NC12S15A_01102008
7
DESIGN CONSIDERATIONS
FEATURES DESCRIPTIONS
ENABLE (On/Off)
The NC15 is a single phase and voltage mode controlled
Buck topology. Block diagram of the converter is shown in
Figure 23. The output can be trimmed in the range of
0.9Vdc to 5.0Vdc by a resistor from Trim pin to Ground.
The ENABLE (on/off) input allows external circuitry to put
the NC converter into a low power dissipation (sleep)
mode. Positive (active-high) ENABLE is available as
standard.
The converter can be turned ON/OFF by remote control.
Positive on/off (ENABLE pin) logic implies that the
converter DC output is enabled when this signal is driven
high (greater than 2.4V) or floating and
disabled when
the
signal is driven low (below 0.8V). Negative on/off logic is
optional and could also be ordered.
Positive ENABLE (active-high) units of the NC series are
turned on if the ENABLE pin is high or floating. Pulling the
pin low will turn off the unit. With the active high function,
the output is guaranteed to turn on if the ENABLE pin is
driven above 2.4V. The output will turn off if the ENABLE
pin voltage is pulled below .8V.
The converter provides an open collector signal called
Power Good. The power good signal is pulled low when
output is not within ±10% of Vout or Enable is OFF.
The ENABLE input can be driven in a variety of ways as
shown in Figures 24, 25 and 26. If the ENABLE signal
comes from the primary side of the circuit, the ENABLE
can be driven through either a bipolar signal transistor
(Figure 24) or a logic gate (Figure 25). If the enable signal
comes from the secondary side, then an opto-coupler or
other isolation devices must be used to bring the signal
across the voltage isolation (please see Figure 26).
The converter can protect itself by entering hiccup mode
against over current and short circuit condition. Also, the
converter will shut down when an over voltage protection
is detected.
NC6A/15A/20A
Vout
Ground
Trim
Enable
Ground
Vin
Figure 23:
Block Diagram
Figure 24:
Enable Input drive circuit for NC series
NC6A/15A/20A
Vout
Ground
Trim
Enable
Ground
Vin
5V
Safety Considerations
It is recommended that the user to provide a very
fast-acting type fuse in the input line for safety. The output
voltage set-point and the output current in the application
could define the current rating of the fuse.
Figure 25:
Enable input drive circuit using logic gate.
NC6A/15A/20A
Vout
Ground
Trim
Enable
Ground
Vin
Figure 26
: Enable input drive circuit example with isolation.
