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LR745

High-Input Voltage SMPS Start-up

Microchip Technology 

LR745

4.0 DESIGN CONSIDERATIONS

To ensure the best design using LR745, evaluate the

value of C1 and the SMPS requirements.

4.1 Calculating the value for C1

Sizing the VCC capacitor, C1, is an important factor.

Making C1 too large will cause the SMPS to power up

too slowly. However, if too small, C1 will not allow the

SMPS to power up due to insufficient charge in the

capacitor to power the IC and MOSFET until the auxil-

iary supply is available. The value of C1 can be approx-

imated by the following equation:

C1 = -V----S---1-Tf---A---R----T---N-–----V----M-1---I-N--

Definitions:

- f = switching frequency

- N = number of clock cycles required to

charge VAUX to VMIN value

- I = PWM operating current

- VSTART = PWM IC start threshold rating

- VMIN = PWM IC minimum VCC operating volt-

age

Consider for example, a PWM IC with a switching fre-

quency of 100KHz, operating current of 20mA, start

threshold of 16V, and a minimum operating voltage of

10V. If 100 clock cycles are required to charge the aux-

iliary voltage to 10V, the minimum value of C1 is calcu-

lated as follows:

C1 = ---1--------0------0-----1---k------H----1------z6-----V------–-1---1-0--0-0---V------2---0----m------A--

4.2 SMPS with wide minimum to

maximum load

An important point is that the LR745’s output voltage,

VOUT, must discharge to below the nominal VOFF trip

point of 13.25V in order for its output to turn off. If the

SMPS requires a wide minimum to maximum output

load variation, it will be difficult to guarantee that VCC

will fall below 13.25V under minimum load conditions.

Consider an SMPS that is required to power small as

well as large loads and is also required to power up

quickly. Such a SMPS may power up too fast with a

small load, not allowing the VCC voltage to fall below

13.25V. For such conditions, the circuit in Figure 4-1 is

recommended.

In Figure 4-1, the VREF pin of the UC3844 is used to

bias the ground pin of the LR745. The VREF pin on the

UC3844 is a 5.0V reference, which stays at 0V until the

VCC voltage reaches the start threshold voltage. Once

VCC reaches the start threshold voltage, VREF will

switch digitally from 0V to 5.0V. During start-up, the

LR745 will be on, and VCC will start to increase up to

16V. Once VCC reaches16V, the UC3844 will start to

operate and VREF will increase from 0V to 5.0V. The

LR745 will see an effective VOUT voltage of 11V (16V

minus 5.0V) because the ground of the LR745 is now

at 5.0V. The LR745 will immediately turn off its output,

VOUT, without having to wait for the VCC voltage to

decrease. The VREF switching from 0 to 5.0V during

start is a common feature in most PWM ICs.

C1 = 3.3F

FIGURE 4-1:

USING VREF FOR GROUND VOLTAGE

VIN

LR7

GND

VOUT

C1

VCC

VREF

PWM IC

DS20005394A-page 8

 2015 Microchip Technology Inc.

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