LTC1439CG datasheet(20/32 Pages) LINER

20 / 32 page

LTC1438/LTC1439

APPLICATIONS INFORMATION

When used as a voltage comparator as shown in Figure

11c, the auxiliary block has a noninverting characteristic.

When AUXFB drops below 1.19V, the AUXDR pin will be

pulled low. A minimum current of 5

µA is required to pull

up the AUXDR pin to 5V when used as a comparator output

in order to counteract a 1.5

µA internal pull-down current

source.

When the voltage present at the AUXON pin is greater than

1.19V the regulator/comparator is on. The amplifier is

stable when operating as a low dropout regulator. This

same amplifier can be used as a comparator whose

inverting input is tied to the 1.19V reference.

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 11a, 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 SFB1 pin

regulates the input voltage to the PNP regulator (see SFB1

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

2V above the required output voltage of the auxiliary

regulator. A Zener clamp diode may be required to keep the

secondary winding resultant output voltage 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 resistor divider is available to provide a 12V output

by simply connecting AUXFB directly to the collector of the

external PNP. The internal resistive divider is switched in

when the voltage at AUXFB goes above 9.5V with 1V built-

in hysteresis. For other output voltages, an external resis-

tive divider is fed back to AUXFB as shown in Figure 11b.

The output voltage VOAUX is set as follows:

OAUX =+









=≥

119

8V AUXDR < 8.5V

12V

AUXDR

12V

OAUX

–

AUXON

AUXFB

ON/OFF

INPUT

VPULL-UP < 7.5V

AUXDR

OUTPUT

1438 F11c

1.19V REFERENCE

LTC1439

Figure 11c. Auxiliary Comparator Configuration

LTC1439

AUXDR

AUXFB

SFB1

AUXON

1438 F11a

VSEC

SECONDARY

WINDING

1:N

ON/OFF

VOAUX

12V

µF

VSEC = 1.19V

> 13V

1 +

()

Figure 11a. 12V Output Auxiliary Regulator

Using Internal Feedback Resistors

Figure 11b. 5V Output Auxiliary Regulator Using

External Feedback Resistors

LTC1439

AUXDR

AUXFB

SFB1

AUXON

1438 F11b

VSEC

SECONDARY

WINDING

1:N

ON/OFF

VOAUX

µF

VSEC = 1.19V

> VOAUX

1 +

()

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