LTC1436AIGN datasheet(19/28 Pages) LINER

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LTC1436A

LTC1436A-PLL/LTC1437A

APPLICATIONS INFORMATION

The divided down voltage at the negative (–) input to the

comparator is compared to an internal 1.19V reference. A

20mV hysteresis is built in to assure rapid switching. The

output is an open drain MOSFET and requires a pull-up

resistor. This comparator is

not active in shutdown. The

low side of the resistive divider should connect to SGND.

SFB Pin Operation

When the SFB pin drops below its ground-referenced

1.19V threshold, continuous mode operation is forced. In

continuous mode, the large N-channel main and synchro-

nous switches are used regardless of the load on the main

output.

In addition to providing a logic input to force continuous

synchronous operation, the SFB pin provides a means to

regulate a flyback winding output. Continuous synchro-

nous operation allows power to be drawn from the auxil-

iary windings without regard to the primary output load.

The SFB pin provides a way to force continuous synchro-

nous operation as needed by the flyback winding.

The secondary output voltage is set by the turns ratio of

the transformer in conjunction with a pair of external

resistors returned to the SFB pin as shown in Figure 4a.

The secondary regulated voltage VSEC in Figure 4a is

given by:

SEC

OUT

≈+

() >+









1 19

where N is the turns ratio of the transformer and VOUT is

the main output voltage sensed by VOSENSE.

Auxiliary Regulator/Comparator

The auxiliary regulator/comparator can be used as a

comparator or low dropout regulator (by adding an exter-

nal PNP pass device).

When the voltage present at the AUXON pin is greater than

1.19V the regulator/comparator is on. Special circuitry

consumes a small (20

µA) bias current while still remain-

ing stable when operating as a low dropout regulator. No

excess current is drawn when the input stage is overdriven

when used as a comparator.

The AUXDR pin is internally connected to an open drain

MOSFET which can sink up to 10mA. The voltage on

AUXDR determines whether or not an internal 12V resis-

tive divider is connected to AUXFB as described below. A

pull-up resistor is required on AUXDR and the voltage

must not exceed 28V.

With the addition of an external PNP pass device, a linear

regulator capable of supplying up to 0.5A is created. As

shown in Figure 12a, the base of the external PNP con-

nects to the AUXDR pin together with a pull-up resistor.

The output voltage VOAUX at the collector of the external

PNP is sensed by the AUXFB pin.

The input voltage to the auxiliary regulator can be taken

from a secondary winding on the primary inductor as

shown in Figure 11a. In this application, the SFB pin

regulates the input voltage to the PNP regulator (see SFB

Pin Operation) and should be set to approximately 1V to

2V above the required output voltage of the auxiliary

regulator. A Zener diode clamp may be required to keep

VSEC under the 28V AUXDR pin specification when the

primary is heavily loaded and the secondary is not.

The AUXFB pin is the feedback point of the regulator. An

internal resistive divider is available to provide a 12V

output by simply connecting AUXFB directly to the collec-

tor of the external PNP. The internal resistive divider is

selected when the voltage at AUXFB goes above 9.5V with

1V built-in hysteresis. For other output voltages, an exter-

nal resistive divider is fed back to AUXFB as shown in

Figure 11b. The output voltage VOAUX is set as follows:

VOAUX = 1.19V(1+R8/R7) < 8V

AUXDR < 8.5V

VOAUX = 12V

AUXDR > 12V

The circuit can also be used as a noninverting voltage

comparator as shown in Figure 11c. When AUXFB drops

below 1.19V, the AUXDR pin will be pulled low. A mini-

mum current of 5

µA is required to pull the AUXDR pin to

5V when used as a comparator output, in order to coun-

teract a 1.5

µA internal current source.

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