LTC1436A-PLL_15 Datasheet, PDF (19/28 Pages) Linear Technology – High Efficiency Low Noise Synchronous Step-Down Switching Regulators

LTC1436A
LTC1436A-PLL/LTC1437A
APPLICATIONS INFORMATION
The divided down voltage at the negative (–) input to the
comparator is compared to an internal 1.19V reference. A
20mV hysteresis is built in to assure rapid switching. The
output is an open drain MOSFET and requires a pull-up
resistor. This comparator is not active in shutdown. The
low side of the resistive divider should connect to SGND.
SFB Pin Operation
When the SFB pin drops below its ground-referenced
1.19V threshold, continuous mode operation is forced. In
continuous mode, the large N-channel main and synchro-
nous switches are used regardless of the load on the main
output.
In addition to providing a logic input to force continuous
synchronous operation, the SFB pin provides a means to
regulate a flyback winding output. Continuous synchro-
nous operation allows power to be drawn from the auxil-
iary windings without regard to the primary output load.
The SFB pin provides a way to force continuous synchro-
nous operation as needed by the flyback winding.
The secondary output voltage is set by the turns ratio of
the transformer in conjunction with a pair of external
resistors returned to the SFB pin as shown in Figure 4a.
The secondary regulated voltage V_SEC in Figure 4a is
given by:
( ) ^VSEC ^≈
N
+
1
V_OUT
>
1.19V
⎛
⎝⎜¹⁺
R6^⎞
R5⎠⎟
where N is the turns ratio of the transformer and V_OUT is
the main output voltage sensed by V_OSENSE.
Auxiliary Regulator/Comparator
The auxiliary regulator/comparator can be used as a
comparator or low dropout regulator (by adding an exter-
nal PNP pass device).
When the voltage present at the AUXON pin is greater than
1.19V the regulator/comparator is on. Special circuitry
consumes a small (20µ A) bias current while still remain-
ing stable when operating as a low dropout regulator. No
excess current is drawn when the input stage is overdriven
when used as a comparator.
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 12a, 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 SFB pin
regulates the input voltage to the PNP regulator (see SFB
Pin Operation) and should be set to approximately 1V to
2V above the required output voltage of the auxiliary
regulator. A Zener diode clamp may be required to keep
V_SEC 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 resistive divider is available to provide a 12V
output by simply connecting AUXFB directly to the collec-
tor of the external PNP. The internal resistive divider is
selected when the voltage at AUXFB goes above 9.5V with
1V built-in hysteresis. For other output voltages, an exter-
nal resistive 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 AUXDR < 8.5V
V_OAUX = 12V
AUXDR > 12V
The circuit can also be used as a noninverting voltage
comparator as shown in Figure 11c. When AUXFB drops
below 1.19V, the AUXDR pin will be pulled low. A mini-
mum current of 5µA is required to pull the AUXDR pin to
5V when used as a comparator output, in order to coun-
teract a 1.5µA internal current source.
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