ncp1578 ON Semiconductor, ncp1578 Datasheet - Page 18

• Manufacturer Part Number: ncp1578
• Description: Synchronous Step Down Controller With 50ma Linear Regulator
• Manufacturer: ON Semiconductor

stable and high bandwidth overall loop. Worst case
component variation should be considered when selecting
Input Capacitor Selection
power supply source. The input capacitors should be placed
as close as possible to the drain of the high-side MOSFET
and source of the low-side MOSFET. The PCB trace style
should be in form of short and wide ones. The voltage rating
and the RMS current rating are the important parameters for
the bulk input capacitor. In typical application, the bulk
capacitor should be selected such that the voltage and
current ratings must above the maximum input voltage and
largest RMS current required by system. As a safety
guideline, the capacitor voltage rating should be at least 1.5
times greater than the maximum input voltage. And the
RMS current rating requirement is approximately half of the
DC load current. The required input capacitor RMS ripple
current rating may be estimated by the following equation:-
capacitor for notebook application due to its low ESR and
good ripple current rating and high voltage rating.
Aluminum electrolytic capacitors are also good choice.
They are relatively low cost but they should be used in
parallel connection to lower the ESR which is intrinsically
high compared with ceramic capacitors.
Output Capacitor Selection
ripple voltage and the initial voltage drop after a high
slew-rate transient. So, only specialized low-ESR
I
It is used to minimize the input voltage ripple from the
Ceramic capacitor is the good choice of the input
The output capacitor's ESR will determine the output
Cin(RMS)
w I
out
V
V
in(min)
out
*
0dB
V
V
in(min)
out
F
Z1
Modulator & Filter Gain
Figure 36. Bode Plot of the Converter Gain
= 0.5 F
2
F
LC
LC
F
ESR
F
Z2
(eq. 8)
http://onsemi.com
= F
F
Open Loop Error Amp Gain
LC
FREQUENCY
P1
NCP1578
= F
Converter Gain
ESR
18
the components of impedance network such that the control
loop phase margin should be greater than 45°.
capacitors should be used for switching regulator
applications. For steady state ripple, both ESR and
capacitance of the output capacitor contribute output ripple
voltage. Normally, ESR is the dominant factor for output
ripple voltage. The output ripple voltage DV
estimated by the following equation:
and
DV
Where:
DI
ESR = Effective Series Resistance of the output capacitor
L = Inductance
C
F
V
V
ripple voltage can be reduced by either using the inductor
with larger inductance or the output capacitor with smaller
ESR value. In general rule of thumb, the inductor ripple
current is typically 30% of the maximum load current and
the ripple voltage is typically 2% of the output voltage.
response of load step up or release transients. The voltage
S
o
out
in
DV
DI
From the above equations, it can be seen that the output
The output capacitor also plays an important role in
L
= Switching frequency
= Regulated output voltage
o
= Input Voltage
F
L
= Inductor ripple current
= DI
= Output capacitance
P2
o
+
+ DI
= 0.5 F
(V
L
L
Compensation Gain
in
X ESR if C
S
L
* V
F
S
ESR )
o
)
V
in
V
out
o
8
is large enough.
C
DI
out
L
F
S
o
(eq. 10)
can be
(eq. 9)