3

LR745

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

OUT

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.

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.