5

LR745

Figure 3: Using V

Stage III

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

IC is operating from the V

V

mum operating V

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

Design Considerations

I. Calculating the value for C1

Sizing the V

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:

where,

f = switching frequency

N = number of clock cycles required to chargeV

V

I = PWM operating current

V

V

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:

C1 = 3.3µF

II. SMPS with wide minimum to maximum load

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

must discharge to below the nominal V

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 V

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 V

such conditions, the circuit in Figure 3 is recommended.

In Figure 3, the V

ground pin of the LR745. The V

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

reaches the start threshold voltage. Once V

start threshold voltage, V

5.0V. During start-up, the LR745 will be on, and V

to increase up to 16V. Once V

will start to operate and V

The LR745 will see an effective V

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

The LR745 will immediately turn off its output, V

out having to wait for the V

switching from 0 to 5.0V during start is a common feature in

most PWM ICs.

GND

V

IN

V

OUT

V

CC

V

REF

C1

PWM IC

UC3844

LR7

C1 =

1

f x (N) x (l)

(V

C1 =

1

100KHz x (100) x (20mA)

(16V -10V)