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DS_NC12S30A_05222008
9
FEATURES DESCRIPTIONS (CON.)
Output Voltage Programming
The output voltage of the NC series is trimmable by
connecting an external resistor between the trim pin and
output ground as shown Figure 31 and the typical trim
resistor values are shown in Figure 32. The output can
also be set by an external voltage connected to trim pin as
shown in Figure 32.
The NC30A/40A module has a trim range of 0.9V to
5.0V. A plot of trim behavior is shown in Figure 33
Cout
-SENSE
+SENSE
GROUND
Rs
Vout
TRIM
Figure 31: Trimming Output Voltage
The NC30/NC40 modules have a trim range of 0.9V to
5.0V. The trim resistor equation for the them is :
Rs (k
Ω) =
9.0
69.12
−
−
Vout
Vout
Vout is the desired voltage setpoint,
Rs is the trim resistance between TRIM and Ground,
Rs values should not be less than 1.8 k
Ω
Output Voltage Rs(Ω)
+0.9 V OPEN
+1.2 V 38.3K
+1.5 V 18.7K
+1.8 V 12.1K
+2.5 V 6.34K
+3.3 V 3.92K
+5.0 V 1.87K
Figure 32: Typical trim resistor values
Rt
Cout
Vt
-SENSE
+SENSE
GROUND
Rs
Vout
TRIM
Figure 33: Output voltage trim with voltage source
To use voltage trim, the trim equation for the NC30 is (please
refer to Fig. 33) :
69.12)1(9.0
)69.121.13(
)(
++−
−
+
=Ω
RsVoutRs
VoutVtRs
kRt
Vout is the desired output voltage
Vt is the external trim voltage
Rs is the resistance between Trim and Ground (in KΩ)
Rt is the resistor to be defined with the trim voltage (in KΩ)
Below is an example about using this voltage trim equation :
Example：
If Vt = 1.25V, desired Vout = 2.5V and Rs = 1 k
Ω
Ω=
++−
−
+
=Ω k
RsVoutRs
VoutVtRs
kRt 72.0
69.12)1(9.0
)69.121.13(
)(
Power Good
The converter provides an open collector signal called Power
Good. This output pin uses positive logic and is open
collector. This power good output is able to sink 5mA and set
high when the output is within ±10% of output set point. The
power good signal is pulled low when output is not within
±10% of Vout or Enable is OFF.
Output Capacitance
There is no output capacitor on the NC series modules.
Hence, an external output capacitor is required for stable
operation. For NC30 modules, two external 6.3V/680μF
output low ESR capacitors in parallel (for example, OSCON)
are required for stable operation.
It is important to places these low ESR capacitors as close to
the load as possible in order to get improved dynamic
response and better voltage regulation, especially when the
load current is large. Several of these low ESR capacitors
could be used together to further lower the ESR.
Please refer to individual datasheet for the maximum allowed
start-up load capacitance for each NC series as it is varied
between series.