NCV97310MW50GEVB
http://onsemi.com
7
APPLICATION INFORMATION
Output Voltage Selection
The voltage outputs for switcher 2 and switcher 3 are
adjustable and can be set with a resistor divider. The FB
reference for both switchers is 1.2 V.
VOUT 2 (VOUT 3)
FBx
= 1 . 2 V
R
UPPER
R
LOWER
The upper resistor is set to 10 k
W
and is part of the
feedback loop. To maintain stability over all conditions, it is
recommended to change the only the lower feedback resistor
to set the output voltage. Use the following equation:
R
LOWER
+
R
UPPER
V
FB
V
OUT
*
V
FB
Some common setups are listed below:
Desired
Output (V)
VREF (V)
R
UPPER
(k
W
, 1%)
R
LOWER
(k
W
, 1%)
1.2
1.2
10.0
NP
1.5
1.2
10.0
40.0
1.8
1.2
10.0
20.0
2.5
1.2
10.0
9.31
3.3
1.2
10.0
5.76
Spread Spectrum
In SMPS devices, switching translates to higher
efficiency. Unfortunately, the switching leads to a much
noisier EMI profile. We can greatly decrease some of the
radiated emissions with some spread spectrum techniques.
Spread spectrum is used to reduce the peak electromagnetic
emissions of a switching regulator.
f
c
9f
c
7f
c
5f
c
3f
c
f
c
9f
c
7f
c
5f
c
3f
c
t
t
V
V
Time Domain
Frequency Domain
Unmodulated
The spread spectrum used in the NCV97310 is an
“up−spread” technique, meaning the switching frequency is
spread upward from the 2.0 MHz base frequency. For
example, a 5 % spread means that the switching frequency
is swept (spread) from 2.0 MHz up to 2.1 MHz in a linear
fashion – this is called the modulation depth. The rate at
which this spread takes place is called the modulation
frequency. For example, a 10 kHz modulation frequency
means that the frequency is swept from 2.0 MHz to 2.1 MHz
in 50
m
s and then back down from 2.1 MHz to 2.0 MHz in
50
m
s.
