ncp5214 ON Semiconductor, ncp5214 Datasheet - Page 19

• Manufacturer Part Number: ncp5214
• Description: 2-in-1 Notebook Ddr Power Controller
• Manufacturer: ON Semiconductor

Input Capacitor Selection for VDDQ Buck Regulator
operation of the buck regulator. It minimizes the input
voltage ripple and current ripple from the power source by
providing a local loop for switching current. The input
capacitor should be placed close to the drain of the
high−side MOSFET and source of the low−side MOSFET
with short, wide traces for connection. The input capacitor
must have large enough rms ripple current rating to
withstand the large current pulses present at the input of the
bulk regulator due to the switching current. The required
input capacitor rms ripple current rating can be estimated
by the following with minimum V
be at least 1.25 times of the maximum input voltage.
Capacitance of around 20 mF to 50 mF should be sufficient
for most DDR applications. Ceramic capacitors are the
most suitable choice of input capacitor for notebook
applications due to their low ESR, high ripple current, and
high voltage rating. POSCAP or OS−CON capacitors can
also be used since they have good ESR and ripple current
rating, but they are larger in size and more expensive.
Aluminum electrolytic capacitors are also a choice for their
high voltage rating and low cost, but several aluminum
capacitors in parallel should be used for the required ripple
current. If ceramic capacitors are used, X5R and X7R types
are preferred rather than the Y5V type since the X5R and
X7R types are ceramic capacitors and have smaller
tolerance and temperature coefficient.
Output Capacitor Selection for VDDQ Buck Regulator
steady state output ripple voltage, load transient
requirement, and loop compensation stability. The ESR
and the capacitance of the output capacitor are the most
important parameters needed to be considered. In general,
the output capacitor must have small enough ESR for
output ripple voltage and load transient requirement.
Besides, the capacitance of the output capacitor should be
large enough to meet the overshoot and undershoot during
load transient. Since steady state output ripple voltage,
transient load undershoot and overshoot are the largest at
maximum V
capacitor should be estimated at the maximum V
condition.
capacitance of the output capacitor are the contributing
factors, however, the capacitor ESR is the dominant factor.
The output ripple voltage is calculated as follows:
The input capacitor is important for proper regulation
Besides, the voltage rating of the input capacitor should
The output filter capacitor plays an important role in
For steady output ripple voltage, both ESR and
V ripple + I L(ripple)
I CIN(RMS) w I OUT
IN
, the ESR and capacitance of output
ESR )
V OUT
V IN
IN
I L(ripple)
:
*
C OUT
V OUT
V IN
APPLICATION INFORMATION
t on
2
(eq. 1)
(eq. 2)
http://onsemi.com
NCP5214
IN
19
V ripple + I L(ripple)
where I
and C
equation:
where L is the inductance and f
frequency. The output ripple voltage can be reduced by
either using the inductor with larger inductance or the
output capacitor with smaller ESR. Thus, the ESR needed
to meet the ripple voltage requirement can be obtained by:
maximum load current and the ripple voltage is typically
2% of the output voltage. Thus, the above inequality can be
simplified to:
both the load−rise and the load−release responses. The
voltage undershoot during step−up load can be calculated
by the equation:
where DI
term is ignored, then it becomes the following inequality:
requirement at step−up load transient can be estimated
from the above inequality.
obtained by the following:
because the excessive stored energy in the inductor is
absorbed by the output capacitor. The overshoot voltage
can be calculated by the following equation:
V undershoot + DI LOAD
V overshoot +
The inductor ripple current can be calculated by the
The inductor ripple current is typically 30% of the
For the load transient, the output capacitor contributes to
The maximum ESR requires to meet voltage undershoot
Then, the required output capacitor capacitance can be
C OUT w
The output voltage overshoot during load−release is
OUT
L(ripple)
LOAD
is the output capacitance.
I L(ripple) +
ESR v
V undershoot −DI LOAD
is the inductor ripple current, t
is the change in output current. If the second
ESR v
LI 2 STEP(peak) ) C OUT V 2 OUT
ESR v
V ripple
(V IN −V OUT )
ESR, for small t on and large C OUT
DI LOAD
0.3
(V IN −V OUT )
0.02
V undershoot
L
ESR )
DI LOAD
L
I LOAD(max)
C OUT
f SW
V OUT
f SW
DI LOAD
SW
C OUT
ESR
V OUT
V IN
V OUT
is the switching
V IN
on
1−
is on−time,
f SW
1−
V OUT
f SW
−V OUT
V IN
(eq. 10)
V OUT
(eq. 3)
(eq. 4)
(eq. 5)
(eq. 6)
(eq. 7)
(eq. 8)
(eq. 9)
V IN