LTC1539 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1539
Linear Technology
'LTC1539' PDF : 32 Pages View PDF
LTC1538-AUX/LTC1539
APPLICATIONS INFORMATION
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:
VOAUX = 1.19V 1+ RR78 < 8V AUX DR < 8.5V
VOAUX = 12V
AUX DR ≥ 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 5µA is required to pull up
the AUXDR pin to 5V when used as a comparator output in
order to counteract a 1.5µA 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 + ...)
SECONDARY
WINDING
1:N
( ) VSEC = 1.19V
1+
R6
R5
> 13V
VSEC
+
R6
AUXDR
SFB1 AUXFB
R5 LTC1538-AUX/
LTC1539
AUXON
+
ON/OFF
VOAUX
12V
10µF
1538 F11a
Figure 11a. 12V Output Auxiliary Regulator
Using Internal Feedback Resistors
SECONDARY
WINDING
1:N
( ) VSEC = 1.19V 1 +
R6
R5
> (VOAUX + 1V)
VSEC
+
R6
AUXDR
SFB1 AUXFB
R5 LTC1538-AUX/
LTC1539
AUXON
R8
R7
ON/OFF
VOAUX
+
10µF
1538 F11b
Figure 11b. 5V Output Auxiliary Regulator Using
External Feedback Resistors
VPULL-UP < 7.5V
ON/OFF
INPUT
LTC1538-AUX/LTC1539
AUXON
AUXDR
AUXFB –
+
1.19V REFERENCE
OUTPUT
1538 F11c
Figure 11c. Auxiliary Comparator Configuration
20
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