NCP1587 ONSEMI [ON Semiconductor], NCP1587 Datasheet - Page 8

• Manufacturer Part Number: NCP1587
• Description: Low Voltage Synchronous Buck Controller
• Manufacturer: ONSEMI [ON Semiconductor]

Input Capacitor Selection
produced during the on time of the upper MOSFET, so it
must have a low ESR to minimize the losses. The RMS value
of this ripple is:
where D is the duty cycle, Iin
& I
maximum value with D = 0.5. Loss in the input capacitors
can be calculated with the following equation:
where P
ESR
capacitance. Due to large dI/dt through the input capacitors,
electrolytic or ceramics should be used. If a tantalum must
be used, it must by surge protected. Otherwise, capacitor
failure could occur.
Calculating Input Start-up Current
equation can be used.
where I
total output capacitance, V
and t
current during max load, then the input fuse should be rated
accordingly, if one is used.
Calculating Soft Start Time
be used.
Where C
capacitor,
C
I
DV is the comp voltage from zero to until it reaches
regulation: ((d * ramp) + 0.9)
ss
Vcomp
p
The input capacitor has to sustain the ripple current
To calculate the input start up current, the following
If the inrush current is higher than the steady state input
To calculate the soft start time, the following equation can
is the soft start current
Vout
is the additional capacitor that forms the second pole.
OUT
CIN
SS
inrush
is the soft start interval.
CIN
is the effective series resistance of the input
c
is the load current. The equation reaches its
is the compensation as well as the soft start
is the power loss in the input capacitors &
is the input current during start-up, C
Iin
P
RMS
CIN
I
inrush
t
ss
+ ESR
+ I
+
+
OUT
(C
DV
C
OUT
p
OUT
CIN
) C
RMS
900 mV
D
is the desired output voltage,
I
t
ss
SS
c
is the input RMS current,
) * DV
Iin
V
(1 * D) ,
OUT
RMS
,
2
,
APPLICATION SECTION
OUT
http://onsemi.com
is the
8
rising to when output voltage starts becomes valid.
reaches regulation; DV is the difference between the comp
voltage reaching regulation and 0.9 V.
Output Capacitor Selection
response of the power supply. In fact, during load transient,
for the first few microseconds it supplies the current to the
load. The controller immediately recognizes the load
transient and sets the duty cycle to maximum, but the current
slope is limited by the inductor value.
drops due to the current variation inside the capacitor and the
ESR. ((neglecting the effect of the effective series
inductance (ESL)):
where V
effects of ESR and the ESR
resistance of the output capacitors.
current during the load transient without discharging it. The
voltage drop due to output capacitor discharge is given by
the following equation:
where V
due to the effects of discharge, L
value & V
ΔV
of these two voltages will determine the maximum deviation
of the output voltage (neglecting the effect of the ESL).
Inductor Selection
the selection of an output inductor. From a mechanical
perspective, smaller inductor values generally correspond to
smaller physical size. Since the inductor is often one of the
largest components in the regulation system, a minimum
inductor value is particularly important in space-constrained
applications. From an electrical perspective, the maximum
current slew rate through the output inductor for a buck
regulator is given by:
regulator’s ability to slew current through the output
inductor in response to output load transients. Consequently,
output capacitors must supply the load current until the
inductor current reaches the output load current level. This
results in larger values of output capacitance to maintain
DV
The above calculation includes the delay from comp
To calculate the time of output voltage rising to when it
The output capacitor is a basic component for the fast
During a load step transient the output voltage initial
A minimum capacitor value is required to sustain the
It should be noted that ΔV
Both mechanical and electrical considerations influence
This equation implies that larger inductor values limit the
OUT-ESR
OUT−DISCHARGE
OUT-ESR
OUT-DISCHARGE
IN
are out of phase with each other, and the larger
DV
is the input voltage.
SlewRate
OUT−ESR
is the voltage deviation of V
+
2
LOUT
+ DI
is the voltage deviation of V
COUT
C
+
OUT
OUT
DI
V
OUT
is the total effective series
OUT
IN
L
* V
ESR
(V
OUT
OUT-DISCHARGE
is the output inductor
2
IN
OUT
COUT
L
OUT
D * V
OUT
due to the
OUT
)
OUT
and
,