NCP1597A
http://onsemi.com
11
APPLICATION INFORMATION
Programming the Output Voltage
The output voltage is set using a resistive voltage divider
from the output voltage to FB pin (see Figure 25). So the
output voltage is calculated according to Eq.1.
V
out
+ V
FB
@
R
1
) R
2
R
2
(eq. 1)
Figure 25. Output divider
FB
R2
R1
V
out
Inductor Selection
The inductor is the key component in the switching
regulator. The selection of inductor involves trade−offs
among size, cost and efficiency. The inductor value is
selected according to the equation 2.
L +
V
out
f @ I
ripple
@ ǒ1 *
V
out
V
in(max)
Ǔ
(eq. 2)
Where V
out
− the output voltage;
f − switching frequency, 1.0 MHz;
I
ripple
− Ripple current, usually it’s 20% − 30% of output
current;
V
in(max)
− maximum input voltage.
Choose a standard value close to the calculated value to
maintain a maximum ripple current within 30% of the
maximum load current. If the ripple current exceeds this
30% limit, the next larger value should be selected.
The inductor’s RMS current rating must be greater than
the maximum load current and its saturation current should
be about 30% higher. For robust operation in fault conditions
(start−up or short circuit), the saturation current should be
high enough. To keep the efficiency high, the series
resistance (DCR) should be less than 0.1 W, and the core
material should be intended for high frequency applications.
Output Capacitor Selection
The output capacitor acts to smooth the dc output voltage
and also provides energy storage. So the major parameter
necessary to define the output capacitor is the maximum
allowed output voltage ripple of the converter. This ripple is
related to capacitance and the ESR. The minimum
capacitance required for a certain output ripple can be
calculated by Equation 4.
C
OUT(min)
+
I
ripple
8 @ f @ V
ripple
(eq. 3)
Where V
ripple
is the allowed output voltage ripple.
The required ESR for this amount of ripple can be
calculated by equation 5.
ESR +
V
ripple
I
ripple
(eq. 4)
Based on Equation 2 to choose capacitor and check its
ESR according to Equation 3. If ESR exceeds the value from
Eq.4, multiple capacitors should be used in parallel.
Ceramic capacitor can be used in most of the applications.
In addition, both surface mount tantalum and through−hole
aluminum electrolytic capacitors can be used as well.
Maximum Output Capacitor
NCP1597A family has internal 1 ms fixed soft−start and
overcurrent limit. It limits the maximum allowed output
capacitor to startup successfully. The maximum allowed
output capacitor can be determined by the equation:
C
out(max)
+
I
lim(min)
* I
load(max)
*
Di
p−p
2
V
out
ńT
SS(min)
(eq. 5)
Where T
SS(min)
is the minimum soft−start period (1ms);
D
iPP
is the current ripple.
This is assuming that a constant load is connected. For
example, with 3.3 V/2.0 A output and 20% ripple, the max
allowed output capacitors is 546 mF.
Input Capacitor Selection
The input capacitor can be calculated by Equation 6.
C
in(min)
+ I
out(max)
@ D
max
@
1
f @ V
in(ripple)
(eq. 6)
Where V
in(ripple)
is the required input ripple voltage.
D
max
+
V
out
V
in(min)
is the maximum duty cycle.
(eq. 7)
Power Dissipation
The NCP1597A is available in a thermally enhanced
6−pin, DFN package. When the die temperature reaches
+185°C, the NCP1597A shuts down (see the
Thermal−Overload Protection section). The power
dissipated in the device is the sum of the power dissipated
from supply current (PQ), power dissipated due to switching
the internal power MOSFET (P
SW
), and the power
dissipated due to the RMS current through the internal
power MOSFET (PON). The total power dissipated in the
package must be limited so the junction temperature does
not exceed its absolute maximum rating of +150°C at
