LTC1439 Linear Technology, LTC1439 Datasheet - Page 20

• Manufacturer Part Number: LTC1439
• Description: Dual High Efficiency/ Low Noise/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

LTC1438/LTC1439
APPLICATIONS
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 V
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 V
20
V
V
OAUX
OAUX
1 19
12V
.
V
U
1
OAUX
OAUX
R
R
INFORMATION
8
7
U
is set as follows:
at the collector of the external
8V AUXDR < 8.5V
W
AUXDR
U
12V
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 A is required to pull
up the AUXDR pin to 5V when used as a comparator output
in order to counteract a 1.5 A internal pull-down current
source.
ON/OFF
+
INPUT
SECONDARY
Figure 11c. Auxiliary Comparator Configuration
V
Figure 11b. 5V Output Auxiliary Regulator Using
External Feedback Resistors
WINDING
SEC
Figure 11a. 12V Output Auxiliary Regulator
Using Internal Feedback Resistors
+
1:N
SECONDARY
V
AUXON
AUXFB
WINDING
R6
R5
SEC
1:N
SFB1
R6
R5
LTC1439
+
–
AUXDR
AUXON
1.19V REFERENCE
SFB1
AUXFB
V
LTC1439
LTC1439
SEC
AUXDR
AUXON
= 1.19V
AUXFB
V
SEC
ON/OFF
= 1.19V
1 +
AUXDR
ON/OFF
R6
R5
R8
R7
1 +
V
> V
PULL-UP
OAUX
R6
R5
1438 F11b
+
> 13V
+
10 F
7.5V
V
12V
OUTPUT
1438 F11a
OAUX
1438 F11c
10 F
V
OAUX