LR745 datasheet(5/5 Pages) SUTEX

5 / 5 page

(Continued from page 14-7)

If for some reason the auxiliary voltage does not reach 10V, V

will continue to decrease. Once V

CC goes below 10V, the PWM

IC will return to its start-up condition. The PWM IC will now only

draw 0.5mA. V

CC will continue to decrease but at a much slower

rate. Once V

CC decrease below 7.0V, the LR7 will turn the

output, V

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

Stage I.

Stage III

At this stage the LR7’s output is turned off and the PWM IC is

operating from the V

AUX supply. The auxiliary voltage, VAUX, can

be designed to vary anywhere between the minimum operating

CC voltage of the PWM IC (10V) to the maximum auxiliary

voltage rating of the LR7 (22V).

Design Considerations

I. Calculating the value for C1

Sizing the V

CC 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

approximately by the following equation:

START

MIN









( )×( )

−

(

)

where,

switching frequency

number of clock cycles required to charge

AUX to VMIN value

PWM operating current

START

PWM IC start threshold rating

MIN

PWM IC minimum V

CC operating voltage

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=

100KHZ

100

20mA

16V -10V









( )×

(

)

(

)

C1= 3.3

µF

II. SMPS with wide minimum to maximum load

An important point is that the LR7’s output voltage, V

OUT, must

discharge to below the nominal V

OFF 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 V

CC 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 as

SMPS may power up too fast with a small load, not allowing the

CC voltage to fall below 13.25V. For such conditions, the circuit

in Figure 3 is recommended.

In Figure 3, the V

REF pin of the UC3844 is used to bias the ground

pin of the LR7. The V

REF pin on the UC3844 is a 5.0V reference,

which stays at 0V until the V

CC voltage reaches the start thresh-

old voltage. Once V

CC reaches the start threshold voltage, VREF

will switch digitally from 0V to 5.0V. During start-up, the LR7 will

be on and V

CC will start to increase up to 16V. Once VCC reaches

16V, the UC3844 will start to operate and V

REF will increase from

0V to 5.0V. The LR7 will see an effective V

OUT voltage of 11V

(16V minus 5.0V) because the ground of the LR7 is now at 5.0V.

The LR7 will immediately turn off its output V

OUT without having

to wait for the V

CC voltage to decrease. The VREF switching from

0 to 5V during start is a common feature in most PWM ICs.

LR745

LR7

PWM IC

UC3844

GND

VIN

VOUT

VCC

VREF

Figure 3: Using V

REF for the LR7 Ground Voltage

1235 Bordeaux Drive, Sunnyvale, CA 94089

TEL: (408) 744-0100 • FAX: (408) 222-4895

www.supertex.com

11/12/01

LR745 датащи(PDF) 5 Page - Supertex, Inc

LR745 датащи(HTML) 5 Page - Supertex, Inc

Html Pages

1 2 3 4 5

датащи скачать

ссылки URL