Rev 1.1a

Copyright

© 1994

11861 Western Avenue

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SG1549/SG2549/SG3549

FIGURE 2 — CURRENT CONTROL FOR A BUCK REGULATOR WITH

CONSTANT DRIVE CURRENT

FIGURE 4 — POWER BOOST AND CURRENT CONTROL WITH THE SG1627

FIGURE 3 — A PUSH-PULL CONVERTER WITH LOW-LINE

EMITTER CURRENT SENSING

Another method of introducing the current shutdown signal is shown in

Figure 2 where the SG1524 is used to activate a constant drive current

to the high-current switch, in this case an SM600. The 2N2222 forms a

constant current generator when driven from the SG1524’s 5.0 volt

reference through a 1K resistor. This transistor is then switched off by

the LO OUTPUT transistor in the SG1549, achieving the fastest re-

sponse to the output of the regulator.

LOW LINE SENSING - In many types of feed-forward or push-pull

converters, current protection may be provided by sensing in an emitter

resistor referenced to ground on the primary side of an output trans-

former. The fast-reacting SG1549 can easily sense secondary overload

as reflected back to the primary and, additionally, provide protection

from unbalanced transformer saturation.

When using the LO CM inputs, the HI CM inputs should be shorted

together. While the LO CM inputs may be connected directly across a

sense resistor, R

required to eliminate high frequency transients. It must be remembered

that the 500

Ωinput impedance at the LO CM terminals will cause the use

of R1 to increase the effective threshold; however, this also offers the

possibility of an easily adjustable threshold by incorporating a potenti-

ometer at the input.

Coupling the output signal from the SG1549 to the control chip may be

done in several ways including the use of either the Compensation or

Shutdown pins on the SG1524 as described earlier.

Another convenient way to tie the output of the SG1549 into the PWM

control in higher power applications is by using the SG1627 Dual

Interface Driver and connecting the LO OUTPUT terminal of the Sg1549

directly to the two Non-Inverting inputs of the SG1627 as shown in Figure

4.

And finally, keep in mind that the LO OUTPUT terminal of the SG1549

will easily drive most high-speed optical couplers should some type of

isolation between current sense and shutdown control be required.

Ambient

Temperature Range

Part No.

Package

Connection Diagram

8-PIN CERAMIC DIP

Y - PACKAGE

8

7

6

5

2

1

3

4

— HI CM INPUT

+ HI CM INPUT

+ LO CM INPUT

— LO CM INPUT

CLOCK RESET

+V

S

LO OUTPUT

HI OUTPUT

SG1549Y/883B

-55

°C to 125°C

SG1549Y/DESC

-55

°C to 125°C

SG1549Y

-55

°C to 125°C

SG2549Y

-25

°C to 85°C

SG3549Y

0

°C to 70°C

8-PIN PLASTIC DIP

M - PACKAGE

SG2549M

-25

°C to 85°C

M Package: RoHS / Pb-free 100% Matte Tin Lead Finish

SG3549M

0

°C to 70°C

M Package: RoHS Compliant / Pb-free Transition DC: 0503

Note 1. Contact factory for JAN and DESC product availability.

2. All parts are viewed from the top.

D Package Pb-free transition DC:0440 100% Pb-free Matte Tin Lead Finish