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LR745N8-G

High Input Voltage SMPS Start-up

Supertex Inc 

LR745

Stage III

At this stage the LR745’s output is turned off and the PWM

IC is operating from the VAUX supply. The auxiliary voltage,

VAUX, can be designed to vary anywhere between the mini-

mum operating VCC voltage of the PWM IC (10V) to the maxi-

mum auxiliary voltage rating of the LR745 (22V).

Design Considerations

Consider for example, a PWM IC with a switching frequency

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 auxiliary voltage to 10V,

the minimum value of C1 is calculated as follows:

1

100KHz

x (100) x (20mA)

C1 =

(16V -10V)

I. 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 auxiliary supply is available. The

value of C1 can be approximated by the following equation:

1

f

x (N) x (l)

C1 = (VSTART - VMIN)

where,

f = switching frequency

N = number of clock cycles required to chargeVAUX to

VMIN value

I = PWM operating current

VSTART = PWM IC start threshold rating

VMIN = PWM IC minimum VCC operating voltage

C1 = 3.3µF

II. 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 3 is recommended.

In Figure 3, 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 mi-

nus 5.0V) because the ground of the LR745 is now at 5.0V.

The LR745 will immediately turn off its output, VOUT, with-

out 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.

Figure 3: Using VREF for the LR745 Ground Voltage

VIN

LR7

VOUT

GND

VCC

VREF

C1

PWM IC

UC3844

● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com

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