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LR745

High-Input Voltage SMPS Start-up

Microchip Technology 

LR745

3.0 APPLICATION INFORMATION

Figure 3-1 shows a simplified typical configuration of a

switch mode power supply, SMPS, using LR745 in the

start-up circuit.

LR745’s VOUT terminal is connected to the VCC line of

a PWM IC. An auxiliary winding on the transformer

generates a VCC voltage to power the PWM IC after

start-up. LR745 supplies power for the PWM IC only

during start-up. After start-up, LR745 turns off and the

auxiliary winding supplies power for the PWM IC.

Figure 3-2 shows the typical current and voltage wave-

forms at various stages from power-up to operation

powered by the auxiliary winding.

3.1 Stage I

Once a voltage is applied on VIN, LR745 starts to

charge the VCC capacitor, C1. The VCC voltage starts to

increase at a rate limited by the internal current limiter

of 3.0mA. The PWM IC is in its start-up condition and

will typically draw 0.5mA from the VCC line. The VCC

voltage will continue to increase until it reaches the

PWM IC’s start threshold voltage, typically 16V.

3.2 Stage II

Once VCC reaches 16V, the PWM IC is in its operating

condition and will typically draw 20mA, depending on

the operating frequency and size of the switching

metal–oxide–semiconductor field-effect transistor

(MOSFET). The output of LR745, VOUT, is internally

current limited to 3.0mA. The remaining 17mA will be

supplied by C1, causing the VCC voltage decrease.

When VCC decreases to 13.25V, LR745 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 min-

imum VCC voltage, typically 10V.

Once the switching MOSFET starts operating, the

energy in the primary winding is transferred to the sec-

ondary outputs and the auxiliary winding, thereby build-

ing up VAUX. It is necessary to size the VCC storage

capacitor, C1, such that VAUX increases to a voltage

greater than 10V before VCC decreases to 10V. This

allows VAUX to supply the required operating current for

the PWM IC.

If for some reason the auxiliary voltage does not reach

10V, VCC will continue to decrease. Once VCC goes

below 10V, the PWM IC will return to its start-up condi-

tion. The PWM IC will now only draw 0.5mA. VCC will

continue to decrease but at a much slower rate. Once

VCC decreases below 7.0V, LR745 will turn the output,

VOUT, back on. VOUT will start charging C1 as described

in Stage I.

3.3 Stage III

At this stage, LR745 output is turned off and the PWM

IC is operating from the VAUX supply. The auxiliary volt-

age, VAUX, can be designed to vary anywhere between

the minimum operating VCC voltage of the PWM IC

(10V) to the maximum auxiliary voltage rating of the

LR745 (22V).

FIGURE 3-1:

SIMPLIFIED SMPS USING LR745

VIN

IIN

High Voltage

VAUX

IAUX

C2 D2

LR7

VOUT

VCC

GND

C1

PWM IC

 2015 Microchip Technology Inc.

DS20005394A-page 5

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