DC1307A Datasheet, PDF (9/22 Pages) Linear Technology – Complete Switch Mode Power Supply

LTM8027
Pin Functions
V_IN (Bank 3): The V_IN pins supply current to the LTM8027’s
internal regulator and to the internal power switch. These
pins must be locally bypassed with an external, low ESR
capacitor (see Table 2).
V_OUT (Bank 1): Power Output Pins. Apply the output filter
capacitor and the output load between these and the GND
pins.
AUX (Pin A7): Low Current Voltage Source for BIAS1
and BIAS2. In many designs, the BIAS pin is connected
to V_OUT by way of the AUX pin. The AUX pin is internally
connected to V_OUT and is placed near the BIAS pins to ease
printed circuit board routing. Although this pin is internally
connected to V_OUT, do NOT connect this pin to the load. If
this pin is not tied to BIAS1 and BIAS2, leave it floating.
BIAS1 (Pin A6): The BIAS1 pin connects to the internal
power bus. Connect to a power source greater than 8.5V.
If the output is greater than 8.5V, connect it to this pin. If
the output voltage is less, connect this to a voltage source
between 8.5V and 15V.
BIAS2 (Pin A3): Internal Biasing Power. Connect to AUX
(if 24V or less) or a voltage source above 3V. Do not leave
BIAS2 floating.
RUN (Pin A4): Tie the RUN pin to ground to shut down the
LTM8027. Tie to 1.4V or more for normal operation. The
RUN pin is internally clamped to 5V, so when it is pulled
up, be sure to use a pull-up resistor that limits the cur-
rent in to the RUN pin to less than 1mA. If the shutdown
feature is not used, tie this pin to the V_IN pin through a
pull-up resistor.
GND (Bank 2): Tie these GND pins to a local ground plane
below the LTM8027 and the circuit components.
RT (Pin B1): The RT pin is used to program the switching
frequency of the LTM8027 by connecting a resistor from
this pin to ground. The Applications Information section of
the data sheet includes a table to determine the resistance
value based on the desired switching frequency. Minimize
capacitance at this pin.
SYNC (Pin C1): The SYNC pin provides an external clock
input for synchronization of the internal oscillator. The R_T
resistor should be set such that the internal oscillator fre-
quency is 10% to 25% below the external clock frequency.
This external clock frequency must be between 100kHz and
500kHz. If unused, the SYNC pin is connected to GND. For
more information see Oscillator Sync in the Application
Information section of this data sheet.
ADJ (Pin A2): The LTM8027 regulates its ADJ pin to 1.23V.
Connect the adjust resistor from this pin to ground. The
value of R_ADJ is given by the equation:
R_ADJ = 613.77/(V_OUT – 1.23)
where R_ADJ is in kΩ.
SS (Pin A5): The soft-start pin is used to program the
supply soft-start function. Use the following formula to
calculate C_SS for a given output voltage slew rate:
C_SS = 2µA(t_SS/1.231V)
The pin should be left unconnected when not using the
soft-start function.
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