LTC1438_15 Datasheet, PDF (20/32 Pages) Linear Technology – Dual High Efficiency, Low Noise, Synchronous Step-Down Switching Regulators

LTC1438/LTC1439
APPLICATIONS INFORMATION
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_OAUX 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 V_OAUX is set as follows:
V_OAUX
=
1.19V ^{⎛⎝⎜}1+
R8^⎞
R7⎠⎟
<
8V
V_OAUX = 12V
AUXDR < 8.5V
AUXDR ≥ 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.
SECONDARY
WINDING
1:N
( ) ^{VSEC = 1.19V 1 +}
R6
R5
> 13V
V_SEC
+
R6
AUXDR
SFB1 AUXFB
R5 LTC1439
V_OAUX
+ ^12V
10µF
AUXON ON/OFF
1438 F11a
Figure 11a. 12V Output Auxiliary Regulator
Using Internal Feedback Resistors
SECONDARY
WINDING
1:N
( ) ^{VSEC = 1.19V 1 +}
R6
R5
> V_OAUX
V_SEC
+
R6
AUXDR
SFB1 AUXFB
R5 _LTC1439
V_OAUX
R8
+
R7
10µF
AUXON ON/OFF
1438 F11b
Figure 11b. 5V Output Auxiliary Regulator Using
External Feedback Resistors
V_PULL-UP < 7.5V
ON/OFF
INPUT
AUXON
AUXFB _–
LTC1439
AUXDR
+
1.19V REFERENCE
OUTPUT
1438 F11c
Figure 11c. Auxiliary Comparator Configuration
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