start-up, the
LR745 turns off and the auxiliary winding is used to supply
power for the PWM IC. Figure 2 shows the typical current
and voltage waveforms at various stages from power up to
operation powered by the auxiliary winding.
Stage I
Once a voltage is applied on V
charge the V
increase at a rate limited by the internal current limiter of
3.0mA. The PWM IC is in its start-up condition and will typi-
cally draw 0.5mA from the V
tinue to increase until it reaches the PWM IC’s start thresh-
old voltage of typically 16V.
Stage II
Once V
tion and will draw typically 20mA, depending on the operat-
ing frequency and size of the switching MOSFET. The output
of the LR745, V
remaining 17mA will be supplied by C1 causing the V
age decrease. When V
will turn off its output, thereby reducing its input current from
3.0mA to 10s of microamperes. At this point, all 20mA will be
supplied by C1. The PWM IC can now operate to a minimum
V
Once the switching MOSFET starts operating, the energy in
the primary winding is transferred to the secondary outputs
and the auxiliary winding, thereby building up V
essary to size the V
increases to a voltage greater than 10V before V
es to 10V. This allows V
current for the PWM IC.
If for some reason the auxiliary voltage does not reach 10V,
V
the PWM IC will return to its start-up condition. The PWM
IC will now only draw 0.5mA. V
but at a much slower rate. Once V
the LR745 will turn the output, V
charging C1 as described in Stage I.
CC
CC
voltage of typically 10V.
will continue to decrease. Once V
CC
reaches 16V, the PWM IC is in its operating condi-
●
Tel: [phone]
CC
OUT
OUT
capacitor, C1. The V
, is internally current limited to 3.0mA. The
terminal is connected to the V
CC
storage capacitor, C1, such that V
CC
AUX
decreases to 13.25V, the LR745
to supply the required operating
CC
●
line. The V
www.supertex.com
CC
OUT
IN
CC
, the LR745 will start to
will continue to decrease
, back on. V
decreases below 7.0V,
CC
CC
voltage will start to
CC
CC
goes below 10V,
voltage will con-
voltage to pow-
AUX
OUT
CC
CC
LR745
. It is nec-
decreas-
will start
line of a
CC
volt-
AUX