LTC1539 LINER [Linear Technology], LTC1539 Datasheet - Page 20

• Manufacturer Part Number: LTC1539
• Description: Dual High Efficiency, Low Noise, Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

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-
LTC1538-AUX/LTC1539
APPLICATIONS
20
Figure 11a. 12V Output Auxiliary Regulator
Using Internal Feedback Resistors
+
SECONDARY
V
WINDING
SEC
1:N
R6
R5
LTC1538-AUX/
U
SFB1
OAUX
LTC1539
AUXDR
AUXON
AUXFB
INFORMATION
V
U
SEC
at the collector of the external
= 1.19V
ON/OFF
1 +
W
R6
R5
ON/OFF
+
INPUT
Figure 11c. Auxiliary Comparator Configuration
> 13V
10 F
V
12V
1538 F11a
OAUX
AUXON
AUXFB
U
LTC1538-AUX/LTC1539
–
+
1.19V REFERENCE
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
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.
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
often useful to analyze individual losses to determine what
is limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
V
V
OAUX
OAUX
AUXDR
+
Figure 11b. 5V Output Auxiliary Regulator Using
External Feedback Resistors
SECONDARY
V
V
WINDING
PULL-UP
SEC
1:N
1 19
12V
R6
R5
.
7.5V
OUTPUT
SFB1
LTC1538-AUX/
V
1538 F11c
LTC1539
1
OAUX
AUXDR
AUXON
AUXFB
V
R
R
SEC
7
8
is set as follows:
= 1.19V
ON/OFF
8V AUX DR < 8.5V
1 +
R6
R5
R8
R7
AUX DR
> (V
OAUX
1538 F11b
+ 1V)
+
10 F
12V
V
OAUX